Posted by Randy Yates on 11/11/06 03:10

"Matthew L. Martin" <nothere@notnow.never> writes:

> G-squared wrote:
>> Matthew L. Martin wrote:
>> > Randy Yates wrote:
>> > > Tom Stiller <tomstiller@comcast.net> writes:
>> > >
>> <snip 'guest'>
>> >
>> > > Actually, it can. I'll have to back-peddle a little here.
>> > >
>> > > S/PDIF is a single-wire interface that embeds the clock with the
>> data.
>> > > Because of this, problems with the interface that aren't serious
>> > > enough to cause bit errors can still affect the sound by affecting
>> the
>> > > quality of the recovered clock. A PLL is used to recover the
>> clock,
>> > > and depending on the type of loop filter used in the PLL, input
>> noise
>> > > from cable degradations could introduce jitter into the digital
>> clock
>> > > and thus the reconstructed analog signal.
>> > >
>> > > How audible is it? That is a whole dissertation unto its own.
>> >
>> > According to test I did years ago when I had access to the proper
>> > equipment the jitter induced sidebands on an impaired cable were
>> 85dB
>> > below the program material (CD test tones). One golden eared guy
>> > duplicated my results with similar equipment, but he claimed he
>> could
>> > hear 85dB below the program material, so the jitter was audible to
>> him.
>> >
>> > Matthew
>>
>> Matthew, on your jitter tests, did the hardware have the PLL clock
>> recovery Stiller mentions ?
>
> Of course.
>
>> If so, was there any way to monitor the PLL
>> error signal to see any relation to the jitter sideband issue ?
>
> That is what the spectrum analyzer was for.
>
>> Should
>> we expect more recent hardware to behave better and if so, how much
>> better in your estimation ?
>
> Any design, new or old, that uses a large enough shift register to
> buffer the incoming signal should have close to no jitter induced side
> bands.

To elaborate just a little, I believe PLLs in these types of
applications have long used a sample buffer which is fed by the
recovered clock and consumed by a VCXO or other low-jitter source. The
control voltage to the VCXO can be very, very smooth, essentially
introducing no jitter, and the short-term stability of a VCXO results
in jitter sidebands that are essentially non-existant.

PS: That's some set of ears on the guy that claimed he could hear
anything 85 dB down, unless he was listening to something really
dynamic like classical.
--
% Randy Yates % "She's sweet on Wagner-I think she'd die for Beethoven.
%% Fuquay-Varina, NC % She love the way Puccini lays down a tune, and
%%% 919-577-9882 % Verdi's always creepin' from her room."
%%%% <yates@ieee.org> % "Rockaria", *A New World Record*, ELO
http://home.earthlink.net/~yatescr

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