[linux-audio-user] noisy capture->playback

Benjamin Flaming lau at solobanjo.com
Sun Nov 2 16:19:20 EST 2003

On Sun, 2 Nov 2003 21:48:58 +0100
David <dplist at free.fr> wrote:

> I wanted to play a guitar through jack-rack and I heard this noise. Did
> it come from jack-rack, a ladspa plugin, jackd, alsa, the soundcard ?
> This is why I tried to isolate the problem, ending with a very simple
> setup : jackd connects the input channel to the output channel PCM Out 1
> assigned to H/W Out 1 (with the "PatchBay/Router" in envy24control). I
> still get the noise.
> I don't have this problem when doing true hardware monitoring of the
> input : be it by assigning H/W In 1 to H/W Out 1 (with the
> "Pathbay/Router") or by assigning Digital Mixer L to H/W Out 1 (with
> the"Patchbay/Router") and unmuting/raising the slider of channel H/W
> In 1 with the "Monitor Mixer" pane in envy24control.

A couple of suggestions for narrowing the problem down further:

1. Record something (ideally something easy to see and hear problems with - such as an oscillator) via alsa, without jack
2. Record the same signal with jack
3. Examine the waveforms visually

If the recorded waveforms look garbled:

4. Post a small fragment or two on a web server
5. Pass along the URL so we have something more precise than "the
sound is full of horrible "blips"." ;)

If the recorded waveforms look okay:

4. Play the recordings via alsa
5. Play the recordings via jack

This should help identify where the problem is.


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