Phil/Robin,
I've been holding off contributing to this thread, but what the heck - here's a few views to fuel the thread (but I don't predend to be an expert in this area, just an enthusiastic amateur...)
1) Is jitter audable and where's the evidence:
From my personal experience, yes [i.e I can hear differences which I would put down to jitter - more of in minute]
However I would suggest you do a quick search on the AES pages for references to papers about jitter. The AES (Audio Engineering Society) is definately a body which doesn't give much time to audiophile snake oil and hype - but does consider the serious engineering behind audio. Unfortunately they charge for back copies, so the only access you can get to papers on line are ones hosted by companies with a vested interest or accademics (e.g. Hawksford - worth looking at his list of papers http://www.essex.ac.uk/ese/research/audio_lab/malcolms_publications.html#Journal)
Anyway, you may be interested in ref & graph 9 of the following: http://www.nanophon.com/audio/jitter92.pdf [Can't actually find the paper referred to online]
2) Can't spdif be implemented in such a way to avoid jitter?
I would agree with this to a reasonably degree - essentially the early implimentations were naive as they didn't understand the impact of jitter and in many cases this remains.
I agreed this comes down to how much engineering is put into the transmission line:
- consumer products bandwidth limit the output to ensure emissions tests are passed with cheap cables - in many cases they arguably constrain it too much
- most links are poorly terminated so that reflections occur. In the case of most cheap consumer gear both ends are capacitively coupled (no problem with that), but use cheap RCA connects etc, which ensure some reflections. It is argued by some that assuming the cable is very short this will distort the signal seen at the receiver as it is the sum of the original plus reflections. [One proposal I have seen is to always use 1.5m of cable to delay the reflection!]
- common older spdif digital receiver chips are good at capturing the signal, but are possbly limited in other respects. E.g. the Crystal 8412 in my DAC (one of the industry standards from a few years ago which is easy to use and hence used by many!) - its PLL doesn't attenuate any jitter in the audio band - corner frequency is at 25 KHz. Additionally the critical frame (L/R) transition clock is derived straight from the input signal rather than from the clock produced by the PLL - dacs often reclock this!. (Some of the audio diy community also suggest the 8412 injects significant noise back into the transmission line if coupled to it without buffering due to its input stage using Smitt triggers)
Bottom line - the implementation of the input receiver is potentially a dominant factor in this conversation.
3) Why do different DAC chips impact things?
These days I think there is a dependance on the DAC conversion technology and many new DAC chips take steps to reduce sensitivity to jitter.
Its relatively simple to comprehend a non oversampled multibit converter which changes its output on a transition of the L/R Clock - in this case if there is jitter in the L/R clock transition then this has a direct relationship to the resulting audio signal - as the correct voltage is produced, but not at the correct point in time. All the simple analysis consider this case.
With single bit and newer DAC technolgies (some of which are a small number of parallel multibits running at higher speed, or some other hybrid of single and multibit), then there appears to be the opertunity to reduce this concern by clever processing/noise shaping stuff. Hence the claims of many newer converter chips to have reduced jitter sensitivity.
4) What's my experience.
Well with my current DAC and high end system I can definately hear the difference between spdif sources. I put this down to the fact that my current DAC has a very basic design for extracting the spdif clocks. Specifically between the Squeezebox and my reference CD transport there is a small but noticable difference (I'll not bore you with the audiphile phrases). However I am happy with this because the cost of the mods I have done on the CD transport exceed the cost of the Squeezebox! [The CD transport is actually a much modified one from a few years ago which has a new low jitter clock and I reclock the spdif signal against this imediately prior to the output buffer.]
In contrast the Squeezebox as a highly integrated device has quite a simple clock circuit and hence is always going to generate a bit more jitter than a modern high end transport.
This is the reason I would recommend audiophiles looking for the best high end sound to use the Squeezebox with a modern dac with reclocking. In this case there are many happy users who are getting a high end hifi experience + the all the usability and convience benefits from the Squeezebox. [However do try any new dac against the Squeezebox as there are a very small number of reports about certain reclocking dacs having problems locking to the Squeezebox in pcm mode]
5) Lets put this all into perspective.
Unless you care about hi end hifi, you won't care about this! (You need to be talking about >$500 dacs before you want to get interested in this)
If you do, then you probably recognise that human hearing is very sensitive and to some degree personal taste comes into it... I say this because I don't believe measurements are the final arbiter of a good audio experience. [I listen through valve (tube) amplification which in simplistic terms measures poorly, but has that certain something to the sound which is highly engaging... In contrast transister power amps of a few years ago measured very well in terms of distortion but sounded awful....]
Adrian
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