here are some screenshots...
the beating bell shape on 440Hz sinus with 84 harmonics
http://i.imgur.com/B4s0M.png
a simple sine table 220Hz
http://i.imgur.com/hPKj1.png
julian
Am 09.04.2012 22:32, schrieb Julian Schmidt:
I think you win! ;)
first i tried switching to integer phase accumulators like RBJ suggested.
It sounded exacly the same.
I really think it is an aliasing problem.
but not due to the wrong wavetable content, but due to a cheap audio
codec with poor filters.
I toyed around with the mip mapping settings of the limited saw table
and an FFT analyzer.
even when i output a single 440hz sine i get harmonics starting at
-48db up to nyquist and beyond.
the amplidude of these harmonics increases as i increase the number of
sines in my wavetable.
if i play 10 harmonics from the saw table at 440Hz beginning from the
4th harmonic I see that harmonics are damped or increased in the shape
of the codecs filter stopband riffle ( 4th and 5th harmonic damped,
than amplified in a bell shape)
the harmonics go up to nyquist (allthoug the higher ones are in the
range ob -60 to -66dB)
with the right amount of harmonics i get a 4dB beating at 6.5Khz at -30dB
and a 30dB beating at around 12kHz at around -42dB again in the shape
of a bell like in the codecs stopband ripple
I think now i will activate the tone control of the codec and see if
that can surpress the bleeding harmonics.
julian
Am 09.04.2012 20:28, schrieb Olli Niemitalo:
On Mon, Apr 9, 2012 at 7:17 PM, Julian Schmidt
<julian_schm...@chipmusik.de> wrote:
setting the phase increment to an integer value solves the problem.
[...]
adding linear or cubic interpolation makes it a little better, but the
pulsing is still very audible.
[...]
The nearer i get to an integer phase increment, the slower the
amplitude
of the harmonics is pulsating. at an integer value it stops.
All of these findings support the hypothesis that it is an aliasing
problem. So it doesn't need to have to do with phase wrapping. Saw
wave and similarly discontinuous waveforms are particularly nasty when
it comes to aliasing problems because the harmonics decay so slowly
(amplitude of Nth harmonic = 1/N). I bet the reason why you had so
little problems with the sine wave is because it was greatly
oversampled (that is, the highest frequency, in this case the
fundamental frequency, is less than sampling frequency / 2).
Do a mental check on what the bandwidth of your wavetable is, and
whether you will get aliasing by playing back the wavetable at a
different rate than 1:1 and by resampling it to the target sampling
frequency.
-olli
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