[sdiy] BLIT/BLEP virtual analogue synthesis

[nicolas]

I am not such an expert, but my understanding is that you can only trust nyquist with regard to the output you listen to.  But in a synth there are lots of intermediate signals we don't listen to.  

For example if that 8kHz sawtooth is beating with an 8.02kHz sawtooth you will clearly hear the beat product in an analogue synth because the higher harmonics are also involved in the beating.  But if those two sawtooths are digital approximations at moderate sample rates, there will be hardly any higher harmonics to beat together and the so resultant beating will be sine-like, which at 20Hz will be hardly audible.

Won't this sort of problem happen throughout the synthesis, giving quite different results from analogue?

Cheers,
Nicolas


--- On Wed, 4/8/10, Antti Huovilainen <ajhuovil at cc.hut.fi> wrote:

> From: Antti Huovilainen <ajhuovil at cc.hut.fi>
> Subject: Re: [sdiy] BLIT/BLEP virtual analogue synthesis
> To: "Tom Wiltshire" <tom at electricdruid.net>
> Cc: "sdiy list" <synth-diy at dropmix.xs4all.nl>
> Received: Wednesday, 4 August, 2010, 2:46 AM
> On Tue, 3 Aug 2010, Tom Wiltshire
> wrote:
> 
> > Has anyone on the list played with either of the above
> techniques?
> > 
> > https://ccrma.stanford.edu/~stilti/papers/blit.pdf
> > http://www.cs.cmu.edu/~eli/papers/icmc01-hardsync.pdf
> 
> Does coauthoring this count? ;)
> http://ieeexplore.ieee.org/search/srchabstract.jsp?arnumber=4117934&isnumber=4116828&punumber=79&k2dockey=4117934@ieeejrns&query=%28%28valimaki+v.%29%3Cin%3Eau+%29&pos=1
> 
> (V. Välimäki and A. Huovilainen, "Antialiasing
> oscillators in subtractive synthesis," IEEE Signal
> Processing Magazine, vol. 24, no. 2, pp. 116.125, March
> 2007.)
> 
> I can send a copy of the draft to interested people, but
> can't put it up for public access. Also see my Master's
> Thesis:
> http://users.tkk.fi/~ajhuovil/Huovilainen_MastersThesis.pdf
> 
> > I've been reading the papers (the two classics above
> amongst others) and doing some experiments, and it seems to
> me that getting decent results at low sample rates (like
> 44.1KHz) is still really difficult. For example, a 8KHz tone
> is going to have about 5.5 samples per period at 44.1KHz.
> This doesn't seem like enough to represent anything much,
> nyquist theory notwithstanding.
> 
> 5.5 samples is enough to represent two harmonics which are
> the only audible harmonics for a 8 kHz sawtooth. Ignore your
> gut feeling and trust Nyquist (remember: audio DAC has a
> very steep digital lowpass filter which removes the image
> frequencies and hence the jaggies in the output waveform).
> 
> > If you've some experience to share, I'd be interested
> to discuss this, either on or off-list.
> 
> Ignore BLIT and minimum phase BLEP and stick to pure BLEP.
> BLIT has numerical and stability issues and minimum phase
> BLEP is in 9%% of cases only a pointless complication (it
> does not remove lookahead like Brandt's paper claims).
> 
> Antti
> 
> "No boom today. Boom tomorrow. There's always a boom
> tomorrow"
>   -- Lt. Cmdr. Ivanova
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