For a school project (a subtask of a thesis), I have to write a popularizing paper. (so not too scientific)
The subject is the comparison of analog and recent digital distortion fx for guitar.
What would be interesting to compare is probably the clipping graphs and frequency responses for showing the harmonics.
Do you guys know any articles or links of comparisons of commercial digital distortion pedals?
What are all the problems of the recent commercial digital distortion fx?
- more intermodulation distortion (is this correct?)
- lower quality because of sampling and quantisation (is this still a problem?)
-
And what is your opinion about the recent dig distortion fx?
Thanks
No matter how many times I have played digital drive and distortion effects, they eventually end up giving me the sh#ts.
I 'went' Boss GT5 for a while... about a month. Couldn't freaking wait to hand it back. I spent all my time with my head virtually in the speaker box tweaking those parameters 'looking' for my sound, always feeling there was this 'missing' element I couldn't quite quantify, only to wake up the next day, plug in and immediately decided I had tweaked it into some weird tone corner and that it sounded like warm puss. I remember staying up late nearly every night I had the thing, trying to get the drive/distortion to sound decent.
As I took more and more patches out of the chain and replaced them with my actual pedals. It just never stopped sounding and feeling wrong/unfamiliar/responding weird/dry harsh and non-intuitively. Now don't get me wrong, that GT would have been hellava useful at the time, I just couldn't dial in anything I could use and feel happy with. The final realization was the 'wrong-ness' of plugging in my good old fav drive alone, with no other effects or patches, and working out that it too was responding oddly. I realized that the second I took it out of the GT loop and went straight in to the amp, all was well with the world again.
The times I have borrowed Pods, I found more I played the Pod the more 'Pod-like' it ends up sounding to me... I usually then notice Poddy sounding recordings on TV adverts radio jingles etc. for the next few days.
Those experiences, especially the expense and ultimate disappointment with the GT series made me the analog lover I am today.
I do use a Pandora for practice and ideas but there is usually a time limit before it tires the ears out.
The way I figure it, imitation raspberry cordial is still IMITATION, no matter how well it is done.
Hope those experiences are of some use to you.
I don't have much experience about digitals... but I totally agree on the "weird feeling-response". the owner of my band's rehearsal room has a T.C.electronics NASA style pedal and I played with it a bit... only bad vibes from it... :P the only cool thing is storing presets... but that's not what this digital vs analog is about..
The biggest issue I would say with digital distortion is that distortion is a very non-linear and not necessarily time invariant effect. So trying to model it with DSPs that really like linear and time invariant systems, it gets "false" sounding.
not to mention most cheap and affordable digital distortions are made from low-bit DSPs which never sound good...
2c
not to sound like the devil's advocate here, but there are a fair amount of software distortion/drive emulation plugins that do a decent job at getting a good "stompbox" distortion. not only that, but they can generally go further into sound design territory (if that's what you're looking for), something most stompboxes cannot do. they're just different tools, imo...
some decent ones for include izotope trash, NI guitar rig, bluetubes drive, a handful of smaller "boutique" plugins that i can't think of the names of.
i agree that digital stompbox/rack distortions generally sound horrible, though...
Digital Distortion is a fine aquaintance.
I became very aquanted with a couple digital devices, have seen, tried...
And noticed every one of them lacks:
The live dynamic.
The reason for it:
1 There's a live dynamic in tube amp / speaker / guitar wood / pickup / strings / air.
2 Digital is 'told' where to go and it says oh, I have a place 'you want to go here'?
Substituting a tube for SS output to speaker is one thing, this can work quite well, taking the output tube distortion characteristics and replacing it with...analog~ !
Ramming a guitar output into a signal decoder, then a 'decider' of tone, then a signal uncoder is another deal entirely, however some question the ability of extremely powerful processing to replicate to some degree the influece that sympathetic vibration has on wood/air/strings and other metals, really it is a rediculous notion that such interactions could be pre-decided.
There is always a predestination with digital. Seemingly endless varieties of replication until the novelty wears off, and the realization sets in that the tones offered are superb and enourmously tweekable, or that the tones offered sound only like a 'live rig' in some ways.
I use and have used multiple digital distortions and amp simulators from the Roland COSM to the J-Station, V-AMP Pro, POD, and Digitech Stomp Pedals.
They have their place and are very flexable units if used properly. One thing most digital Amp simulations and digital distortions lack is dynamics. Amp sims can and digital distortions can be a little anemic. But what they lack in dynamics you gain in flexibility you can have a multitude of tones and effects at a press of a foot switch. The band I was in played "dance music" everything from Santana, ZZ, Tabares, Heatwave, Lighthouse, Billy Ocean, Kool and the Gang etc. so I needed to have a wide variety of tones. If I were playing only a certain genre i.e. vintage rock (which I love) then hands down it would be a tube rig.
Here are some interesting things I've found fiddling with digital amp simulators.
The most important and difficult thing about digital amp sims (or any digital device) is to NOT overdrive the input into digital distortion. Most digital amp sims don't have an input clip light so the only way you know when the input is being overdriven is when it sounds shrill. Many guitarists don't realize this and run their pedals cranked into the amp sim, like they do with a tube amps or SS to overdrive the amplifier's preamp stage. This is a no-no for digital amp sims / digital devices. You can still run your overdrives, effect and booster pedals before the digital hardware but have to be aware that the output levels of the last pedal is not overdriving the the amp sim input stage into digital distortion.
