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Old 11th December 2006, 07:00 PM   #1
preiter is offline preiter  United States
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Default Why do we care about jitter?

Why is jitter a problem in audio?

I mean, I know what jitter is, and why it is important. In the industry where I work, we measure jitter in picoseconds. That's why I can't understand why jitter large enough to be perceived in an audio signal is not trivially easy to solve.

In the digital systems I am used to, signals pass through a long pipeline, being reclocked every step of the way. Jitter only matters if it is large enough that the signal is not valid when the clock happens. Does the data from the CD (or whatever your digital source is) really not get reclocked all the way from the laser to the DAC? If so, why not?

Digital signals are passed around serially. The actual digital signal (from a CD) is 32 bits per sample. 32 x 44.1KHz is 1,411,200 bits per second. That means that each bit only has a .709 uS window to bounce around in (any larger and bits are going to be misread entirely). Is jitter on that order of magnitude audible?

So what am I missing here? Why do people spend thousands of dollars on a CD transport?
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Old 11th December 2006, 07:04 PM   #2
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Default Re: Why do we care about jitter?

Quote:
Originally posted by preiter
Digital signals are passed around serially. The actual digital signal (from a CD) is 32 bits per sample. 32 x 44.1KHz is 1,411,200 bits per second. That means that each bit only has a .709 uS window to bounce around in (any larger and bits are going to be misread entirely). Is jitter on that order of magnitude audible?
[/B]
You first talk about jitter on the order of picosecond, but then ask why microsecond jitter would be audible? You do realize microsecond are 6 orders of magnitude larger, right?
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Old 11th December 2006, 07:11 PM   #3
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Default Re: Why do we care about jitter?

Quote:
Originally posted by preiter
Why is jitter a problem in audio?
[snip]So what am I missing here? Why do people spend thousands of dollars on a CD transport?

Good point. In another thread here, several people did all kinds of tests with CD transports, from very expensive to 15 $ cheap PC drives. In each case copying from any drive to any other, repeatedly, even to itself, always got an exact bit-for-bit copy. They tested bit-for-bit.

So, if you reclock your data at the DAC, whatever is before it, including the drive, doesn't bother at all. Assuming, of course, that the drive is accurate enough in frequency to keep the DAC filled. That appears to be the case.

The rest is all anecdotal.

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Old 11th December 2006, 07:22 PM   #4
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Jitter is important because it can be audible. CD playback systems are real-time, so the clock jitter directly affects the D/A converter output. Read this article for more:
http://www.positive-feedback.com/Issue22/nugent.htm

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Old 11th December 2006, 07:23 PM   #5
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Default Re: Re: Why do we care about jitter?

Quote:
Originally posted by janneman



Good point. In another thread here, several people did all kinds of tests with CD transports, from very expensive to 15 $ cheap PC drives. In each case copying from any drive to any other, repeatedly, even to itself, always got an exact bit-for-bit copy. They tested bit-for-bit.

So, if you reclock your data at the DAC, whatever is before it, including the drive, doesn't bother at all. Assuming, of course, that the drive is accurate enough in frequency to keep the DAC filled. That appears to be the case.

The rest is all anecdotal.

Jan Didden
The problem with this strategy is that there are no perfect DAC reclockers. They dont exist.

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Old 11th December 2006, 07:26 PM   #6
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Quote:
Originally posted by audioengr
Jitter is important because it can be audible. CD playback systems are real-time, so the clock jitter directly affects the D/A converter output. Read this article for more:
http://www.positive-feedback.com/Issue22/nugent.htm

Steve N.
Empirical Audio
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Steve,

Sure, but at least I can save 5k or so by getting a cheap transport, spend 2k on the best DAC reclocker I can find and use the other 3k to take the wife on a cruise. Everyone wins (except the high-end transport peddlers. Tough luck).

Jan Didden
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Old 11th December 2006, 07:30 PM   #7
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Default Re: Re: Re: Why do we care about jitter?

Quote:
Originally posted by audioengr


The problem with this strategy is that there are no perfect DAC reclockers. They dont exist.

Steve N.
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Steve,

You know better than that. It's the DAC clock that determines the final jitter. That DAC clock isn't perfect, nothing in this world is. So?

Jan Didden
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Old 11th December 2006, 07:36 PM   #8
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Default Re: Re: Why do we care about jitter?

Quote:
Originally posted by ezkcdude


You first talk about jitter on the order of picosecond, but then ask why microsecond jitter would be audible? You do realize microsecond are 6 orders of magnitude larger, right?
The systems with picoseconds of jitter are not audio systems. I only mentioned that to demonstrate how easy it should be to deal with jitter on an order of magnitude that would be audible.
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Old 11th December 2006, 07:38 PM   #9
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Default Re: Re: Re: Re: Why do we care about jitter?

Quote:
Originally posted by janneman


Steve,

You know better than that. It's the DAC clock that determines the final jitter. That DAC clock isn't perfect, nothing in this world is. So?

Jan Didden

What are you taking about? There are no clocks inside 90% of commercially available DAC's, except some with asynch. upsampling clocks, which dont qualify. The few with native reclocking circuits are far from perfect. In other words, they do not eliminate the effects of jitter on the input of the DAC. I have experience with many of these because I have modded them.

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Old 11th December 2006, 07:41 PM   #10
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I perused the data sheet for the CSS8414 and it reclocks the incoming data with a PLL. It seems to me that should do a pretty good job of reducing the jitter.

Exactly what amount of jitter do people claim is audible? I just have trouble understanding why this is a problem that would cost more than $20 to fix.

Also, I have seen some systems that reclock the signal at a much higher rate? Why would you do that? Aren't you just reclocking the jitter-skewed signal in the wrong place?
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