Re: [linux-audio-dev] audio interference or ... ?

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Subject: Re: [linux-audio-dev] audio interference or ... ?
From: David Olofson (audiality_AT_swipnet.se)
Date: pe loka   15 1999 - 16:52:04 EDT


On Fri, 15 Oct 1999, Paul Barton-Davis wrote:
> do you think it reasonable to conclude that this is caused by video
> noise ?

If I could hear it, I'd recognize it if it's video noise. Very
characteristic to analog + digital systems, and brings inspiring C64
memories back... :-) (BTW, note that I generally *hate* noise!)

> or is there still some chance that scheduling nonsense caused
> by the X server needing to run may be to blame ?

Most probably not, if it's soft noise. That would in most cases
result in very loud distortion due to the DMA hitting old data, or
the CODEC being stopped. (Significantly louder and sharper than hard
clipping!)

- Identified as sharp spikes in the output data.

It *could*, however, result in the DAC stalling at the current level
until new data arrives. IIRC, the AudioPCI can be programmed to
behave that way.

- Identified as flat sections in the signal.
  Use a sine wave of 1-5 kHz or so as a test signal.

> for what its worth, increasing the frames
> processed per cycle from 64 to 1024 makes no apparent difference in
> the noise.

That sounds like video noise to me...

Oh, BTW, you *are* ramping to new levels with sample accuracy when
the controllers are moved, right? If you don't, you'll get clicks
and/or a soft, buzzing sound when moving the faders... If you have a
Roland JV-1080 around, slide the pan controller some while playing a
note. (Don't know if that flaw is in all versions [XP, 2080...],
though.) IIRC, the EMU8000 (AWE) chip has the same bug on some or all
of the controllers as well.

//David

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