Another thing is, to get good tones from an amp sim, you MUST "dial it in" or tweak the settings to YOUR axe. This can take hours. Most amp sims don't come with good factory settings and this can take some time. I plugged my friend's Strat into one of my favorite amp sim settings which was set for humbuckers and it sounded like crap. we had to fiddle with the tone controls and find the right amp/cab emulator that worked. But even all this tweaking sometimes, like with some amps, an acceptable tone just cant be dialed in.
Good luck on your thesis!
Quote from: Sir H C on March 09, 2009, 11:39:37 AM
The biggest issue I would say with digital distortion is that distortion is a very non-linear and not necessarily time invariant effect. So trying to model it with DSPs that really like linear and time invariant systems, it gets "false" sounding.
But you can make a look-up table of a tube response. Only problem is that it's static; I think.
I think the biggest reason why guitarist don't like digital distortion is that they heard digital distortions with cheap processors. Because of this the ADC and DAC is not good enough; and the processing power is not enough for complicated algorithms.
Do you think that with good enough processors, it is possible to make decent digital distortion?
Thank you all for your replies. Keep em coming.
Quote from: petemoore on March 09, 2009, 01:23:09 PM
Digital Distortion is a fine aquaintance.
I became very aquanted with a couple digital devices, have seen, tried...
And noticed every one of them lacks:
The live dynamic.
The reason for it:
1 There's a live dynamic in tube amp / speaker / guitar wood / pickup / strings / air.
2 Digital is 'told' where to go and it says oh, I have a place 'you want to go here'?
Substituting a tube for SS output to speaker is one thing, this can work quite well, taking the output tube distortion characteristics and replacing it with...analog~ !
Ramming a guitar output into a signal decoder, then a 'decider' of tone, then a signal uncoder is another deal entirely, however some question the ability of extremely powerful processing to replicate to some degree the influece that sympathetic vibration has on wood/air/strings and other metals, really it is a rediculous notion that such interactions could be pre-decided.
There is always a predestination with digital. Seemingly endless varieties of replication until the novelty wears off, and the realization sets in that the tones offered are superb and enourmously tweekable, or that the tones offered sound only like a 'live rig' in some ways.
You say: "
the influece that sympathetic vibration has on wood/air/strings and other metals, really it is a rediculous notion that such interactions could be pre-decided."
I don't get this. There is still a guitar that you are using; so still wood/air/strings. No?
But if you were referring to the modelling fx; I understand.
But I don't want to discuss the modelling stuff. Just a plain distortion. (although that's some kind of modelling too; because it models a tube in a way)
For me, part of the beauty of a singing tube amp is the interaction between edgy pickups and a driven power tubes. Breakup that is just 'so' with a bend. And most importantly, interaction from the proximity of the player, amp and the guitar. Riding all of that properly is a real joy. Kind of like surfing.
So, do you think proximity dynamics can be 'modeled'?
Quote from: tranceracer on March 09, 2009, 01:58:57 PM
I use and have used multiple digital distortions and amp simulators from the Roland COSM to the J-Station, V-AMP Pro, POD, and Digitech Stomp Pedals.
They have their place and are very flexable units if used properly. One thing most digital Amp simulations and digital distortions lack is dynamics. Amp sims can and digital distortions can be a little anemic. But what they lack in dynamics you gain in flexibility you can have a multitude of tones and effects at a press of a foot switch. The band I was in played "dance music" everything from Santana, ZZ, Tabares, Heatwave, Lighthouse, Billy Ocean, Kool and the Gang etc. so I needed to have a wide variety of tones. If I were playing only a certain genre i.e. vintage rock (which I love) then hands down it would be a tube rig.
Here are some interesting things I've found fiddling with digital amp simulators.
The most important and difficult thing about digital amp sims (or any digital device) is to NOT overdrive the input into digital distortion. Most digital amp sims don't have an input clip light so the only way you know when the input is being overdriven is when it sounds shrill. Many guitarists don't realize this and run their pedals cranked into the amp sim, like they do with a tube amps or SS to overdrive the amplifier's preamp stage. This is a no-no for digital amp sims / digital devices. You can still run your overdrives, effect and booster pedals before the digital hardware but have to be aware that the output levels of the last pedal is not overdriving the the amp sim input stage into digital distortion.
Another thing is, to get good tones from an amp sim, you MUST "dial it in" or tweak the settings to YOUR axe. This can take hours. Most amp sims don't come with good factory settings and this can take some time. I plugged my friend's Strat into one of my favorite amp sim settings which was set for humbuckers and it sounded like crap. we had to fiddle with the tone controls and find the right amp/cab emulator that worked. But even all this tweaking sometimes, like with some amps, an acceptable tone just cant be dialed in.
Good luck on your thesis!
Do you know why the lack dynamics and sound anaemic?
Quote from: petemoore on March 09, 2009, 01:23:09 PM
Ramming a guitar output into a signal decoder, then a 'decider' of tone, then a signal uncoder is another deal entirely, however some question the ability of extremely powerful processing to replicate to some degree the influece that sympathetic vibration has on wood/air/strings and other metals, really it is a rediculous notion that such interactions could be pre-decided.
Exactly right Pete.
I know where your coming from.
