[LAD] on the soft synth midi jitters ...

Folderol folderol at ukfsn.org
Tue Oct 5 20:22:23 UTC 2010

On Tue, 5 Oct 2010 22:00:11 +0200
fons at kokkinizita.net wrote:

> On Tue, Oct 05, 2010 at 02:50:10PM +0200, David Olofson wrote:
> > Not only that. As long as the "fragment" initialization overhead can be kept 
> > low, smaller fragments (within reasonable limits) can also improve throughput 
> > as a result of smaller memory footprint.
> 'Fragment initialisation' should be little more than
> ensuring you have the right pointers into the in/out
> buffers.
> > Depending on the design, a synthesizer with a large number of voices playing 
> > can have a rather large memory footprint (intermediate buffers etc), which can 
> > be significantly reduced by doing the processing in smaller fragments.
> > Obviously, this depends a lot on the design and what hardware you're running 
> > on, but you can be pretty certain that no modern CPU likes the occasional 
> > short bursts of accesses scattered over a large memory area - especially not 
> > when other application code keeps pushing your synth code and data out of the 
> > cache between the audio callbacks.
> Very true. The 'bigger' the app (voices for a synth, channels for
> a mixer or daw) the more this will impact the performance. Designing
> the audio code for a fairly small basic period size will pay off.
> As will some simple optimisations of buffer use.
> There are other possible issues, such as using FFT operations.
> Calling a large FFT every N frames may have little impact on
> the average load, but it could have a big one on the worst case
> in a period, and in the end that's what counts.
> Zyn/Yoshimi uses FFTs for some of its algorithms IIRC. Getting
> the note-on timing more accurate could help to distribute those
> FFT calls more evenly over Jack periods, if the input is 'human'.
> Big chords generated by a sequencer or algorithmically will still
> start at the same period, maybe they should be 'dispersed'...
> Ciao,

I'm all in favour of a bit of dispersal.

When I started out with a Yamaha SY22 and Acorn Archimedes it was all
too easy to stuff too much down the pipe at once. However, doing some
experimenting, I was surprised at how much you could delay or advance
Note-On events undetectably although it depended to some extent on the
ADSR envelope.

I don't need to do that any more, but old habits die hard, so if I'm
copy-pasting tracks I tend to be deliberately a bit sloppy.

I'm also a bit puzzled by people complaining about jitter. I don't have
any exceptional kit, but in reality I can't say I've ever noticed it.
Latency yes, but that's easily corrected with a bit of post record

Will J Godfrey
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.

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