Author Topic: any FV-1 cabsim?  (Read 429 times)

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potul

any FV-1 cabsim?
« on: November 23, 2017, 10:32:47 AM »
Hi All

I'm aware it's not possible to do convolutioni and use IR with the FV-1, so I guess it's not feasible to get a well donce cabsim with it... But I was wondering if at least some "poor man's" cabsim is possible. At the end... when looking at the simple DIY analog cabsims they are nothing more than filters...

So my question is, has anyone attempted to code some sort of cabsim with the FV-1?

If no, and I would need to start from scratch... what sort of filter should I be looking at to get a response similar to a cabsim?

Mat

Digital Larry

Re: any FV-1 cabsim?
« Reply #1 on: November 23, 2017, 12:32:59 PM »
FV-1 is set up to do single pole recursive filters and 2-pole state variable filters pretty efficiently.  I've also seen 2-pole direct form IIRs done using delay RAM for the memory storage.

The old Hughes and Kettner Red Box "cab sim" direct box was just a 2-pole resonant low pass filter IIRC which could easily be made using the state variable structure.  My take on what people mean these days by "cab sim" is something that works with an impulse response.  You might be able to implement something that used 3 or 4 instructions per impulse response sample in an FIR.  You'd need to scale the input way down to prevent overload after summing up 32 samples.

Let's suppose you could get 32 samples (128 total instructions/4) at 32 kHz, that is a 1 msec impulse response to create your filter coefficients.  My guess is that this is not going to be adequate to get a lot of accuracy at low frequencies.
Digital Larry
Holy City Audio - home of SpinCAD Designer
http://www.holycityaudio.com

ElectricDruid

Re: any FV-1 cabsim?
« Reply #2 on: November 23, 2017, 04:10:57 PM »
You could do something equivalent to this in the FV-1:



That's only a 2-pole LPF followed by a 2-pole HPF. That's going to be easy enough, I'd have thought.

Genuine impulse response sims are going to take more data-munching than the FV-1 will do, so you're back to faking it like we do in analog....

Tom

potul

Re: any FV-1 cabsim?
« Reply #3 on: November 23, 2017, 04:20:46 PM »
You could do something equivalent to this in the FV-1:



That's only a 2-pole LPF followed by a 2-pole HPF. That's going to be easy enough, I'd have thought.

Genuine impulse response sims are going to take more data-munching than the FV-1 will do, so you're back to faking it like we do in analog....

Tom

That's exactly what I had in mind.... a simple set of some 2-pole filters... I'm not trying to do any genuine IR based cab sim ... I just want to have an equivalent to these simple DIY cabsims we used to like not so long ago... :)

I will try to replicate the topology of the Bulldog, or maybe the Condor (http://runoffgroove.com/condor.html), that uses an LPF, a HPF and a notch.

from the runoffgroove page:

Quote
Bridged-T notch filter. Produces the 400Hz notch found in the P10R response. Please note that the extra resistor in parallel with the capacitor to ground is not only for biasing the op-amp, but it also produces a high-pass shelving effect. High frequencies are then 6dB above low frequencies, as shown in the speaker response curve. To reduce the notch depth, you can replace the 1n and 15n caps with 1n5 and 10n, or even 2n2 and 6n8, respectively.

High-pass filter. This is the network between the first two op-amp stages. It simulates the low frequency rolloff of the speaker cabinet, plus the 90Hz peak. For deeper bass, you can replace the two 47nF caps with 56nF or 68nF. For less bass, use 39nF or 33nF.

Low-pass filter. These are the two identical networks between second and fourth op-amp stages. They simulate the steep high frequency rolloff of a guitar speaker, in addition to the peak near 3kHz. For more high frequency content, you can reduce the four 22k resistors to 18k or less. To cut more highs, increase them to 27k or more.

ElectricDruid

Re: any FV-1 cabsim?
« Reply #4 on: November 24, 2017, 04:24:23 PM »
There'll be code for the simple two-pole filters on the SpinSemi website too. I'd have thought you can get close by bolting some stuff together and tweaking the coefficients to taste/by experimentation.

