[PD] Does Pd have a "sound"?
Matt Barber
brbrofsvl at gmail.com
Mon Feb 15 16:53:35 CET 2016
Re: cubic interpolation. Yes and no. Pd and csound both use the same
Lagrange interpolator, which gives discontinuities at segment boundaries,
but the segments it generates are actually a bit closer to what you would
expect from sinc interpolation. SC3's Hermite interpolator, which matches
two points and first derivatives at the boundaries gets rid of the
discontinuities but at the price of some waveform distortion. The Hermite
interpolator is also not continuous at the 2nd derivative on boundaries and
is prone to sudden changes in concavity, while the Lagrange's 2nd
derivative discontinuities are removable; there are no sudden changes.
You can see this in the screenshot I attached, which demonstrates five
interpolators in action.
At the very top is the SR/4 cosine wave which serves as the source for the
interpolators. At the bottom left is what we'd expect from a sinc
interpolator (I haven't implemented it yet, but it should be very close to
a cosine wave).
In red are 1) Pd's [tabread4] cubic Lagrange interpolator using an
array-reading abstraction [array-read4], and 2) The 4-point cubic Hermite
interpolator [array-read4h]. You can clearly see the 1st-derivative
discontinuities at the peaks in the former, and the 2nd-derivative
discontinuities at zero crossings of the latter.
In purple are 1) A 6-point quintic Lagrange interpolator [array-read6], 2)
A 6-point quintic interpolator [array-read6h] which matches four points and
first derivatives, and 3) A 6-point quintic interpolator [array-read6h2]
which matches two points, first derivatives, and second derivatives.
One important thing to notice is how the Lagrange interpolations are much
closer in overall shape to the cosine wave at bottom left. The cost of
matching derivatives is a compromise in the shape of the waveform between
breakpoints.
On Mon, Feb 15, 2016 at 9:57 AM, Claude Heiland-Allen <claude at mathr.co.uk>
wrote:
> On 14/02/16 22:27, Matti Viljamaa wrote:
>
>> Do you think Pd has a characteristic sound to it? Or whether
>> discussion board threads claiming Pd (and Max) have a distinct (and
>> not good) sound just have people who havenâ€™t listened to good
>> patches?
>>
>
> Some issues with Pd that affect sound character:
>
> 1. cos~ (and osc~) use a small table with linear interpolation, which
> means there is quite a lot of interpolation noise - I wrote about it here:
> http://mathr.co.uk/blog/2015-04-21_approximating_cosine.html
>
> 2. vcf~ (and probably other recursive filters) use single precision
> floating point in the feedback loop (pd-double might be different) which
> causes weird rounding artifacts - I wrote about it here:
> http://lists.puredata.info/pipermail/pd-list/2010-08/082104.html
>
> 3. cubic interpolation (tabread4~ etc) in Pd uses an (imho) incorrect
> algorithm - it makes a curve that goes through 4 points instead of matching
> the derivatives at the nearest 2 points, which leads to sharp corners at
> the original sample points with associated aliasing artifacts - this has
> been discussed on the lists many times in the past, for example here:
> http://lists.puredata.info/pipermail/pd-list/2008-06/062864.html and:
> http://lists.puredata.info/pipermail/pd-list/2010-03/077278.html
>
> 4. sig~ (and implicit sig~ from float messages to signal inlets) is steppy
> and only takes effect at block boundaries - compare with .kr in SC3 which
> is (afaik) linearly interpolated between each block boundary
>
> 5. Pd doesn't print enough digits to perfectly reconstruct floating point
> values when round-tripping through files, so (eg) biquad~ coefficients can
> become imprecise if you don't write them outside Pd in a text editor
>
> 6. other systems tend to come bundled with more nice-sounding stuff like
> bandlimited oscillators etc, with Pd you tend to have to find externals
> yourself (deken should make that easier now)
>
>
> Claude
> --
> http://mathr.co.uk
>
>
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