'Twas brillig, and Tanu Kaskinen at 29/06/09 11:40 did gyre and gimble:
2009/6/29 Davide Cescato <[email protected]>:
Actually I now have a somewhat related question, for another unrelated task
I have in mind: recording voip phone calls. What I currently can do is
recording from the microphone input, which picks up both my voice and the
other person's voice coming through the speakers. However, the path "out
through the speakers and in through the microphone" adds a lot of noise to
the other person's voice.
So, is it possible to somehow set up PA to let me capture a mixed stream
containing both the internal audio stream carrying the other person's voice
and the microphone input, so that both voices can be captured in good
quality? Of course, I would wear headphones to hear the other person's voice
and to block it from going through the speakers.
No, PA doesn't have a feature to mix multiple recording sources.
However, if the voip app is capable of recording through PA (as
opposed to using the sound card directly) then you could record the
two sources and do the mixing yourself somehow. Recording through PA
shouldn't be any significant feat for alsa applications (just use the
"default" device like with output), but AFAIK skype can't do it
currently.
Recording from multiple sources can have some timing issues due to
clock drift between the sources. If the nominal sample rate is 44100Hz
and the real rate difference is 12Hz between the devices (I think this
might be quite ordinary case) then the drift is about one second per
hour, if I calculated this correctly. For short calls this shouldn't
be too much. Also, if the output device (from which you record using
the monitor source) and the mic device are on the same card, they may
share the clock, in which case no drift happens.
Just to add a little more here:
What Tanu says is totally correct with regard to clock skew (well, as
far as I understand it too!).
Now pulse does have a somewhat related capability with regards for
compensating for this skew in the form of module-combine. What this
module does is take two sinks (i.e. output devices - alsa cards etc) and
combine them into one virtual sink. You can then play on this sink and
have your sound physically output on two independent cards at once. This
module deals with the complications of resampling due to clock skew that
is also present in this scenario.
In theory, what you are doing is somewhat similar, but kind of the
opposite case, you want to take two sources (the mic + the monitor
source of the "sink input" (i.e. the playback stream of just the voip
app - not the monitor source of the sink itself as it may contain other
sound from other apps too) and join them to have a kind of joined source
which you then simply record.
I'm not sure what Lennart's ultimate plans are in this regard, but it
would be a useful feature IMO to be able to do this.
In theory a third party app can look for any streams for a given
application (VoIP app) and automatically find the relevant sources and
record them in a combined stream that is created and destroyed on the
fly. The only special case handling the app would have to do is to
handle when a source disappears from under it gracefully (e.g. when the
app stops playing sound, the stream will die and thus the monitor source
that goes with it will also die, killing our combined source - typically
traditional apps don't like it when pulse kills the stream, so you'd
have to be careful to handle this correctly).
This may or may not be practically useful as the app itself may do
numerous post processing after sampling input before sending it to the
other party. From a legal perspective, what you record is not
necessarily what the other person actually heard, but it's probably good
enough for most purposes.
It's quite a lot of work tho', so I wouldn't expect this kind of support
in pulse anytime soon, if at all :) (unless you get your hands dirty and
do it yourself :D)
Just thought I'd throw it out there into the discussion pool and see if
it could swim :p
Col
--
Colin Guthrie
gmane(at)colin.guthr.ie
http://colin.guthr.ie/
Day Job:
Tribalogic Limited [http://www.tribalogic.net/]
Open Source:
Mandriva Linux Contributor [http://www.mandriva.com/]
PulseAudio Hacker [http://www.pulseaudio.org/]
Trac Hacker [http://trac.edgewall.org/]
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