That's the 'suck factor' that ruined even my favorite old analog drive when plugged in to the GT with no other patches running. The signal got analog digital converted - then again at the out put. Best mystery tone/dynamics kill yet! You see it sounded familiar but it lost all of it's life dynamics and sweetness. Much like an Mp3 with a low bitrate of a famliar song, but of course more than that too. A guitarists signal is interactive it's a work in progress, a two way street with the signal chain.
All the digital stuff I have used eventually feel 2 dimensional in a 3D world.
Maybe if I was in a wedding covers band or something I wouldn't care.
Quote from: carrejans on March 09, 2009, 03:48:46 PM
Quote from: Sir H C on March 09, 2009, 11:39:37 AM
The biggest issue I would say with digital distortion is that distortion is a very non-linear and not necessarily time invariant effect. So trying to model it with DSPs that really like linear and time invariant systems, it gets "false" sounding.
But you can make a look-up table of a tube response. Only problem is that it's static; I think.
I think the biggest reason why guitarist don't like digital distortion is that they heard digital distortions with cheap processors. Because of this the ADC and DAC is not good enough; and the processing power is not enough for complicated algorithms.
Do you think that with good enough processors, it is possible to make decent digital distortion?
Thank you all for your replies. Keep em coming.
Lookup tables are static, also think about the dynamic range, how big a signal before the ADC starts clipping, or do you compress here thereby altering the sound/response of the circuit vs. an analog variant. I love to slam some fuzzes with insane voltages from other pedals, and not sure you can do that with an digital version.
With enough power yes a great one could be made, the problem is that you have to control latency, work hard on the front end to make it robust but not intrusive, and work harder on the digital side.
In the end, it is probably still cheaper for Danelectro to do an analog circuit for this than digital.
Quote from: Toney on March 09, 2009, 04:22:29 PM
Quote from: petemoore on March 09, 2009, 01:23:09 PM
Ramming a guitar output into a signal decoder, then a 'decider' of tone, then a signal uncoder is another deal entirely, however some question the ability of extremely powerful processing to replicate to some degree the influece that sympathetic vibration has on wood/air/strings and other metals, really it is a rediculous notion that such interactions could be pre-decided.
Exactly right Pete.
I know where your coming from.
That's the 'suck factor' that ruined even my favorite old analog drive when plugged in to the GT with no other patches running. The signal got analog digital converted - then again at the out put. Best mystery tone/dynamics kill yet! You see it sounded familiar but it lost all of it's life dynamics and sweetness. Much like an Mp3 with a low bitrate of a famliar song, but of course more than that too. A guitarists signal is interactive it's a work in progress, a two way street with the signal chain.
All the digital stuff I have used eventually feel 2 dimensional in a 3D world.
Maybe if I was in a wedding covers band or something I wouldn't care.
I doubt that it has anything to do with the ADC and DAC. The sampling and quantisation is not audible, I think. The rates are very high.
But keep the discussion going. :)
And I'm still looking for an articles or so of comparisons of commercial digital distortion pedals? Plots or charts.
With good enough A/D and D/A converters, as well as a lot of processing power and good algorithms, you can get some really good sounds.
You know the Axe-FX by Fractal Audio, right?
All the rave right now in the rack world. And MAN, that thing can sound righteous. Go check out their clips.
As pointed out earlier, digital distortion "sucks" because what we're used to are low-line products that are sold to the masses by advertising a lot of DIFFERENT sounds, not necessarily a lot of GOOD sounds.
Andy
Quote from: eleanor296 on March 09, 2009, 04:49:02 PM
With good enough A/D and D/A converters, as well as a lot of processing power and good algorithms, you can get some really good sounds.
You know the Axe-FX by Fractal Audio, right?
All the rave right now in the rack world. And MAN, that thing can sound righteous. Go check out their clips.
As pointed out earlier, digital distortion "sucks" because what we're used to are low-line products that are sold to the masses by advertising a lot of DIFFERENT sounds, not necessarily a lot of GOOD sounds.
Andy
No, never heard of the Axe-fx; but checking it out now.
I also believe that the big problem is that we are used to those low-cost digital distortions.
Does anyone knows which methods or algorithms the commercially available digital distortions use?
I read interesting articles of Julius O. Smith about "numerical solutions of nonlinear ordinary differential equations". Has this been done in commercial pedals?
For those who are interested; I will post the titles of some real interesting papers about implementing digital distortion. They are all very recent publications. I can mail them to you, if you want.
- "Discrete-time models for nonlinear audio systems" - Schattschneider & Zölzer (1999) [-> Volterra]
- "Simulation of the diode limiter in guitar distortion circuits by numerical solution of ordinary differential equations" - Yeh & Abel & Smith (2007) [-> Numerical]
- "Numerical methods for simulation of guitar distortion circuits" - Yeh & Abel & Vladimirescu & Smith (2008) [-> numerical]
- "Simplified, physically-informed models of distortion and overdrive guitar effects pedals" - Yeh & Abel & Smith (2007) [-> simplification]
- "Wave digital simulation of a vacuum-tube amplifier" - Karjalainen & Pakarinen (2006) [-> WDF]
- "Non-linear digital implementation of a parametric analog tube ground cathode amplifier" - Santagata & Sarti & Tubaro (2007) [-> phenomenological model]
Digital may/might possibly have some future...
Analogue definitely has a past/immortality and present, and future...
