In article ,
Lostgallifreyan

Jitter has a surprising effect in using audio outputs with DC blocking
caps renoved for DC coupling for laser projector galvo scan amp drive.

FWIW so far as I know correctly implimented spdif includes a transformer or
equivalent. Mainly to block loops and common mode, but it would also block
dc. So act similarly to a dc blocking cap IIRC.

[big snip]

Afraid I don't know what some of the terms you use like 'ILDA' and 'WDM'
mean in terms of the waveform details. However...

Anyway, what I found was that this interface has two main driver
variants, one of them a WDM version. If you run this pattern without
blanking the beam so you see the draws as the beam is quickly
positioned for the next drawn element in the pattern, you can see a
partial loop, like a bight of rope, as it leaps from the bottom of the
circle to a small vertical line drawn above it. Scanner galvos have a
lot of mass, they're like heavy duty fast moving- magnet meters, and I
don't know how much this relates to audio systems, but it appears as if
the resonance in a scanner being fixed, reacts with the small timing
errors in the non-WDM driver to form a dramatic and erratic shift in
the 'bight' formed by the scanners. It affects both axes, and the
vertical axis saw it shift in a range as great as 10 to 15 percent of
full scale.

How anyone interprets all this is up to them.

My main interpetation is that the waveform shape 'as scanned' is altered.
That is the equivalent of saying 'jitter exists and can be caused by the
transmission system having a frequency response that isn't flat'. So not a
surprise.

I only mention it because it happened, and I found it interesting. There
is no doubt that removing jitter makes the resonance play nice, but
whether this would be important in sound playback I don't know.

The significant point is that 'playback' requires a DAC which is clocked in
some way. If:

A) The DAC uses a clock that does *not* in the medium and short term depend
on the embedded pilot tone in the spdif.

B) The spdif 'jitter' is reasonably small compared with the bit-length
cycle

Then the 'jitter' on the signal is essentially suppressed by the 'replay'
system.

So you'd need to measure 'jitter' on the *output* from the *replay system*
- not just observe it on the spdif waveform - to assess the level so far
as any listener is concerned.

IIUC The systems you described drive the scanning directly, or by some
subsystem 'receiver' whose details are not clear to me from what you wrote.
In essence what you described would then be like looking at the spdif
waveform on a scope - not looking at the DAC output.

Hence like yourself I have no idea if what you say actually has any
relevance to the audio case.

I'd imagine that our hearing might react to it as sight reacts to
dither. It might even sound 'better' in many cases, if we notice it at
all.

Alas the problem these is as I previously commented. Many things "might be
so", but only some of them (or none) *are so*. Indeed, imagining something
doesn't even show it is 'possible'... :-)

I don't doubt that at a suitably high level 'jitter' would be audible. Wow
and flutter if bad enough are easily audible. But that doesn't tell us if
what people claim is 'true'. :-)

Slainte,

Jim

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