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发表于 2009-11-21 00:09:44
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不好意思，必须要偏题了。因为看了那个帖子，实在太有意思了。
某网友说JITTER对听感的影响不大，商业产品不会超过5NS，在这数值以下，耳朵是听不出来的。
然后，DAN予以严厉驳斥：
5 nsec of jitter is terrible!
First on a very basic level:
For a 16 bit system, you have a "quantization grid" of 65563 levels. For a say +/- 1V signal, each quantization level is 2V/65536 = 30.5uV (micro volts). The fastest analog signal within 44.1KHz system is 22KHz full scale sine wave, nothing is faster.
Such a signal has the fastest slope when you cross the midpoint of the signal (the zero crossing). At that point the slope for that +/- 1V signal is 138160 volts per second. In other words, each 1nsec on such a slope cost you an error of 138uV.
But given that each quantization step is 30.5uV, each 1nsec of error is in fact 138uV / 30.5 uV = 4.5 quantization levels. Or you can say that a timing error of 0.222nsec (220 psec - pico seconds) is an error of one quantization level. Of course, that 220psec is for a full scale sine wave at 22KHz.
If the sine wave is say 11Khz, the jitter can be 444psec for 1 quantization level off. And a signal of a lower level then full scale can also have more time error for missing 1 bit. At the limit, you have DC input where jitter makes no errors at all, if you are late or early the DC did not change so accurate timing is a non issue.
On a more complex level:
Some architecture requires low jitter to end up with a low noise floor. Jitter is a complicated issue, and there all sorts of types and issues. There is random jitter, signal dependent jitter, non random interference (such as "tones") and more.
But 5nesc is a lot of jitter. It makes your 16 bit machine into a less then 13 bit machine. It will behave much better when playing lower frequencies and levels, but at the extreme fast and high level signals it is less then 13 bits!
The amplitude and frequency content of the music does matter a lot. As a designer I need to take care of the worst case. When listening, the conclusion has a lot to do with the music. The faster and louder the music signal , the more impact the jitter has. That is good to keep in mind. I can find you a CD where a couple of nsec is no big deal ,and another CD where 2nsec is just real bad. So it is no surprise that people can have conflicting views on the audibility of jitter.
Yet it matters very much WHERE that jitter is. It is only important to have the low jitter AT THE CONVERTER, right where the digital is converted to analog. That is the "conversion jitter" and that is the jitter that matters. Moving data around can tolerate 100 times the jitter level with no sonic impact. We call that "data transfer jitter". If we have say huge jitter on say the spdif cable, but we get to "clean it" before it gets to the critical circuitry, then we are doing fine.
And BTW, the real difficulties are much more pronounced at say 20 bit performance where the jitter requirement is 16 times tighter then at 16 bits! As a rule, the signal getting to the DA has much higher jitter than the jitter at the critical circuitry, and the DA clocking circuitry needs to clean it up.
Regards
Dan Lavry
Lavry Engineering
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发表于 2009-11-20 18:34:24
发表于 2009-11-20 20:10:24
