Subject: Re: [ecasound] more audio stamp examples
From: Kai Vehmanen (k@eca.cx)
Date: Thu Dec 14 2000 - 07:03:31 EET
On Wed, 13 Dec 2000, Jeremy Hall wrote:
> Is this thing like a tag applied to some special data, allowing it to be
> friends with another chain?
You can think of audio stamps as locations for storing one buffer of audio
data (-b:x -> one buffer contains x sample frames). Every '-eS:x' creates
a new audio stamp location, identified by the 'stamp-id', which is
interpreted as an integer number. In addition, audio stamps are not
specific to chains, but are a chainsetup-wide concept. In other words, a
audio stamp stored in chain "1" is usable by all operators present in the
same chainsetup.
> or maybe a colour? you colour some of the input data and then can use it
> for input to a controller?
Ok, I see what you mean. Audio stamps are not coloured or marked
(like what routers do for ip-packets' tos-fields), but audio is
actually duplicated and copied to a new location while it's passing the
'-eS:x' operator.
Similarly, audio stamp clients (or consumers) get a pointer to a buffer
of samples with a certain 'stamp-id'.
One problem we still have is the order of processing events. At the
moment, chains are processed in a certain order. But this isn't something
you necessarily should rely on. It might be, that some ecasound version
introduces a mix engine that processes chains in parallel. This causes
problems for all uses of audio stamps, for which order of events is
important. Even now we have one problem caused by event ordering. Inside
one chain, all controller values are updated before any chain operators
are processed. So when you issue:
ecasound -a:1 -i drum_loop.wav -o /dev/dsp -eS:1 -efl:400 -ksv:1,20,20000,1,1
...the first loop goes like this:
1. read a buffer of 'drum_loop.wav'
2. copy the audio to chain "1"
3. update all controller values (nothing written to stamp '1' yet,
so -ksv will produce control value of 0 which is mapped to 20Hz)
4. process chainops
4.1 -eS:1 stores the audio buffer to stamp-buffer '1'
4.2 lowpass filter is applied to signal with cutoff-freq 20Hz
5. mix from chain buffer to output ("1" -> /dev/dsp
... so there's a one buffer delay in stamp processing. Now one solution -
although not trivial to implement - would be put both both controllers and
chainops in the same ordered list, but this seems as a difficult thing to
implement without slowing down all other use cases.
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