[linux-dvb] Re: Antwort: Re: Best sound card with S/PDIF output for VDR and AC3/DolbyDigital sound?

[Georg Acher <acher@in.tum.de>]

On Mon, Jul 30, 2001 at 11:44:31PM +0200, Carsten Koch wrote:
> I personally believe that quality differences in optical
> cables are a myth created by the vendors of the more
> expensive cables. I bought a DM 6 optical cable for my
> DVD player from Reichelt. I have no problems at all with
> it, the sound is excellent and there are no dropouts.
> I think it should not be too hard to transmit the bits
> without errors. As long as that is the case, what difference 
> can a higher quality cable make (except for maybe looking
> better)?

Optical cables have (due to their large diameter compared to real high speed
fibers) a "smoothing" effect on the edges of the signal. In addition to the
(relatively) low bandwidth of the transmitter/receiver-combo (TORX*/TOTX* or
equivalent), this leads to measurable jitter at the receiver. A better cable
may generate less jitter, which in turn doesn't generate so much noise in
modern Delta-Sigma-DACs (a high-frequency jitter of 5ns is easily audible,
no expensive measurement tools needed...).

That's the theory (also applies to expensive coaxial cables for SPDIF).

But: Every SPDIF-receiver has a jitter-killer, that attenuates high
frequency jitter (the one mentioned above) by more than 40dB, practically
removing the jitter and the need for expensive cables. Of course, this fact
isn't told by the high-end-equipment salesmen ;-)

-- 
         Georg Acher, acher@in.tum.de         
         http://www.in.tum.de/~acher/
          "Oh no, not again !" The bowl of petunias          


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