BBD Based Effects, DSPs

[freebird1127]

Hey All, hope you're doing well...

I found a schematic on Mark Hammer's site, a reverb based on BBD chips (The MN3007 and MN3101).  Has anyone built this effect?  What do you think?  Does anyone have experience with these chips?  What did you think?  In general, what do we think of BBD effects?  Pros and cons?

Thanks!
Evan Haklar

[Mark Hammer]

IIRC, there are a few errors in the schematic.  All reparable (so the thing ought to work when they are corrected) but the design will not work as shown (which is probably why Tom Gamble took it down).

[Peter Snowberg]

There are many BBD effects out there from flangers, to chorus, to echos. I love them, but I tend to like anything that I think sounds good. BBDs have their own sonic signature and digital gets really close but the combination of the BBD characteristics and the filtering (and often companding) required to get the best out of them makes them unique sonically. They're not as good for long delays, but that's their main weak point.

Take care,
-Peter

[Mark Hammer]

BBDs are a bit like kids.  There are things they contribute that are relatively impossible to attain in any other way.  At the same time, in much the same way that simply having a kid entails obligatory changes to your living space, your income level, your sleep habits, grocery shopping, etc., BBDs bring a set of obligatory considerations with them that crack the whip over one's head as far as design and layout go, and often with respect to cost, size, component-scoring and power considerations.

That's not a reason to shy away from them, but it can be a big (and for some people, prohibitive) step to go from one- or two-transistor fuzzes to making just about anything with a BBD.

There are BBD based projects that are costlier and more time-consuming and skill-demanding than others, so I wouldn't say there are NO easy BBD projects.  Certainly from a safety perspective, I would happily entertain building something with BBDs before I'd dream of tackling anything with tubes and their voltage ranges.  But having said that, there are certainly more things to think about and factor in when building or designing anything around a BBD.  As well, given the increasing difficulty and cost of obtaining some types of BBDs, screwing up and damaging one because one went a little beyond one's skill-range, is more frustrating than burning out a FET or breaking the lead off the last cap of that value in your parts bin.

People have two sorts of generic attitudes about BBDs.  Some take their benchmark as pristine audio, so BBDs were fine for a while back in the days of 8-bit A/D but come up very short now, and so are poo-pooed.  I understand this perspective.  Others do not use pristine audio as their standard but simply look for something that provides more audio possibilities with as few impediments to productively using those possibilities as possible.  So, yeah, there is the noise and headroom limitations and bandwidth crap to deal with, but reasonable compromises can be made for all of these things.

[freebird1127]

Okay, I gotcha.  As with almost anything new, there are compromises either way...

Basically what I thought when I was looking for a BBD reverb was making a reverb that didn't require spring/tank circuitry.  Is there a better way you might suggest, apart from DSP microprocessors (for now) ?

Ev

Oh yeah, Mark, do you know where/what the error is, or the symptom?
Thx

[Peter Snowberg]

BBDs are good for echos... but real reverb takes either mechanical or digital signal processing. Sorry, not trying to be symantic... the use of the term reverb has been fairly cloudy, but in short.... echos rely on a single delay line while reverbs rely on multiple delay lines in parallel and generally some very specific filtering.

Take care,
-Peter

[Mark Hammer]

Yep.  "Reverb" and "echo" get interchanged the same way that "vibrato" and "tremolo" got interchanged for years.

We've been through this before, but I'll see if I can summarize it quickly.  Reverb does not produce or result in discrete identifiable repetitions, because it is a product of multiple close-range reflections.  You can simulate this with a BBD a little bit if there is more than one delay-time operating.  The MN3011 and MN3214 attempted to do this (and one assumes were designed with it in mind) by providing multiple taps at unrelated delay times (i.e., none of the taps were multiples of any others), which could be combined and intelligently fed back to the input to produce and output that did not consist of clearly identifiable repetitions.

Sadly, these chips have pretty much gone the way of the dodo and are very expensive when you can find them.  Moreover, there aren't a whole lot of projects/dsigns for them out there in webland.  I've seen some for the MN3011 (one was in ETI) but none for the MN3214.

