It's sort of disappointing that it has to be so expensive, but you do what you have to to get the quality and consistency of the sound. The question of whether there is an optimal interpolating kernel (which I'd like to work on) isn't really going to help you in the near term. It's enough just to find a solution that works well and performs well.
You've got a scalability issue when using multiple instances of delreadsinc~: Each new instance recalculates the table when created. The table is stored in the struct _sigvdsinc so it is also duplicated for each instance beyond the first. So, I propose a few other choices:
- Calculate the table during setup and add a class method that
returns a pointer to that table, so that a new object doesn't calculate the table. It just points at the existing table. This option would use memory, even if the object is never used, and would not free the table under any circumstances (not great, but easy) 2. Add a class method that returns NULL if the table has not been created or a pointer to the existing table and a class method to set the pointer. The first instance of the object used would run the initialize_sinc_table function, but each additional instance would not. The table hangs around for future use, even if the instances are deleted. 3. Add the class methods in 2. plus methods to keep a count of the number of instances. Then, when the last instance is deleted, the table is freed and the pointer re-assigned to NULL.
By my approximation, that table uses about (4*4096*10) 160 kB (or 320 kB in double precision). I'm not real clear on the differences in stack, static array allocation, and dynamic allocation in behavior/performance. I know it's there, just can't seem to digest it when I look at it. You might have had some reasons in mind to allocate the array the way you did--I'd like to learn more about this
Best, Chuck
On Wed, Apr 28, 2021 at 2:27 PM Max abonnements@revolwear.com wrote:
Hi Chuck,
Clemens isn't on the list (yet), he sent me some measurements he made in response to your post.
All at 48kHz.
https://cloud.yuca.biz/s/WZseAmBQm9z6NdL
he says he can't hear a difference with 10 samples cutoff@pi and 14 samples cutoff@0.85*pi It sounds better than than delread4~ from 6 samples cutoff@pi upwards.
greetings, Max & Clemens
On 28.04.21 17:56, Charles Z Henry wrote:
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 abonnements@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://github.com/sebshader/shadylib (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 abonnements@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
- the code in delwrite~ isn't agnostic towards the interpolation since
it already provides the padding for the 4 point interpolation
- 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://lists.puredata.info/pipermail/pd-list/2019-06/125391.html https://lists.puredata.info/pipermail/pd-list/2019-06/125391.html
[2] https://github.com/chairaudio/pure-data/tree/feature/delreadsinc https://github.com/chairaudio/pure-data/tree/feature/delreadsinc
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