Jitter... what's that all about then?

[Mike]

The stuff mentioned about jitter all over the place is often quite amusing. I wonder how many posts contain that stupid(*) word, and how may people who've bandied it about know what it actually is or does?

In digital audio, should we be taking about absolute jitter, period jitter, or cycle to cycle jitter? etc. Should we be worrying about jitter period or jitter frequency? etc.

Do any of us (including me!) actually understand any of this? Or are we just throwing another 'buzz word' about?

In the interest of starting a possibly controversial discussion, here's a few things I've come across that should get the ball rolling...

• Cables can reduce jitter.
• Cables can cause jitter.
• Jitter can be heard (what does it sound like?)
• Jitter can't be heard, so doesn't matter

I'm now going to run for cover!


(*) I'm not denying the phenomenon, I just think it's a stupid word!

[Alex_UK]

This'll be fun!

[Themis]

Well, I will let others comment on jitter on digital audio.

I want to say is that jitter is an unwanted signal variation that affects ALL signal-transmitting protocols. This includes telecommunications, data packet transmissions (including devices like HD) and all communications whatever the protocol (USB, IP, SATA, put-your-favorite-protocol-here).
In high-speed digital streams the clock is never transmitted with the data (for various architectural reasons, including transmission speed). The clock for these signals is calculated by the receiver. It is thus, generated by a special algorithm called "clock and data recovery".

Thus, jitter is NOT a digital audio phenomenon, it has to do with data transmission. It is well known, is measured and (as it is unwanted) it is addressed by the "data consuming" application.

(ok, you can wake up, now ! )

[Mike]

Quite right, Dimitri. I'm a telecommunications engineer by trade, which is where I first came across it. The actual word "jitter" I believe did not originate on this side of 'the pond' where we talked in terms of 'phase shift'... Jitter (as an engineering term) was rather derided in fact. I remember offering my interpretation of the word and receiving some rather withering looks!

However...

In high-speed digital streams the clock is never transmitted with the data (for various architectural reasons, including transmission speed). The clock for these signals is calculated by the receiver. It is thus, generated by a special algorithm called "clock and data recovery".

But what about 'synchronous systems' where the data transmission is governed by a single clock? Or is that what you mean by "clock and data recovery"?

Synchronous data transmission is very, very common in telecoms and yet I don't think it happens at all with digital audio. Anyone?

I've heard of high end gear using 'master clock' type gizmo's, but I don't know if this is similar? Either way, they always seem to be high end systems at extraordinary prices. And yet there is a system available the world over which is relatively cheap and can be directly traced back to one or two extremely accurate atomic clocks!

Wonder why no-one's thought of that? Or maybe they have but don't want to miss out on those high end price tags!

[Themis]

But what about 'synchronous systems' where the data transmission is governed by a single clock? Or is that what you mean by "clock and data recovery"?

Synchronous systems, just have additional exchanges for synchronizing their respective clocks. In other words, they send some kind of "clock" data separately.
They use asynchronous communications, but they reconstruct (synchronize) their data based on the clock data sent. We use this only on specific critical applications. Synchronization it's too computing-cycle consuming to be used on a regular basis.
Some use 'synchronous systems' as a synonym for 'isochronous communications'. In this case, yes, the system reconstructs the clock from the interval of the data received. It is, indeed, what I meant by "clock and data recovery". To me, they are still "asynchronous" communications (but perhaps I'm wrong?).

At least that's how I see it from a system (consuming) level.

The "master clock" solution (afaik) can be seen as some kind of "synchronous" implementation: it allows to share a clock and thus not having two different clock errors. It doesn't eliminate all timing errors, it justs eliminates "some" kind of errors: the ones due to the differences between the "producing" and the "consuming" clock.
It is a (good) solution. I would have preferred some kind of "logical" layer, like the one used in "synchronizing" systems. The advantage of logical layers is to provide an error recovery. I've allways wonder why the guys who designed the actual audio protocols didn't think of it. Perhaps they were more "hardware oriented" than "software". Pity.
Even the actual audio USB implementation doesn't have an error recovery.

But, again, I'm not a specialist in audio communications. There must be some difficulties that I cannot grasp.

[Mike]

Synchronous systems, just have additional exchanges for synchronizing their respective clocks. In other words, they send some kind of "clock" data separately.

True... but in a highly localised system like a HiFi it should be possible to actually use the SAME clock!

Which, with a little bit of careful application, could completely solve:

The "master clock" solution (afaik) can be seen as some kind of "synchronous" implementation: it allows to share a clock and thus not having two different clock errors. It doesn't eliminate all timing errors, it justs eliminates "some" kind of errors: the ones due to the differences between the "producing" and the "consuming" clock.

And...

We use this only on specific critical applications.

Is, in fact, quite common!

Here's a bit of 'light reading' which might get your grey matter working!

Availability of a world-wide accurate clock; http://en.wikipedia.org/wiki/Global_...me_measurement

Where I get my ideas about 'synchronisation'; http://en.wikipedia.org/wiki/Synchro...ital_Hierarchy

*WARNING* You may suffer 'rapid onset narcolepsy'!

[Mike]

HeHeHe... we seem to have gone strait to discussing 'off the wall' possible solutions here!

What about the JITTER?... c'mon ya b'stards! What is it?, What causes it?, What do we do about it?

What about "jitter reducing cables"?... I've seen quite a few claims for that one! As well as "reducing hash", "reducing world poverty" etc. etc...

LETS TALK ABOUT JITTER!!!!

[Themis]

I will try to find references of how the jitter sounds.
(in my personal experience, it sounds abnormally "warm" in the midrange. You wouldn't expect it, would you ?)

[leo]

Jitter reducing cables
You'll make your heads explode guys with all this jitter talk

[Mike]

Bah!...

I got all excited when I noticed you'd replied. But yer as evasive as all the other buggers!