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List:       kde-multimedia
Subject:    Re: Recording PCM audio with aRts
From:       Stefan Westerfeld <stefan () space ! twc ! de>
Date:       2001-06-10 5:14:57
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   Hi!

On Thu, Jun 07, 2001 at 07:07:54PM +0100, Rik Hemsley wrote:
> I'd like to write a quick-and-simple audio recorder which uses aRts.
> 
> Having looked through the available documentation on arts-project.org,
> I don't really have a clue about how to proceed.
> 
> Is there more documentation, or can someone give me a quick overview
> of how I would go about selecting, opening, reading and closing an
> input device ?

* selecting:

aRts currently uses no explicit selection of input sources, neither for midi
nor for audio. Rather the current strategy is through the (Audio|Midi)Manager
objects. This works like that:

  1. you start recording (playing), describing what you do
  2. there are predefined points where you can record data from (play data to),
     which the user can connect your application to through artscontrol
  3. the next time your application records (plays) again, it will be connected
     to the same point again

It might be that you might want to change something in there, but thats the
way it works right now.

* opening, reading, closing:

Well, recording is still somewhat on the aRts TODO list, but here is an
overview what works and what does not.

A. recording with server-side objects

Creating a Synth_AMAN_RECORD (which does that AudioManager thing), and
connecting it to a Synth_CAPTURE_WAV is the simplest style of recording.
It will create a wave file in /tmp/mcop-<username>/... - I did hardcode
that for now because of security considerations (what if you get an .arts
file by mail that "records" some file which already exists and thus overwrites
it? or creates a file which you - as stupid user - will never find again?).

Recording with server-side objects has the advantage that it is simple and
well-supported, but has the disadvantage that it does write files - if you
write them - server side. You can easily create your own objects which then
run server-side and do the recording there.

There is no explicit open() and close() within the flow system by the way.

B. recording with the aRts C API

There is arts_record_stream(int rate, int bits, int channels, const char *name);
(the last parameter is used for the audio manager thing again). However,
recording via C API is not implemented yet (returns E_NOIMPL). We'd need an
equivalent to SimpleSoundServer::attach() here, to make recording
implementable. And some AudioToByteStream implementation. The problem is that
resampling from float to something else is right now not easy to do within
aRts (I need to fix that), so that AudioToByteStream is not a one-liner.

C. recording with CSL

CSL does support recording to a degree - it doesn't do resampling yet. But
well, there you have another API where you can open/read/close things. One
day, the aRts C API might be implemented on top of CSL, and then recording
would work.


So... as conclusion I'd say, it would be great if you write that simple
recording application (we really really need that), and fix things on
the way. (/me currently always uses artsbuilder with .arts files to do
recording, which is probably not the ideal thing).

   Cu... Stefan
-- 
  -* Stefan Westerfeld, stefan@space.twc.de (PGP!), Hamburg/Germany
     KDE Developer, project infos at http://space.twc.de/~stefan/kde *-         
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