Robert Harley posted in the D/A section of this forum an interesting item that fits with this section:
TAS reader David Sanford sent me this e-mail in response to my reply to his letter in TAS 182:
Thanks for publishing my letter in the June/July 2008 issue.
I don't really question the audibility of jitter. However, I have to be skeptical when someone tells me that timing variations between clock cycles of 7 picoseconds, (which is 7 billionths of a millisecond) are audible. I need a bit more scientific evidence than what one person claims he can hear.
I have been skeptical of people's perceptions ever since my high school physics lab. The teacher passed around two objects and we were to write down which one we thought was heavier. The first object was a small clear glass jar containing mercury. The second object was a larger clear glass jar containing some solid objects. Both jars had strings attached and each student was allowed to inspect the jars simultaneously any way he liked including holding them by the strings. Just over three-fourths of the class wrote down that the smaller jar containing the mercury was heavier. After we all gathered around and weighed the jars we found that the larger jar in fact weighed twice as much as the smaller jar!
Why were most of the students wrong, when the weights of the jars weren't even close? I don't know, but I realized that day that you should question the accuracy of human perceptions. In high end audio logic, however, many would argue that the fact that the larger jar actually measured heavier isn't important. What is important is that the students perceived the smaller jar to be heavier and therefore it must be.
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PostPosted: 09, Jun Mon 2008 10:36 am Post subject: Reply with quote
If I understand Johnson's point, then Sanford (who may not have seen Johnson's message) misses it. Sanford, I believe, is thinking analog. But this is digital. In digital signals, time has meaning that is different than what it means to humans in the analog domain. Thus, we are not talking about direct human perception of picosecond time, but the digital meaning of picosecond time.
If for example, we had a time-based encoding system operating in the terahertz range, then we might assign different frequencies to different digitally encoded numbers. A difference of even 1 picosecond in such a system would change the digital representation of the signal. A 7 picosecond variation might completely change the signal (close to 100% distortion, perhaps?).
Now in the case of CD, the frequencies are much lower, and I don't know enough about the encode/decode schemes to know whether the data error introduced by 7 picosecond timing variations is material. The point is, there are two issues: 1) is there a data error introduced by jitter and 2) is the error perceivable. The former seems at least plausible, and until we know the type and magnitude of the error I don't see why we should assume it would be hard to hear.
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PostPosted: 09, Jun Mon 2008 10:40 am Post subject: Reply with quote
Sanford, btw, may make another interesting point (unintentionally): his student buddies were very accurate perceivers (of density) and bad explainers (their theory was out of alignment with their perceptual system). We should not confound these two.