[icecast] Low frequency feedback?

Devin Campbell devin at moonglade.com
Fri May 30 07:35:22 PDT 2003



While crafting a rather lengthy reply (included below for information's
sake) I believe I stumbled on to the real problem.  At some point I was
playing around with the Ices2 samplerate to see how that affected sound
quality and CPU usage, etc.  I ended up leaving this set to 32000 in the
ices.xml.  What I didn't realize was that Winamp 2.91 said the stream was 44
kHz as it played.  I changed Ices to reencode at 44100 and after listening
to the stream for over an hour and a half I see no sign of the problem.

So, is Ices doing something funny during the reencoding process or is there
something wrong with Winamp 2.91 listening to a 32 kHz, quality 0 OGG stream
or is it just my setup?

Devin

> Owtch, my ears. :( I find that 16 kHz beep most irritating, it hurts
> even at low volumes. ;P

I agree. :)  When I say that this 16 kHz beep is coming from Cakewalk, I'm
fairly confident of that, but the lower hum was there before I captured that
sample in Cakewalk.  Let me expand on the procedure that I came by that MP3.

I'm streaming OGG files with Ices2 and Icecast2.  When I listen to the
stream with Winamp 2.91 the low end hum slowly creeps into the stream (takes
about 5-10 minutes).  After 10 minutes or so the hum is overpowering.  Since
I don't want to force people to listen to the stream for 10 minutes to see
the problem, I captured the audio coming through my mixer to lay down a
track in Cakewalk.  So, the 16 kHz tone either came from the Windows mixer
or Cakewalk during the capture process because it exists afterward, but not
before.  That is, unless Winamp is doing something that blocks out the high
frequency before it gets to my speakers.... the EQ has 16k pumped up fairly
high so if it's there I think I would have heard it.

So, the MP3 is just a static MP3 file with a captured sample of the problem.
It looks like the 16 kHz signal was a product of Cakewalk's conversion to
MP3.  Here's an OGG that sounds more true to what I'm hearing on the stream.
http://www.starbeamllc.com/images/feedback.ogg

The thing that bothers me is how the sound slowly grows from nothing to
almost overpowering.  Gives me the impression of some sort of feedback loop.

I've listened to several other OGG streams (listed on Oddsock's site) and
found a few with similar bitrates, but I haven't noticed the problem on any
of their streams.

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