What could cause digital audio signal oscillation?

Archived from groups: rec.audio.pro (More info?)

peakester@earthlink.net wrote:
> While recording a harmonica into my Alesis HD-24 I got a pronounced
> pulsing/oscillation type signal. At first we though there was spit in
> the harmonica. When I went into the recording room it was clearly not
> present in the harmonica. I traced the problem to the return from the
> HD-24 coming through a toslink connection into the factory ADAT optical
> input of my Tascam DM-24 console. When I switched to an analog signal,
> it no longer happened. I returned to the digital input and tried
> decreasing the digital attenuation. I got less signal but no less of
> the undesired signal affect.
>
> The toslink cable was purchased at a consumer audio store but was not a
> cheapie.
>
> The problem was most audible only on certain notes on the harp and as
> stated, was not part of the recorded signal.
>
> Any thoughts?

I'd bet that you are somehow summing two identical signals within your
digital mixer, one direct and one returned from your disk recorder. If
there's some slight latency between the two, it would affect the audio
exactly as you've described it.