Re: [Audacity-nyquist] newest rectifier plugs

[<edg...@we...>]

Thanks to Alex S. Brown:

> If you try to keep the whole sound in memory and use snd-fetch to move
> through it, the RAM use for your script will grow dramatically with the
> length of the sound.

The intention was just the opposite: I wanted to use snd-fetch to avoid
keeping the whole sound in memory.

> Depending on your algorithm, there may be ways around this effect, but
> generally snd-fetch does not let you go backwards or do random access
> within the sound.

I only whould need a simple sequencial function that reads one sample
after the other, sum it up in a single varible (or two in case of stereo)
and immediately after reading and adding the sample the snd-fetch memory
could be freed again (or garbage collected).

I know there is a problem that snd-fetch affects the samples so I'm not
sure wether I have to snd-copy the whole sound into memory before I can
work on it with snd-fetch. This of course whould turn the whole
snd-fetch theory quite useless.

Unfortunately I cannot reduce the sample frequency to 100 Hz or similar
so I assume a snd-fetch analyzer routine whold be very slow.

> The DSP in XLisp chapter is excellent to start working with those functions,
> but the memory utilization of these functions takes some time to understand.

I know ... (but this is no valid excuse not to learn it)

> There may be a built-in function of Nyquist that does a similar calculation

Unfortunately there is not, I had the same problem several times ago, at
the moment, as far as I know, only the maximum peak is available. The
snd-avg function for example calculates the geometrical average (RMS) what
is fine for dynamic compressors but unfortunately useless in this case.

> If anyone has suggestions, please jump in! I hate posting a reply only
> about the problems of a suggested approach.

doesn't matter - thank you for your reply

- edgar

message to david: just got the code changes, I will test it immediately,
answer follows in  about half an hour.



the rest is transcription

aud...@li... schrieb am 29.11.05 19:50:13:
> 
> I cannot speak to your DC/filter issues, but I have used the snd-fetch function
> quite a bit working on sound detection. I would warn you that snd-fetch is best
> used as a one-pass method to examine and manipulate a sound. If you try to keep
> the whole sound in memory and use snd-fetch to move through it, the RAM use for
> your script will grow dramatically with the length of the sound. Basically,
> Nyquist will read the entire sound into memory and hold onto the whole thing
> until the script is done.
> 
> Depending on your algorithm, there may be ways around this effect, but generally
> snd-fetch does not let you go backwards or do random access within the sound. I
> believe Nyquist will accumulate all the samples in RAM if you try to hold onto
> the sound variable while using snd-fetch to move through it.
> 
> The DSP in XLisp chapter is excellent to start working with those functions, but
> the memory utilization of these functions takes some time to understand. The
> plug-ins that ship with Audacity actually suffers from some of these problems.
> Others on this mailing list helped me a lot to fix up my plug-ins.
> 
> I think your proposal of using snd-fetch to add the samples into a single
> variable would either destroy the sound variable or accumulate all samples in
> memory if you tried to keep a copy of the sound variable. There may be a
> built-in function of Nyquist that does a similar calculation, without the
> memory problems. Something like snd-avg might help, although sometimes these
> functions have the same memory-use problems as snd-fetch.
> 
> If anyone has suggestions, please jump in! I hate posting a reply only about the
> problems of a suggested approach.
> 
> Quoting edg...@we...:
> [snip]
> > Now there are two problems. First, Nyquist is designed to work in one pass
> > only. This not a bad idea because this way cou can work with a minimum of
> > memory because all audio data can be left on the hard disc, in principle
> > only the actual sample is needed. But this leads to the next problem: if
> > you want to write a two pass filter you have to keep the sound in memory
> > between the first and the second pass by assigning it for example to a
> > global variable what quickly can lead to a memory overflow depending to
> > the length of the audio.
> > 
> > There could be a solution by using the snd-fetch function with that you can
> > access single sample values. For example like fetch the first sample and
> > save its value in a variable, then fetch the second sample, add it to the
> > value in the variable and store the result again in the same variable. By
> > adding up all samples one after the other in the same variable and dividing
> > the variable value at the end by the number of samples that is given by the
> > len variable you could calculate the average off all samples with only one
> > variable.
> > 
> > But this is only a theory because I never have worked with the snd-fetch
> > function and neither with the xlisp object system that is needed for this.
> > Maybe Steven or Sami (or whoever reads this) knows how to do. Or maybe
> > this is a reason to learn more about the xlisp object system.
> > 
> > The snd-fetch function and how this could work is explained in the Nyquist
> > manual under "DSP in XLisp".
> 
> ----------------------
> Alex S. Brown, PMP
> ale...@al...
> http://www.alexsbrown.com/
> 
> 
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