This topic reminded me of a tidbit on digital distortion pedal design, I reverse engineered the analog in/out section of the Line 6 AM-4 amp modeler (same board as DM-4 fuzzbox modeler ) a while back and was interested to find that despite being a mono pedal, it uses a stereo processor. What they did was there is normal gain going to one channel, and the other channel was fed with 47x the gain (a huge ~5v P-P signal hitting the ADC, from a moderate guitar level input), and was clipped by some diodes in the opamp's feedback loop pre processing. The hi gain sounds, it's reasonable to assume, choose the high gain, analog-ily pre-distorted channel.
Thought that was interesting as a peek into the designer's thinking, that if you were to apply hi gain digitally to a small signal, you'd lose resolution exponentially, as if you're heavily clipping something, you are hearing just a small window around the signal's zero crossing, and if that gain happens in the digital domain, poof, there goes all the dynamic resolution, and with it a measure of the instrument's character and liveliness
This is my own subjective, no-proof-whatsoever opinion, but I believe that the act of sampling a signal, no matter hom many times per second, along with the tiny but real time it takes a processor to 'work' on the signal, leads to an inorganic sound that seems seperate or 'independent' of the entire 'system' of the guitarist+FX+amp combination.
I think having something react in real time, to your ENTIRE signal, is vital to an organic feeling and sounding rig.
We all know that everything matters, and it all interacts, so in my mind cleaving out the guitar line, turning it into ones and zeros, sending that through a formula and reconstituting the result as the guitar signal, takes away the 'soul' and deprives your entire 'system' of the natural and real time changes that occur throughout.
Again, my opinion...
Jay Doyle
I think digital effects don't necessarily need to sound exactly like analog ones. I just think of them as something different, with it's own characteristics.
In science, one of the basic principles is that the more articulately and realistically you can describe a phenomenon in detail, the closer you can get to explaining it, and eventually predicting and controlling it. So, if I know in detail how a cancer cell behaves under various circumstances, the better able I become to explain why it behaves that way, and devise means to influence it.
Distortion is often portrayed in terms of static waveforms. But as I keep reminding people, a guitar is not a steady state signal generator. So, the tone is a product of the interaction between the moment-to-moment properties of the signal, and what happens to the circuit properties, as a function of that ever-changing signal. And its not just the current signal level, relative to some diode forward voltage. It is the current signal level relative to the last 2-3 milliseconds, and what that might or might not do to the battery, the behaviour of electrolytic caps, etc., PLUS the spectral content of the signal. And remember as well that thicker strings have more output than thinner ones.
So, while comments regarding the use of "cheap DSP" chips are marginally applicable here, it is not so much the cheapness of the chip as much as it is the extent to which the chip is able to apply realistic algorithms in real time in a realistic way. That requires speed, and yes resolution, but the world's most expensive 48-bit DSP allocated exclusively to producing distortion digitally will get you nowhere fast unless the algorithms are realistic and comprehensively descriptive of what normally happens to the signal in the analog/real world. remember, that is what one is trying to emulate in the digital domain. Once upon a time, the limitations really WERE in the technology itself, with 8-bit processing at 16khz sample rates and such. Even if you knew exactly what happened to the signal, moment to moment, the technology couldn't productively use that descriptive/algorithmic information. At this point, however, the limitations are really in the human mind and the algorithmic description/depiction of what happens when guitar signmal produce distortion. The chips themselves are more than fast and precise enough to do the job. After all, they can reproduce already-recorded guitar distortion to our satisfaction.
What people dislike most about digital distortion, I think, is not the tone produced. You can get some really nice tones. Rather it is the way that the tone produced is, or feels, less responsive to the dynamics and shifting properties of one's playing. In other words, you can't wring as many different sounds out of digital distortion with your fingers and pick as you can out of an analog distortion.
So, ultimately, what is missing is a complete description for the DSP chip of what to do when you encounter X, Y, and Z in the player's picking. Case in point, there are often requests for identifying what distortion box was used on various 60's hits. People try this one and that one and nothing ever seems to come close. All unit-to-unit variation aside (and that is not inconsiderable), they forget that many of those hits were played by studio musicians on big body jazz guitars using floating bridges and medium-to-heavy gauge strings. The signal hitting the fuzzbox was entirely different, especially in terms of its attack-vs-decay-phase properties, so the behaviour of the circuit was also different. Are current digital models articulate enough to produce such nuanced perfamcne? My hunch is "Not yet".
Again, this is not to say it is impossible. The challenge really IS in the describing rather than the chips now. If you want better digital distortion, you need to spend more time thinking about, and mapping, the properties of analog distortion under a wide range of conditions. As it happens, most experts tend to be unable to describe how they do something, so we've been hampered in the task of creating the appropriate algorithms because we have been hampered in our ability to describe the phenomenon itself. It's not just what shows up on the scope screen. It's what you DID to produce that.
I played digital sims, both hardware and software for years. I recently built some ROG amp pedals as I was starting to get into this hobby. It is like night and day. The pedals beg me to play them, they just sound and feel good. The sims on the other hand beg me to tweak them, hoping to get a good sound. I feel like an idiot for not learning how to build years ago!
I do think it comes down to 2 things. The anolog stuff is just more responsive to picking, guitar volume, etc. And, not having any latency at all really, really helps it to feel, uh, immediate. I swear my playing has improved a bunch from the shear pleasure of playing through pedals and not my DAW.
As far as the math, I'll take your guy's word for it...
I think you need to be careful about what it is your talking about: simulating distortion or simulating an amplifer-speaker system (that may or may not be overdriving at any part of it's signal chain).