Good luck...;)

T.

Digital Larry

Re: any FV-1 cabsim?
« Reply #5 on: November 24, 2017, 08:03:23 PM »
There are blocks for two pole state variable filters (high/low/bandpass) and a separate notch block (for no particularly good reason other than laziness on my part) in the latest version of SpinCAD Designer (see signature line).  It shouldn't take much time to get something basic going on and then it's just a matter of tweaking things.
Digital Larry
Holy City Audio - home of SpinCAD Designer
http://www.holycityaudio.com

potul

Re: any FV-1 cabsim?
« Reply #6 on: November 25, 2017, 04:55:01 AM »
There are blocks for two pole state variable filters (high/low/bandpass) and a separate notch block (for no particularly good reason other than laziness on my part) in the latest version of SpinCAD Designer (see signature line).  It shouldn't take much time to get something basic going on and then it's just a matter of tweaking things.

Yep, ... this is what I had in mind. I will try and test

potul

Re: any FV-1 cabsim?
« Reply #7 on: November 27, 2017, 05:26:19 AM »
There are blocks for two pole state variable filters (high/low/bandpass) and a separate notch block (for no particularly good reason other than laziness on my part) in the latest version of SpinCAD Designer (see signature line).  It shouldn't take much time to get something basic going on and then it's just a matter of tweaking things.

Hi Larry

I was playing a bit with the SpinCAD blocks.. and I have a doubt. The Frequency Control Input in these blocks, what does it represent? Is this the frequency coeff, or directly Hz, or maybe % vs Sampling freq?

In other words, if I want to have a filter with a giv en Hz, how do I calculate what to give to the block?

Mat



potul

Re: any FV-1 cabsim?
« Reply #8 on: November 28, 2017, 08:24:28 AM »
There are blocks for two pole state variable filters (high/low/bandpass) and a separate notch block (for no particularly good reason other than laziness on my part) in the latest version of SpinCAD Designer (see signature line).  It shouldn't take much time to get something basic going on and then it's just a matter of tweaking things.

Hi Larry

I was playing a bit with the SpinCAD blocks.. and I have a doubt. The Frequency Control Input in these blocks, what does it represent? Is this the frequency coeff, or directly Hz, or maybe % vs Sampling freq?

In other words, if I want to have a filter with a giv en Hz, how do I calculate what to give to the block?

Mat

I was digging into the code of SpěnCAD and I could not fully get how the frequency control input is managed in the filters. I see you used an EXP and SOF to scale it, and then just use it in the filter as a coeff. So no direct relationship with the real frequency, but I suppose good enough for a pot control. BTW, are the LPF and HPF (2/4P ones) SVF filters in disguise?

On the other hand, I see there are fixed SVF blocks where I can provide the frequency directly, and they seem to work fine. I've done some testing today with them, and I have a prototype of a cabsim emulating the ROG Condor. I just need to figure out what resonance to use... If I have time later I will post some clips.

Mat




potul

Re: any FV-1 cabsim?
« Reply #9 on: November 28, 2017, 08:56:49 AM »
Ok,

I could not wait until going home... so during lunch time I did this. It's just the output file generated by SpinCAD, but can give an idea. As soon as I can I will test it in the real FV-1
It starts without cabsim, and at around second 10 it kicks in. There's some reverb all over.

https://soundcloud.com/mateu-ros/out3

Mat

Digital Larry

Re: any FV-1 cabsim?
« Reply #10 on: November 29, 2017, 09:58:35 PM »
I was digging into the code of SpěnCAD and I could not fully get how the frequency control input is managed in the filters. I see you used an EXP and SOF to scale it, and then just use it in the filter as a coeff. So no direct relationship with the real frequency, but I suppose good enough for a pot control. BTW, are the LPF and HPF (2/4P ones) SVF filters in disguise?

