On Wed, Apr 28, 2021 at 10:56:58AM -0500, Charles Z Henry wrote: My 2 cents...
The 4-point interpolation scheme gets radically better if the signal it's used on is oversampled (error goes down asymptotically by 24 dB for each doubling of sample rate) - so my own strategy is simply to 4x upsample everything I send through tabread4~ or delread4~. This moves the "problem" to that od designing an upsampling filter, which is much easier than a general interpolator. cheers Miller > I read through the code in your d_delay.c for the delread_sinc implementation. > It's a 22-point interpolator--very high quality and it looks like a > good implementation > > The Hermite spline is the best 4-point cubic interpolator, but the > point of those polynomial interpolators is just to be cheap, and good > enough for regular usage. It doesn't require a table, so it also > doesn't need as much cache space > > I don't think it's *the* optimal 4-point interpolator. It's just the > best cubic polynomial 4-point interpolator. You could re-use your > sinc interpolator code for other shorter interpolation kernels, ones > that are better than cubics but more expensive to calculate. > > At least for me, it's still an open mathematical problem that I've > come back to, from time to time. I started to wonder if I could > expand the error terms in the truncated sinc spectrum in the form of > 1/w, 1/w^2, 1/w^3, .... and then cancel them out by polynomial > correctors. An alternative to windowing the sinc function, this could > be a more precise way to set the rate of attenuation in the stop-band, > while still retaining all the smoothness of the sinc shape. > I get a break from teaching in about 3 weeks. I'm looking at the math > right now, and I don't think I can solve it quickly (or maybe at all). > > How short does your sinc interpolator with Blackman-Harris window need > to be when you start to get the timbre change effect? > > Best, > Chuck > > On Sun, Apr 25, 2021 at 7:18 PM Max <abonneme...@revolwear.com> wrote: > > > > Interesting. I've included that in the test patch. > > It exposes the same samplerate dependent timbre change. So far only the > > Sinc function solves the issue. > > > > > > On 25.04.21 23:15, Sebastian Shader wrote: > > > I have a [vdhs~] object in my library that does hermite spline > > > interpolation for a delay line (like tabread4c~). > > > https://urldefense.com/v3/__https://github.com/sebshader/shadylib__;!!Mih3wA!X_0yWoTK9_e2FJ67U92nwUpVsf4J8D52RrtLxDbQec742PCPmHmQs5HJnxR0$ > > > > > > (also on deken) > > > > > > I had to re-make delread~ as delread4c~ because delread~s delay lines > > > are not exposed in the .h files > > > > > > -seb > > > > > > -----Original Message----- > > > From: Max <abonneme...@revolwear.com> > > > To: pd-list@lists.iem.at <pd-list@lists.iem.at> > > > Sent: Sun, Apr 25, 2021 5:45 am > > > Subject: [PD] 4-point interpolation changes timbre depending on sample > > > rate > > > > > > Hi list, > > > > > > the 4-point interpolation in tabread4~ has been a popular topic in the > > > past, going back to at least 2008. [1] > > > > > > A similar issue is in delread4~. In fact a simple resonator changes its > > > timbre quite drastically by just changing the sample rate of the audio > > > interface. Attached is a test patch. > > > > > > The issue becomes audible when choosing an odd delay time and compare > > > the result between an odd and an even sample rate (e.g. 44.1k vs. 48k). > > > > > > This is not good. In fact this is a serious defect. Imagine you want to > > > market a product like a synth plugin (based