There are things you can do in the digital domain that can't readily be done in the analog domain. Some things can't be done at all in the analog domain. And there are companies now taking advantage of this to great effect. For example the Source Audio SoundBlox Multiwave Guitar Distortion (http://www.sourceaudio.net/products/soundblox/multiwave_distortion.php) -- that's an all-digital distortion box that does a number of things analog has a hard time with (or can't do at all). Specifically the multi-band distortion (can be done in analog, but is harder -- you can't build perfect filters in analog, and incremental improvements come at great expense). And their foldback distortion algorithm.
Here's a great paper they released on the Multiwave approach to digital distortion: http://www.sourceaudio.net/whitepapers/multiwave_distortion.pdf
If all you're concerned with is mimicking analog sounds in the digital domain you're only seeing a very small piece of what can be done with digital distortion. The guys at Source Audio, they "get it". Digital gives you new possibilities and they're exploring those, my opinion here, to great effect. Why make digital reproduce what's already done in the analog domain? That's so short sighted really. You've got other companies out there doing nice bit -reduction and sampling-type distortion. Totally unique, digital domain sounds. Onwards and upwards!
Quote from: Mark Hammer on March 10, 2009, 01:16:22 PM
In science, one of the basic principles is that the more articulately and realistically you can describe a phenomenon in detail, the closer you can get to explaining it, and eventually predicting and controlling it. So, if I know in detail how a cancer cell behaves under various circumstances, the better able I become to explain why it behaves that way, and devise means to influence it.
Distortion is often portrayed in terms of static waveforms. But as I keep reminding people, a guitar is not a steady state signal generator. So, the tone is a product of the interaction between the moment-to-moment properties of the signal, and what happens to the circuit properties, as a function of that ever-changing signal. And its not just the current signal level, relative to some diode forward voltage. It is the current signal level relative to the last 2-3 milliseconds, and what that might or might not do to the battery, the behaviour of electrolytic caps, etc., PLUS the spectral content of the signal. And remember as well that thicker strings have more output than thinner ones.
So, while comments regarding the use of "cheap DSP" chips are marginally applicable here, it is not so much the cheapness of the chip as much as it is the extent to which the chip is able to apply realistic algorithms in real time in a realistic way. That requires speed, and yes resolution, but the world's most expensive 48-bit DSP allocated exclusively to producing distortion digitally will get you nowhere fast unless the algorithms are realistic and comprehensively descriptive of what normally happens to the signal in the analog/real world. remember, that is what one is trying to emulate in the digital domain. Once upon a time, the limitations really WERE in the technology itself, with 8-bit processing at 16khz sample rates and such. Even if you knew exactly what happened to the signal, moment to moment, the technology couldn't productively use that descriptive/algorithmic information. At this point, however, the limitations are really in the human mind and the algorithmic description/depiction of what happens when guitar signmal produce distortion. The chips themselves are more than fast and precise enough to do the job. After all, they can reproduce already-recorded guitar distortion to our satisfaction.
What people dislike most about digital distortion, I think, is not the tone produced. You can get some really nice tones. Rather it is the way that the tone produced is, or feels, less responsive to the dynamics and shifting properties of one's playing. In other words, you can't wring as many different sounds out of digital distortion with your fingers and pick as you can out of an analog distortion.
So, ultimately, what is missing is a complete description for the DSP chip of what to do when you encounter X, Y, and Z in the player's picking. Case in point, there are often requests for identifying what distortion box was used on various 60's hits. People try this one and that one and nothing ever seems to come close. All unit-to-unit variation aside (and that is not inconsiderable), they forget that many of those hits were played by studio musicians on big body jazz guitars using floating bridges and medium-to-heavy gauge strings. The signal hitting the fuzzbox was entirely different, especially in terms of its attack-vs-decay-phase properties, so the behaviour of the circuit was also different. Are current digital models articulate enough to produce such nuanced perfamcne? My hunch is "Not yet".
Again, this is not to say it is impossible. The challenge really IS in the describing rather than the chips now. If you want better digital distortion, you need to spend more time thinking about, and mapping, the properties of analog distortion under a wide range of conditions. As it happens, most experts tend to be unable to describe how they do something, so we've been hampered in the task of creating the appropriate algorithms because we have been hampered in our ability to describe the phenomenon itself. It's not just what shows up on the scope screen. It's what you DID to produce that.
Mark, I'm glad you wrote up a big long response. I was about to do the same but was dreading how long my response would be. This topic has so much breadth to cover that it could easily fill up a typical engineering text. Still, I'd like to tack onto what you've already stated.
Doing "real-time" digital signal processing (in quotes because it's always at least a sample behind) for something as dynamic as a distortion is no simple task (duh!). As you've already stated, it's all about the algorithm used. I've seen some great attempts and some not so great ones. One of my favorite bad attempts was one I read about in a students paper who did a multi-fx unit as their senior project. Their distortion algorithm was literally "if x is greater than 2, out = 2, if x is less than -2, out = -2". Essentially, they were doing a static hard clip, which I never heard but guarantee it sounded TERRIBLE!
