Jitter : Toslink versus Copper

Robert Katz Leave a Comment

From: Grant Lyman

My comments are:
Bob, thanks for your informative web site and your great comments on the Mastering and Sonic newsgroups. I am hopeful I could get your opinion on a few questions I have. First, have you ever heard Toslink or Glass to sound as good as AES/EBU or SPDIF. My initial tests seem to indicate Toslink and even Glass tend to close the sound stage and thin out the sound. I have heard you mention in the past that the terminations on Toslink and Glass are critical for proper performance. Have you tested what you consider to be properly terminated Toslink or Glass against AES/EBU or SPDIF what was your conclusion.

I thank you, and value your opinion

Grant Lyman
Santa Barbara, CA

Dear Grant:

This is ultimately a jitter question, you know. My answer is that the apparent sonic differences between interface technologies such as Toslink, glass, and copper are IRRELEVANT when doing transfers or when passing signal from one processor to another. You can forget about that question with COMPLETE CONFIDENCE–since all of the technologies are capable of passing perfectly good data, within their specified cable lengths. Remember: the clock is not transferred along with the data. Only the data is transferred to the processor’s circuits.

The apparent sonic differences between interface technologies come into play in only ONE place… and that is at the input to the converters (A/D and D/A).

If the D/A is susceptible to jitter on its digital inputs (as most are), then you will hear differences between toslink (plastic fiber), glass fiber, and copper (hard wire). Some D/As reject jitter better than others, and that will determine the extent you can hear these differences. REMEMBER: This is only important to that particular listening session (the D/A only) and not to any other circumstance.

In the case of an A/D, if placed on internal sync, then its jitter (and subsequent distortion) is totally determined by its internal clock ciruits. But if you have to lock an A/D converter with external “AES” sync, the interface technology chosen may affect the stability of the A/D. Locking an A/D with wordclock produces far less jitter because there is no audio on the wordclock line, it is a pure clock. Wordclock is the second-best way to lock an A/D short of using internal sync. In the case of AES/EBU, the audio and clock are on the same line, and the audio (and other data) can cause interference during the critical clock extraction process. The different technologies (toslink, glass, copper) have different bandwidths, and reduced bandwidth (as with plastic fiber) can cause greater interface jitter.

In any case, it is preferable to put the A/D on internal sync for the lowest distortion. 2012: One possible exception. If the low frequency jitter (below the crossover point of the PLL) is better in the source wordclock generator than the internal jitter of the internal clock, then external sync can be better than internal. See papers at the Grimm website for this development.

Only by placing the master clock of the entire system within the D/A converter and feeding all devices as slaves to that clock can we eliminate these “ephemeral” differences. That way the D/A is immune to clock-induced problems on its AES or SPDIF inputs.

How can we reconcile this issue of requiring the master clock be inside the DAC, yet the A/D has to be on internal sync for lowest distortion? You can only have one master clock in a system. The answer is to design an INTEGRATED A/D and D/A system where the master clock is on a buss, feeding all the critical internal jitter-sensitive elements with a low-jitter buss-interface.

That’s what I’ve done recently in my system and I can attest that the “ephemeral” sonic differences have disappeared. (sigh of relief). At this time, I believe that integrated A/D/A systems with this technology can currently only be obtained from two vendors, TC Electronics (System 6000) and Prism. A consumer company called “Muse” has also adapted this technology on the consumer side, so there is hope. But it is sad and ironic that the audio industry has been so slow to adopt this technology, when the problem and solution have been known for years.

Hope this helps,

Bob

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