[LAU] Tutorial on sidechain compression in Qtractor and a question about screencasting

Philipp Überbacher hollunder at lavabit.com
Tue Aug 24 14:48:51 UTC 2010

Excerpts from Jeremy Jongepier's message of 2010-08-24 15:09:12 +0200:
> On 08/24/2010 01:34 PM, Philipp Überbacher wrote:
> > Excerpts from Jeremy Jongepier's message of 2010-08-24 10:22:57 +0200:
> >> Hello all,
> >>
> >> I've uploaded a new tutorial to YouTube:
> >> http://www.youtube.com/watch?v=pnAqk054g7E
> >>
> >> So using sidechain compression in Qtractor with the SC3 LADSPA plugin.
> >>
> >> Concerning screencasting, anybody any experience with this? For this
> >> tutorial I used recordMyDesktop, but it captures video to Theora, so
> >> after editing and uploading the quality degrades considerably. The big
> >> advantage of recordMyDesktop though is that it has JACK support and that
> >> the overhead is pretty small so I can still run my setup at a decent
> >> latency.
> >> I've also started playing around with FFmpeg. The video quality is
> >> waaaaay better if you capture to a lossless format but I'm having my
> >> thoughts on the audio part. I compiled FFmpeg myself with libjack-dev so
> >> it has JACK support. But FFmpeg apparently has troubles to keep up with
> >> latencies below 20ms, lots of xruns on the FFmpeg side (JACK doesn't
> >> complain though). And I have the feeling FFmpeg lags on the audio side.
> >> So I'm thinking about capturing the video part with ffmpeg and the audio
> >> part with jack_capture and mux the stuff later. But if anyone has any
> >> other suggestions, let me know.
> >>
> >> Best,
> >>
> >> Jeremy
> >
> > Hi Jeremy.
> > I know little about screencasting, but I do know that recordmydesktop
> > can either encode on the fly or afterwards. Maybe it's possible to use
> > and convert its raw data, whatever format that may be, to something else?
> >
> > The tutorial seems quite nice. I do have a little trouble with it
> > because you seem to have a really high resolution screen, and my biggest
> > screen has 1280x800. I'm also not sure it's a good thing that you can't
> > see the mouse, recordmydesktop has an option for that.
> >
> > Seems to be quite a hassle to set sidechain compression up. The essence
> > being: know which sc3 port corresponds to what and what to route where.
> > I sadly couldn't get that from the video because of aforementioned
> > problems.
> >
> > Regards,
> Hello Philipp,
> Concerning the mouse, that's a noob move.
> I'm using a 1920x1080 resolution, which is also the maximum YouTube HD 
> resolution and this should yield the best results: 
> http://www.google.com/support/youtube/bin/answer.py?hl=en&answer=132460

Ok, but on a much smaller screen it's hard to read/see things. It's hard
for me to see connections or which settings you used for the compressor.

> And the SC3 ports, I explain it in the video, the first two inputs of 
> the 3-channel Synth Duck track are for the stereo audio input you'd like 
> to compress and the third input is for the sidechain compression source, 
> in this case a kick from Hydrogen.
> For the outputs, 1 and 3 are the 
> compressed audio signals that I feed into the Synth Mix track.

Thanks Jeremy,
I managed to reproduce it with jack-rack, h2 and aeolus.
Yes, it's perverse to sidechain duck an organ, but I figured it would
provide a nice, constant sound, and thus be ideal for testing.

I had to enable 'create per-instrument outputs' in h2 to get the kicks

The routing in jack-rack is a bit funky.
in_1 is the sidechain input, in_2 the left and jack_rack_5655:sc3_1-1_i1
the right input.
So you hook up the h2 kick to in_1 and the signal to be ducked to the
next two ins.
You'll get the ducked signal left part from out_1 and right from out_2,
as you would expect.

The funky part is when the plugin is disabled though, in which case it
will route in_1 straight to out_1 and in_2 to out_2. If you go from
jack-rack straight to you speakers it results in the kick in the left
and the left signal in the right speaker.

Another thing worth mentioning with sc3 is that the 'chain balance'
parameter is most significant, you need to turn it up to hear any
ducking effect.

Thanks Jeremy, I wouldn't have bothered investigating without your video

> I don't know where recordMyDesktop stores its raw/cache files, but afaik 
> I have encode on the fly disabled.
> Jeremy

It should be possible to figure out where it stores the raw data. I don't
know how to convert it to whatever yt prefers.


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und alle Fragen offen." Bertolt Brecht, Der gute Mensch von Sezuan

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