Short-delay  BBDs with a single tap CAN be encouraged to sound a little bit more like a reverb, though, by judicious use of filtering.  Remember that one of the audible differences between echo and reverb is that you can hear and identify the repeats with echo.  By heavily lowpass filtering both the wet signal and the recirculation signal, any BBD-based echo unit can sound a little more like reverb, though obviously not exactly like springs or plate.  The muffling of the recirculation is particularly helpful since, in the real world, harmonic content in reflections dies out pretty quickly, while low end lingers a while longer.

Winter keeps teasing us with its departure here, but once it is truly gone, I'm looking forward to finally being able to work in the garage for more than 5min and completing a mini-spring reverb unit, made from parts you can buy at the hardware store, in a small chassis you can probably stick in your gutar case.  This will be an ersatz dual spring unit.  Hopefully, by that time we'll have some sort of digicam I can use to illustrate because this won't be easy to describe in words alone.

[Nasse]

http://www.spelektroniikka.fi/audiokytkis.htm

OT OT this comment is off topic but could not resist to post the link. Following the link what you find is a digital version of "Reverb" and "echo" get interchanged thingie, this time made with popular and interesting Princeton digital chip for d.i.y ers.

Text is mainly finnish but there is a pcb pattern and lots of construction info and part list etc etc. This text is in English, and what it says is about the topic somehow. But dunno if it is true  :wink:

"This vintage sound digital reverb unit is exellent for guitarists with amplifiers without (spring) reverb. You don't need special power supply, any "normal" unregulated transformer is OK for this delay (output 8.4 … 12 VDC / 200 mA max.)"

The schem is slightly difficult to follow, and have not fully understood the idea of this schem. But if I understand the echo feedback is "fixed" (no pot to adjust it), but maybe the filter part is interesting for some readers. What do you think? The text in Finnish language claims no wonders but says this is a cheap one for hobbyist :wink:

[scratch]

I too have been looking for a non-spring 'reverb' project for some time. There have been 'announcements' in this forum for such projects, one based on multiple PT2399 (6) (guessing that would be to get those different, short delays like the multi-tapped BBD's) and another based on a DSP, but these don't appear to have made the light of day for whatever reason.

[Peter Snowberg]

The DSP version hasn't made it to the light of day thanks to slow work creating a lack of cash to build with. Maybe I'll just get a regular day job.... if I do, the DIY DSP will appear sooner rather than later. I just can't bring myself to asking people if they "want fries with that?" If it comes to that... somebody please shoot me.

You can build DSP reverbs using the Alesis AL3201B and their A/D & D/A. The cost is about the same as getting a 3 spring tank, but the chips are all surface mount with 0.05" pitch leads. They're still not hard to deal with if you have a small iron.

I would love to see a six piece PT2399 reverb. That would be a cool effect for certain.

Take care,
-Peter

[puretube]

...patience....

[freebird1127]

Yeah I've begun working with DSPs in school now so if I have some spare time, I'll try working out some reverb code.  I'm a veteran programmer but still new to DSPs... we're working with the ADSP2181 from Analog Devices.  Anyone familiar with this chip or with coding DSPs have any suggestions I should take note of for this project??

[Peter Snowberg]

For sure you should check out these links:

http://www.stanford.edu/~dattorro/EffectDesignPart1.pdf
http://www.stanford.edu/~dattorro/EffectDesignPart2.pdf
http://www.stanford.edu/~dattorro/EffectDesignPart3.pdf

Part 1 caused a BIG stir when it was published in the AES Journal because it apparently gives away some Lexicon reverb trade secrets. The article is no longer available from AES. Download it now... who knows where it may go in the future.

I've never used any DSPs from Analog, but I've been playing with Motorola DSK56K chips since the 56001 was in production prototype. IMHO it's no replacement for analog circuits, just another option with unique capabilities and it's own sonic signature.

Take care,
-Peter

[freebird1127]

Thanks tons, all.