On Mon, 23 Jan 2006, Chris wrote:
On 23 Jan 2006, at 18:49, Bill Unruh wrote:
On Mon, 23 Jan 2006, Chris wrote:
On 23 Jan 2006, at 18:35, Bill Unruh wrote:
On Mon, 23 Jan 2006, Chris Birkinshaw wrote:
On 21 Jan 2006, at 15:24, Chris Birkinshaw wrote:
I have a M Audio Transit USB souncard, and have found
a high pitched noise comes out of my speakers when
starting jackd. This noise is not apparent when simply
playing through the device using alsaplayer.
Has anyone else got this? Has anyone else not got this
with their Transit?
In the end I fixed the problem by disabling audio input in my jack
startup line. Not sure why this would have been an issue!
Sounds like feedback. Ie, you have the input being fed to the output
which
is fed back to the input.
This wasn't the case, there was still noise present with nothing connected
to any inputs either physically or in jack. ie. the noise appears as soon
as jackd is started.
One of the features of feedback is precisely that the noise is there even
with no input. As you know when the microphone squeals, the person does not
need to be talking for the squeal to be there.
For feedback to work you have to have a loop in the system. Can you explain
where the loop is in my system as you have baffled me?
The input from the soundcard goes into the computer or the soundcard, which
then produces output out the output jack of the sound card. Now if there is
any feedthrough of the output into the input, you have a loop.
I am not saying that this is what is happening on your case, just that that
is what it sounds like.
It could also be bug in the jack implimentation, where the output buffer
feeds info into the input buffer.
Or it could be something else entirely.
The thing that makes me doubt feedback is that the instability tends to
drive everything to max amplitude. Just as the microphone squeal tends to
get very loud, the feedback squeal tends to get very loud.
But you asked for possible causes.
Additionally I have to disagree with your statement about feedback with no
input. With no input to a positive feedback loop there can be no feedback (in
your example the input is ambient noise in the room. )
Feedback is an unstable system. Any small input will cause it to flip into
a mode where the system rings constantly. Thereafter no additional input is
needed. In the case of the sound card, switching on the system can almost
always cause some noise which triggers the instability.
No, it is NOT amplification ( although amplification is necessary). It is
self sustained.
Best regards,
Chris
--
William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | [EMAIL PROTECTED]
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
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