The real programmers know that in order to properly model an analog-type distortion, you have to understand and model everything in order to obtain the non-linear distortion characteristic you are after. For diode based distortion stuff, it's about accurately modeling the I-V curve of whatever particular diode you like as well as the resistance and capacitance that shifts as you change the signal levels in the device. With a transistor device, you have to design your model according to a set of equations (and limits) that define the operation of a transistor. The tricky part here is modeling the dynamic changes that occur within a transistor as you use a small wiggle at the base to get a much larger one at the collector (and the steady-state parameters as well) so that everything sounds and acts natural. If you don't model the natural exponential nature of a transistor device, for example, you start running into problems like unnatural sounding decay. Plus one has to factor in all of the components surrounding a transistor, these obviously play a vital role in the final sound heard at the output of your distortion box.
Sigh... There's a lot to write about this topic and not enough time!! I've got a 40 page paper due Thursday :icon_eek:..
Quote from: JDoyle on March 10, 2009, 12:37:36 PM
This is my own subjective, no-proof-whatsoever opinion, but I believe that the act of sampling a signal, no matter hom many times per second, along with the tiny but real time it takes a processor to 'work' on the signal, leads to an inorganic sound that seems seperate or 'independent' of the entire 'system' of the guitarist+FX+amp combination.
I think having something react in real time, to your ENTIRE signal, is vital to an organic feeling and sounding rig.
An interesting fact: each foot your ear is away from your speaker is going to add ~1ms delay between playing and hearing the note. Analog latency...
Quote from: Processaurus on March 11, 2009, 12:16:26 AM
Quote from: JDoyle on March 10, 2009, 12:37:36 PM
This is my own subjective, no-proof-whatsoever opinion, but I believe that the act of sampling a signal, no matter hom many times per second, along with the tiny but real time it takes a processor to 'work' on the signal, leads to an inorganic sound that seems seperate or 'independent' of the entire 'system' of the guitarist+FX+amp combination.
I think having something react in real time, to your ENTIRE signal, is vital to an organic feeling and sounding rig.
An interesting fact: each foot your ear is away from your speaker is going to add ~1ms delay between playing and hearing the note. Analog latency...
Plus you have amplitude and phase differences because of part (and speaker) tolerances :D.
Quote from: Cliff Schecht on March 11, 2009, 12:40:56 AM
Quote from: Processaurus on March 11, 2009, 12:16:26 AM
Quote from: JDoyle on March 10, 2009, 12:37:36 PM
This is my own subjective, no-proof-whatsoever opinion, but I believe that the act of sampling a signal, no matter hom many times per second, along with the tiny but real time it takes a processor to 'work' on the signal, leads to an inorganic sound that seems seperate or 'independent' of the entire 'system' of the guitarist+FX+amp combination.
I think having something react in real time, to your ENTIRE signal, is vital to an organic feeling and sounding rig.
An interesting fact: each foot your ear is away from your speaker is going to add ~1ms delay between playing and hearing the note. Analog latency...
Plus you have amplitude and phase differences because of part (and speaker) tolerances :D.
those (speaker) tolerances as well as the x feet distance do add in a digital simulation set up too!
Quote from: Mark Hammer on March 10, 2009, 01:16:22 PM
In science, one of the basic principles is that the more articulately and realistically you can describe a phenomenon in detail, the closer you can get to explaining it, and eventually predicting and controlling it. So, if I know in detail how a cancer cell behaves under various circumstances, the better able I become to explain why it behaves that way, and devise means to influence it.
Distortion is often portrayed in terms of static waveforms. But as I keep reminding people, a guitar is not a steady state signal generator. So, the tone is a product of the interaction between the moment-to-moment properties of the signal, and what happens to the circuit properties, as a function of that ever-changing signal. And its not just the current signal level, relative to some diode forward voltage. It is the current signal level relative to the last 2-3 milliseconds, and what that might or might not do to the battery, the behaviour of electrolytic caps, etc., PLUS the spectral content of the signal. And remember as well that thicker strings have more output than thinner ones.
So, while comments regarding the use of "cheap DSP" chips are marginally applicable here, it is not so much the cheapness of the chip as much as it is the extent to which the chip is able to apply realistic algorithms in real time in a realistic way. That requires speed, and yes resolution, but the world's most expensive 48-bit DSP allocated exclusively to producing distortion digitally will get you nowhere fast unless the algorithms are realistic and comprehensively descriptive of what normally happens to the signal in the analog/real world. remember, that is what one is trying to emulate in the digital domain. Once upon a time, the limitations really WERE in the technology itself, with 8-bit processing at 16khz sample rates and such. Even if you knew exactly what happened to the signal, moment to moment, the technology couldn't productively use that descriptive/algorithmic information. At this point, however, the limitations are really in the human mind and the algorithmic description/depiction of what happens when guitar signmal produce distortion. The chips themselves are more than fast and precise enough to do the job. After all, they can reproduce already-recorded guitar distortion to our satisfaction.
What people dislike most about digital distortion, I think, is not the tone produced. You can get some really nice tones. Rather it is the way that the tone produced is, or feels, less responsive to the dynamics and shifting properties of one's playing. In other words, you can't wring as many different sounds out of digital distortion with your fingers and pick as you can out of an analog distortion.
So, ultimately, what is missing is a complete description for the DSP chip of what to do when you encounter X, Y, and Z in the player's picking. Case in point, there are often requests for identifying what distortion box was used on various 60's hits. People try this one and that one and nothing ever seems to come close. All unit-to-unit variation aside (and that is not inconsiderable), they forget that many of those hits were played by studio musicians on big body jazz guitars using floating bridges and medium-to-heavy gauge strings. The signal hitting the fuzzbox was entirely different, especially in terms of its attack-vs-decay-phase properties, so the behaviour of the circuit was also different. Are current digital models articulate enough to produce such nuanced perfamcne? My hunch is "Not yet".
