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Posted by Matthew L. Martin on 11/11/06 15:34
Randy Yates wrote:
> "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.
That is correct. That's why jitter has always been essentially a
non-issue in home use. I've always been amused that some people think
that jitter can only be eliminated (as if!) by an in circuit device.
> 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.
Yes sir. Some early designs were a bit less elegant, but jitter was
usually not a problem. In my test I had to impair the cable to get
enough jitter to get above the instrument's noise floor.
> 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.
Even then, I would doubt it.
Matthew
--
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You can't win
You can't break even
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