[PD] 4-point interpolation changes timbre depending on sample rate

[Sebastian Shader]

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 at revolwear.com>
To: pd-list at lists.iem.at <pd-list at 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://lists.puredata.info/pipermail/pd-list/2019-06/125391.html

[2] https://github.com/chairaudio/pure-data/tree/feature/delreadsinc

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