Again, this is not to say it is impossible. The challenge really IS in the describing rather than the chips now. If you want better digital distortion, you need to spend more time thinking about, and mapping, the properties of analog distortion under a wide range of conditions. As it happens, most experts tend to be unable to describe how they do something, so we've been hampered in the task of creating the appropriate algorithms because we have been hampered in our ability to describe the phenomenon itself. It's not just what shows up on the scope screen. It's what you DID to produce that.
Thank you Mark for this explanation. It really got me thinking. And that's what I want. ;-)
Quote from: iaresee on March 10, 2009, 03:00:00 PM
I think you need to be careful about what it is your talking about: simulating distortion or simulating an amplifer-speaker system (that may or may not be overdriving at any part of it's signal chain).
There are things you can do in the digital domain that can't readily be done in the analog domain. Some things can't be done at all in the analog domain. And there are companies now taking advantage of this to great effect. For example the Source Audio SoundBlox Multiwave Guitar Distortion (http://www.sourceaudio.net/products/soundblox/multiwave_distortion.php) -- that's an all-digital distortion box that does a number of things analog has a hard time with (or can't do at all). Specifically the multi-band distortion (can be done in analog, but is harder -- you can't build perfect filters in analog, and incremental improvements come at great expense). And their foldback distortion algorithm.
Here's a great paper they released on the Multiwave approach to digital distortion: http://www.sourceaudio.net/whitepapers/multiwave_distortion.pdf
If all you're concerned with is mimicking analog sounds in the digital domain you're only seeing a very small piece of what can be done with digital distortion. The guys at Source Audio, they "get it". Digital gives you new possibilities and they're exploring those, my opinion here, to great effect. Why make digital reproduce what's already done in the analog domain? That's so short sighted really. You've got other companies out there doing nice bit -reduction and sampling-type distortion. Totally unique, digital domain sounds. Onwards and upwards!
For my Master's Thesis, I am currently designing a (multi)-effects processor for hexaphonic pickups. In this way I also reduce the intermodulation distortion. I think it might even give better results than the distortion box from Source Audio.
Thanks for the link; I never heard of them.
Quote from: Cliff Schecht on March 10, 2009, 07:39:47 PM
Sigh... There's a lot to write about this topic and not enough time!! I've got a 40 page paper due Thursday :icon_eek:..
Thanks for contributing to this thread. And good luck with the paper; it can be hard sometime. :P
Check out: http://www.simulanalog.org/
There are some white papers and a Guitarsuite VST plugin available based on their research :)
Quote from: carrejans on March 11, 2009, 06:28:19 AM
Thank you Mark for this explanation. It really got me thinking. And that's what I want. ;-)
My pleasure.
What lead
me to this was some reading I had done years ago on the history of recording. It seems that when Thomas Edison was doing the county fair circuit with his wax cylinder recording machine (where you would pay money to record your voice, or that of your girlfriend...presumably to impress your girlfriend), reporters couldn't say enough about how "lifelike" the recordings were. Of course, we laugh at that claim now, but at the time people were simply impressed by the very
fact of a replayable recording. With time, the novelty wore off, at which point we can assume that some people started being able to say "Well, you know, it doesn't sound
exactly like the person's voice in real life". At which point, we can safely assume that other people started going "Hmmm, well just exactly what IS the difference between these recordings and 'real life', and what could we do to nudge the recorded voice closer in that direction?".
And from there we have the steady progression of audio history, as we discover stuff like bandwidth, group delay, intermodulation, and a whole host of parameters of both the signal itself and the technology used to process that signal. So, for instance, there IS no crossover distortion in "real life", but there IS such a thing in the technology used to produce/reproduce sound, and it interferes with the "realism" of sound. Similarly, aliasing is an artifact introduced by the use of clock-based sampling. It does not exist in the acoustic world, but when we try and emulate the acoustic world, we go "Geez, you know that doesn't sound
quite right", at which point we recognize that what lower sampling rates and lower-resolution sampling introduce to the signal
interferes with it sounding "life-like". So, we embark upon improving the sampling technology to the point where it does not interfere with what the human ear needs to hear in order to eliminate audible differences between the real acoustic world and pathetic attempts to mimic it.
That's what I mean by this relationship between description, explanation, prediction, and control. First, you characterize what something IS. Then you say WHY it is that way. Then you can say when it will be like this or that, at which point you can
make it be like this or that. But it all starts with careful observation and thorough description.
As for hex distortion, and in the spirit of what I said above, let us consider what does and doesn't take place when a circuit that can introduce harmonic content is applied to a
multi-string, vs
single string, signal source. You've mentioned intermodulation, but let's go beyond that. Note that the sum total signal of the normal pickup is composite of all the contributions that individual strings make to overall signal amplitude, whereas the clipping threshold of the normal distortion circuit is fixed. What this means is that the extent of clipping is a function of not only how hard the string is plucked, but how many strings are strummed and which ones. Thicker strings will move one inexorably closer to any clipping threshold, and if an unwound string comes along for the ride, then the harmonic changes applied to the signal get applied to it as well. This is, in fact, the very basis for the TS-808 design and the mid-hump. It was an attempt to compensate for the "extra push" provided by wound strings that would introduce disproportionate clipping across the fretboard. By introducing a disadvantage to the wound strings through filtering, the intent was to produce relatively equivalent clipping from low to high E.