On the other hand, I see there are fixed SVF blocks where I can provide the frequency directly, and they seem to work fine. I've done some testing today with them, and I have a prototype of a cabsim emulating the ROG Condor. I just need to figure out what resonance to use... If I have time later I will post some clips.

Mat

Some of the filter blocks may have some control processing to give them a more log vs. control position frequency control curve.  And some would just scale the filter coefficient directly.  How does this work theoretically?  I can't exactly remember.  The filter with log control in it was probably taken from a voltage controlled filter example.  Adjust by ear with judicious use of scale/offset!

I'm pretty sure the 4 pole filters are two 2 pole SVFs in a row.  Supposedly there is a way to make a 4 pole SVF directly in a single structure but I haven't implemented anything like that yet.

Thanks,

DL
Digital Larry
Holy City Audio - home of SpinCAD Designer
http://www.holycityaudio.com

potul

Re: any FV-1 cabsim?
« Reply #11 on: November 30, 2017, 03:07:14 AM »
Some of the filter blocks may have some control processing to give them a more log vs. control position frequency control curve.  And some would just scale the filter coefficient directly.  How does this work theoretically?  I can't exactly remember.  The filter with log control in it was probably taken from a voltage controlled filter example.  Adjust by ear with judicious use of scale/offset!

I'm pretty sure the 4 pole filters are two 2 pole SVFs in a row.  Supposedly there is a way to make a 4 pole SVF directly in a single structure but I haven't implemented anything like that yet.

This confirms what I was able to reverse-engineer from the code. The adjustable block are using some scaling of the coefficient and that's it. The fixed SVF filters use the frequency directy (the coeff gets calculated on SpinCAD before generating the ASM), and they should be ok for this usage, I don't want to have tweakable frequency.

By the way, I was looking for the .spincad files for the filter blocks and I couldn't find them. I assume you coded them directly in java, am I right?

Digital Larry

Re: any FV-1 cabsim?
« Reply #12 on: November 30, 2017, 08:47:29 AM »
By the way, I was looking for the .spincad files for the filter blocks and I couldn't find them. I assume you coded them directly in java, am I right?

By my answer you'll conclude that I haven't spent much time on this recently... "probably"!   ;D

All of the blocks that were coded directly in Java can be found here:

https://github.com/HolyCityAudio/SpinCAD-Designer/tree/master/src/com/holycityaudio/SpinCAD/CADBlocks

I coded all of the initial blocks in Java which I found so excruciating after awhile that I forced myself to develop the SpinCAD language to allow using marked up Spin ASM.  Those were the good old days.  I had no idea what I was doing but managed to prevail nevertheless.
Digital Larry
Holy City Audio - home of SpinCAD Designer
http://www.holycityaudio.com

ElectricDruid

Re: any FV-1 cabsim?
« Reply #13 on: November 30, 2017, 03:03:55 PM »
Ok,

I could not wait until going home... so during lunch time I did this. It's just the output file generated by SpinCAD, but can give an idea. As soon as I can I will test it in the real FV-1
It starts without cabsim, and at around second 10 it kicks in. There's some reverb all over.

https://soundcloud.com/mateu-ros/out3

Mat

Not bad for a first shot, but it sounds too resonant to me. There's a slightly thinned-out, whistly character that I associate with high resonance and passband drop-off.

T.

potul

Re: any FV-1 cabsim?
« Reply #14 on: November 30, 2017, 05:35:09 PM »
Ok,

I could not wait until going home... so during lunch time I did this. It's just the output file generated by SpinCAD, but can give an idea. As soon as I can I will test it in the real FV-1
It starts without cabsim, and at around second 10 it kicks in. There's some reverb all over.

https://soundcloud.com/mateu-ros/out3

Mat

Not bad for a first shot, but it sounds too resonant to me. There's a slightly thinned-out, whistly character that I associate with high resonance and passband drop-off.

T.

True... I just tested it in the FV-1 and it sounded awfully resonant, both low and high frequencies. I will try tomorrow with a more subtle setting.