b3ll
commented
5 years ago Ok, so I tried both of these, both together and individually and I didn't notice anything different, unfortunately. These things do seem like they're correct regardless (especially zeroing out the ever increasing output filter, and the emptying bucket when there's no clock ticks).
However, I did some more analysis and it looks like the issue is even worse when the delay time is being modulated.
Assuming that what I'm doing is correct (this should be a single delay line, with step size of 256, modulating a delay value from 0.00166 to 0.00535 over a period of 1hz with a simple sine wave:
const float modFrequency = 1.0f;
const float minDelay = 0.00166f;
const float maxDelay = 0.00535f;
float modDelayBase = (maxDelay + minDelay) / 2.0f;
float modDelayDepth = (maxDelay - minDelay) / 2.0f;
bucketSize = 256;
auto *leftChannel = buffer.getWritePointer(0);
float leftClock[numSamples];
for (int i = 0; i < numSamples; i++) {
float modDelayValue = sin(2 * M_PI * _leftTime);
float delayTime = modDelayBase + (modDelayDepth * modDelayValue);
leftClock[i] = ((bucketSize * 2.0) / delayTime) / sampleRate;
_leftTime += (modFrequency / sampleRate);
_leftTime -= (int)_leftTime;
}
_leftDelayLine->process(numSamples, leftChannel, leftClock);It ends up still shelving a lot of artifacts in the top end of the spectrum (regardless of what note is being played). I attached a sound file with before / after, when you hear the delay being enabled you'll hear a buzzing noise (alongside the delayed signal). There's also a video showing the filter output after playing without the delay enabled, playing the same note again without the delay enabled, and then the delay being enabled. You'll see the top end is oscillating.
Part of me wants to think that somewhere along (perhaps the start of) the BBD implementation, some noise gets introduced into the delay and then that noise is persisted indefinitely. When I initially read this I thought that the Xout growing unlimited could've been the issue, but I don't think that's the cause. The buzzing is 100% related to the modulation since if I set a slower or faster modulation rate, the buzzing changes to match.
jpcima
@b3ll, I'll describe in some detail what I'm doing here, and I let you experiment what may work or not. Considering the paper is not really clear at all times, I've taken a few guesses.
If the works reaches a point of apparent stability, I'll merge it. For now, I've added 2 commits in the
unit-delaybranch.Filter couldn't operate according to the structure shown, because the unit delay variable was not kept, meaning either algorithm or structure has been incorrect. Most probably algorithm, so I reformulated the filter with use of unit delays (as
Xout_mem).You have the processing made of 2 loops: (as denoted in the paper)
kloop is the outer loop, runs at the sample rate (beingnin code, don't mistake it)nloop the inner one, operating at BBD clock raten-loop implements as: https://github.com/jpcima/bbd-delay-experimental/blob/543cd7ea6e4452ce421451f07262a6bc4eb70889/bbd_line.cc#L72Further, this
n-loop does two different jobs on even/odd ticks:You will observe odd-case will perform this add: https://github.com/jpcima/bbd-delay-experimental/blob/543cd7ea6e4452ce421451f07262a6bc4eb70889/bbd_line.cc#L89
I have found suspicious that BBD's output
Xoutcan be allowed to grow unlimited, as this sum is never reset.From the paper, I was able to read this information.
If I correctly interpret "over one interval of the
kside", it suggest to reset theXoutsum at each frame. Hence I did. https://github.com/jpcima/bbd-delay-experimental/blob/e7f197e8c26f3c60f963006d955b84dd82d7844e/bbd_line.cc#L70-L71Thinking of it, it means at some frames, accumulation may remain 0 if BBD doesn't tick (the case of a slow clock). But also intuitively, it makes sense: in analog world, when BBD doesn't tick, the bucket will empty itself (capacitor discharge), and it's the similar idea when zero are fed into the lowpass following.
Hope this little comment makes sense. I didn't try this action, you can tell me if this has worked or not.