From bug 952860: (In reply to :Karl Tomlinson (back Jan 28) from comment #4) > Comment on attachment 8357168 [details] [diff] [review] > Use a DC blocker on the square and triangle oscillators. r= > > (In reply to Paul Adenot (:padenot) from comment #3) > > The paper from which I implemented this clearly stated that leaky integrator > > / dc blockers would be needed for this technique to work. > > Yes, I understood leaky integrators were necessary for floating point > rounding > and at frequency changes. Perhaps they are also enough smooth out the > integration discretization error. The paper suggests varying the leak > coefficient when the frequency changes. Perhaps that's an option for future > improvement. > > > > But we should have some leaky integration regardless. > > > I guess 0.999 would be time constant of about 20-25 ms at 44.1 kHz. > > > i.e. a high pass filter at about 40-50 Hz. > > > > I now remember why I chose 0.995: its cut frequency is lower than 20Hz, > > which is kind of the limit under which it does not really matter anymore > > (considering audio equipment and ears). > > I guess my time constant to filter cut-off conversion must be wrong. 0.999 > should give a lower cut-off and less distortion, so keep that if it works > well > enough. The choice now is a bit arbitrary and we can tune later when other > issues are addressed. > > > Anyways, leaky integrating should work, but I switch to merely using `sLeak` > > in this patch, and we still get a bad DC offset. > > Are you are suggesting that leaky integration is different from using > DCBlocker, or just that the parameter is different? I would have expected > 0.995 to kill offsets more rapidly, so I don't understand. > > - mSquare += BipolarBLIT(); > + mSquare = 0.999 * mSquare + BipolarBLIT(); > > - mSaw += UnipolarBLIT() - dcoffset; > + mSaw = 0.999 * mSaw + UnipolarBLIT() - dcoffset; > > should be equivalent to this patch, and seems to be in my testing. You can > go > ahead and assume my r+ if you are happy to land a patch like that, with a > coefficient of sLeaky or closer to 1, whatever you think best. (I won't be > around next week.) > > > I think the problem we have > > is that we don't do anything when changing frequency, whereas I think we > > should be having new integration initial condition for the new frequency / > > phase combination. I'm also pretty sure my maths for the initial integration > > condition at t = 0 were wrong. > > One problem is that the integration sample of the BLIT() should be in the > middle of a phase increment at the current frequency. Currently the BLIT is > performed at t, to integrate from t - 1/(2*samplerate) to > t + 1/(2*samplerate), assuming the frequency is constant over that period. > The phase increment is applied to increment the phase from t to > t + 1/samplerate assuming the frequency is constant over that period. > When the frequency changes, one of these assumptions is incorrect. > I think either the calculation of the BLIT or the phase increment should be > offset by 1/(2*samplerate). > > This also helps consider the initial conditions, which should be for the > point > 1/(2*samplerate) before the BLIT sample. > > The triangle is more complex because the double integration means that DC > offsets add up before they are smoothed by the leaking. The mSquare > integration may need to be leakier for the triangle than for the plain > square. > > I also expect some jumps on frequency changes due to harmonic count changes > changing the positions of the bumps. Resetting initial conditions may halve > this error if done well but it is tricky to set the initial conditions well > near the step in the function. Perhaps blending in the harmonic count > changes > may be an option. > > > In the meantime, we can probably take a patch like this. > > Yes. I'm marking this version r- because the leaky integrator is much > more concise, and easier to see the equation, than using DCBlocker. and then: (In reply to :Karl Tomlinson (back Jan 28) from comment #5) > Created attachment 8358245 [details] [diff] [review] > blit integration experiments > > I'm just putting up this patch to record what I experimented with. > These things at least we should do at some point: > > * Leaky integrate mSquare for the triangle wave. > > * Remove C6 because mSquare has no DC offset. > > * Move the mSignalPeriod adjustment on mTriangle into the integrand so it is > used while it is based on the current frequency. > > These were other experiments that I'm not sure about or not sure I have right > yet: > > * Integrate UniBLIT at half sample interval offset. > > * Get another harmonic for BPBLIT so that frequencies > nyquist/2 > generate output. > > * Integrate at BPBLIT at twice the sample rate.
We don't use BLIT anymore, marking WONTFIX.
Status: NEW → RESOLVED
Last Resolved: 4 years ago
Resolution: --- → WONTFIX
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