on libpd) which sounds > > > different depending on if the daw is opened in 44.1 or 48 kHz. > > > > > > Cyrille Henry has coded tabread4c~ with a four-point cubic interpolation > > > in the nusmuk library, but there is no delread4c~ equivalent in nusmuk. > > > > > > Clemens Wegener has coded delreadsinc~ which implements a > > > Whittaker???Shannon interpolation (Sinc function). This implementation > > > does sounds consistent in any sample rate. It also sounds much better at > > > very slow speeds inside a pitch shifter where delread4~ produces serious > > > artefacts. The Sinc function requires a larger padding for the > > > interpolation. > > > > > > There are a couple of questions regarding on how to contribute this to Pd. > > > Vanilla objects are currently: > > > > > > delwrite~ = the sink > > > delread~ = control rate tap > > > delread4~ (vd~) = audio rate tap with a four-point interpolation > > > > > > 1. the code in delwrite~ isn't agnostic towards the interpolation since > > > it already provides the padding for the 4 point interpolation > > > > > > 2. if we add more interpolation methods to delread / tabread, the > > > cleanest way would be to just have one tap object and the interpolation > > > can be changed by a message and argument to it. currently there are > > > implementations for the miller 4pt, cubic 4pt and Sinc. > > > > > > Unfortunately delread4 already carries the 4 from 4-point interpolation > > > in its name, so probably it would be the best to deprecate that and find > > > a new name like varidelay~ or so. > > > > > > > > > > > > [1] Review of tabread4~ threads in the archive > > > https://urldefense.com/v3/__https://lists.puredata.info/pipermail/pd-list/2019-06/125391.html__;!!Mih3wA!X_0yWoTK9_e2FJ67U92nwUpVsf4J8D52RrtLxDbQec742PCPmHmQsx32PNF8$ > > > > > > <https://urldefense.com/v3/__https://lists.puredata.info/pipermail/pd-list/2019-06/125391.html__;!!Mih3wA!X_0yWoTK9_e2FJ67U92nwUpVsf4J8D52RrtLxDbQec742PCPmHmQsx32PNF8$ > > > > > > > > > > [2] > > > https://urldefense.com/v3/__https://github.com/chairaudio/pure-data/tree/feature/delreadsinc__;!!Mih3wA!X_0yWoTK9_e2FJ67U92nwUpVsf4J8D52RrtLxDbQec742PCPmHmQs9PZY2Zx$ > > > > > > <https://urldefense.com/v3/__https://github.com/chairaudio/pure-data/tree/feature/delreadsinc__;!!Mih3wA!X_0yWoTK9_e2FJ67U92nwUpVsf4J8D52RrtLxDbQec742PCPmHmQs9PZY2Zx$ > > > > > > > > > > _______________________________________________ > > > Pd-list@lists.iem.at <mailto:Pd-list@lists.iem.at> mailing list > > > UNSUBSCRIBE and account-management -> > > > https://urldefense.com/v3/__https://lists.puredata.info/listinfo/pd-list__;!!Mih3wA!X_0yWoTK9_e2FJ67U92nwUpVsf4J8D52RrtLxDbQec742PCPmHmQsyoult3i$ > > > > > > <https://urldefense.com/v3/__https://lists.puredata.info/listinfo/pd-list__;!!Mih3wA!X_0yWoTK9_e2FJ67U92nwUpVsf4J8D52RrtLxDbQec742PCPmHmQsyoult3i$ > > > > > > > > _______________________________________________ > > Pd-list@lists.iem.at mailing list > > UNSUBSCRIBE and account-management -> > > https://urldefense.com/v3/__https://lists.puredata.info/listinfo/pd-list__;!!Mih3wA!X_0yWoTK9_e2FJ67U92nwUpVsf4J8D52RrtLxDbQec742PCPmHmQsyoult3i$ > > > > > > _______________________________________________ > Pd-list@lists.iem.at mailing list > UNSUBSCRIBE and account-management -> > https://urldefense.com/v3/__https://lists.puredata.info/listinfo/pd-list__;!!Mih3wA!X_0yWoTK9_e2FJ67U92nwUpVsf4J8D52RrtLxDbQec742PCPmHmQsyoult3i$ > _______________________________________________ Pd-list@lists.iem.at mailing list UNSUBSCRIBE and account-management -> https://lists.puredata.info/listinfo/pd-list