But all of that is a) in the analog world, not digital, and b) in the monophonic world, not hex. So the question immediately comes up "How could I sidestep the intermodulation that occurs with a monophonic signal, but at the same time mimic the additive effect that strumming 3 wound strings normally has on a distortion box?". In other words, "Smoke on the Water"
should sound more distorted played on the E, A, and D strings than played on the D, G, and B strings. What this suggests is that six parallel non-interacting processing pathways is not necessarly the route to go. There needs to be some "meta" data which allows the parameters of the individual processing paths to be summed in intelligent ways.
There. I thnk I just made you brew another pot of
much stronger coffee. :icon_lol: You're going to be up late tonight!
"Again, this is not to say it is impossible. The challenge really IS in the describing rather than the chips now. If you want better digital distortion, you need to spend more time thinking about, and mapping, the properties of analog distortion under a wide range of conditions. As it happens, most experts tend to be unable to describe how they do something, so we've been hampered in the task of creating the appropriate algorithms because we have been hampered in our ability to describe the phenomenon itself. It's not just what shows up on the scope screen. It's what you DID to produce that."
Bingo. It's the same reason we can't get jfets or mosfets to sound exactly like tubes. Or a silicon fuzz to be the same as a germanium one. We don't know exactly why the germ does what it does. Is it frequency response, leakage, both? So it makes it hard to simulate something we can't accurately describe.
Tube amps seem to have softer distortion than transistor stomp boxes. Tubes run at higher voltages. Higher voltages increase headroom, not soft overdrive. Simulate diode to ground clipping on PSpice and it looks beautifully round and soft yet comes up way short in real life to our ears....
You got it Mark, we aren't measuring and accurately describing what we are trying to emulate.
Quote from: JDoyle on March 10, 2009, 12:37:36 PM
This is my own subjective, no-proof-whatsoever opinion, but I believe that the act of sampling a signal, no matter hom many times per second, along with the tiny but real time it takes a processor to 'work' on the signal, leads to an inorganic sound that seems seperate or 'independent' of the entire 'system' of the guitarist+FX+amp combination.
I think having something react in real time, to your ENTIRE signal, is vital to an organic feeling and sounding rig.
We all know that everything matters, and it all interacts, so in my mind cleaving out the guitar line, turning it into ones and zeros, sending that through a formula and reconstituting the result as the guitar signal, takes away the 'soul' and deprives your entire 'system' of the natural and real time changes that occur throughout.
Again, my opinion...
Jay Doyle
Agreed. I'm kind of an audiophile. Most high end stereo guys and gals think digital still has problems. Most don't think a $100 DVD player cuts the mustard. Especially the AD and DA converters. Heck, most don't think a $1000 DVD is really all that good! So, I would seriously question the quality of the converters in a POD or my computers soundcard. And I can't believe all these so called analog delays on the market with the digital chip and a low pass to simulate the analog warmth. No thanks. I would prefer to NOT run my guitar signal through a $5 AD and DA converter.
I boils down to; "Can you make a more life-like sculpture of my face using legos or clay?"
The real answer is, if the legos were small enough you couldn't tell the difference. But they don't make 'audio legos' that small... yet.
Quote from: MikeH on March 11, 2009, 03:47:28 PM
The real answer is, if the legos were small enough you couldn't tell the difference. But they don't make 'audio legos' that small... yet.
Good analogy. But I think they do. Sampling at 96 kHz with 24 bits of quantization is more than adequate for our lame ears. It's just not available in consumer audio formats yet.
Quote from: Caferacernoc on March 11, 2009, 11:38:22 AM
Quote from: JDoyle on March 10, 2009, 12:37:36 PM
This is my own subjective, no-proof-whatsoever opinion, but I believe that the act of sampling a signal, no matter hom many times per second, along with the tiny but real time it takes a processor to 'work' on the signal, leads to an inorganic sound that seems seperate or 'independent' of the entire 'system' of the guitarist+FX+amp combination.
I think having something react in real time, to your ENTIRE signal, is vital to an organic feeling and sounding rig.
We all know that everything matters, and it all interacts, so in my mind cleaving out the guitar line, turning it into ones and zeros, sending that through a formula and reconstituting the result as the guitar signal, takes away the 'soul' and deprives your entire 'system' of the natural and real time changes that occur throughout.
Again, my opinion...
Jay Doyle
Agreed. I'm kind of an audiophile. Most high end stereo guys and gals think digital still has problems. Most don't think a $100 DVD player cuts the mustard. Especially the AD and DA converters. Heck, most don't think a $1000 DVD is really all that good! So, I would seriously question the quality of the converters in a POD or my computers soundcard. And I can't believe all these so called analog delays on the market with the digital chip and a low pass to simulate the analog warmth. No thanks. I would prefer to NOT run my guitar signal through a $5 AD and DA converter.
I
would prefer to run my guitar signal through a $5 A/D and D/A, because for that price you can get a stereo audio codec with 24bit/96kHz DAC and ADC's. If you're complaining about sound quality with one of these parts then it's the software, not the hardware (or badly designed hardware).
Example $5 audio codec IC: http://focus.ti.com/lit/ds/symlink/tlv320aic23b.pdf