Jitter on digital connections?

[TomLondon]

Alright, I went to the friendly Hifi shop rather close to home last week to buy a new amplifier and speakers. The sales person tried to convince me that I needed a high-end CD player as well, because connecting just the digital output of my Bluray player would not sound reasonable. He quoted "jitter" as the reason, but his explanation sounded way less than convincing.

Soooo... this is what I found out so far:

- The SP/DIF interface between the Bluray and the amplifier runs a fairly standard block protocol which transmits data in large blocks. Data transmission is protected by checksums - so either the bits arrive correctly from the CD, or they do not arrive at all. So from all I can see, the DAC will see exactly the same bit stream from a 50 pounds Bluray player, as from a 5000 pounds CD player.

- The data is transmitted in blocks, and the interface can run at multiple speeds - 44.1 and 48kHz as the most common frequencies. Because of the block protocol, the data is not arriving in a constant stream, but in chunks. Hence, the DAC needs to have a buffer. It also needs to generate a clock signal to get this data converted into voltage at the right speed (unless someone really wants to derive a signal with a completely different frequency from the bus clock - which sounds like a fairly outlandish idea).

So - can anybody explain where this jitter is supposed to come from?

Thanks a lot!

 

[matt49]

I think you're absolutely right not to be worried by this.

In theory, because SPDIF is unidirectional, it can't replace missing data, but that's only a theoretical issue. It's also conceivable that the receiving device won't reclock the incoming data with 100% accuracy, and it'll be dependent on the accuracy of he clock in the sending device, and this could be a source of jitter. But the amounts of clock jitter are usually so small as to be inaudible.

Matt

 

[Vladimir]

All decent DACs have Anti Jitter Circuitry (AJC) so worry not and enjoy the music. Let the developers do the worrying.

 

[davedotco]

This is an interesting subject.

Firstly I am personally quite convinced that the transport can have an effect on sound quality during real time playback. This is my opinion on listening to many transports over many years.

Real time playback of a CD is at the mercy of all kinds of factors that compromise the accuracy of the data stream from the very beginning, I think they can make a substantial difference.

In addition I have no doubt that the spdif interface transfers all the bits correctly but I believe that there might be timing issues in the output of some players.

Whether these issues are consistently audible or not I can not say, but I have seen some data that suggests that timing issues are real and measureable, I have not carried out any proper listening tests to determin if these are audible or not.

 

[Vladimir]

A CDP has a buffer stage so it is not really real time like a turntable or cassette.

 

[matt49]

A CDP has a buffer stage so it is not really real time like a turntable or cassette.

Vlad, a CDP has a buffer stage to compensate for disk read errors.

Dave is talking about something quite different: timing errors in data transmission between a digital source and a DAC. These are measurable. Reclocking by the receiving device can compensate to a greater or lesser extent. In almost all cases they'll be inaudible.

I think it's more likely that if you've heard significant differences between DACs it has to do with their management of upstream PSU noise. I could be completely wrong, of course.

Matt

 

[Vladimir]

PC > DAC has even heftier buffering and timing issues are nonexistant in todays DACs. Jitter, noise, timing, it's all been solved. Computers are no longer a mystery to us humans.

 

[andyjm]

PC > DAC has even heftier buffering and timing issues are nonexistant in todays DACs. Jitter, noise, timing, it's all been solved. Computers are no longer a mystery to us humans.

There is often confusion between jitter impacting data integrity, and jitter introduced into the sample clock. I am unaware of any S/PDIF implementation that is so bad as to cause bit errors, however every S/PDIF implementation will introduce clock jitter to a greater of lesser extent.

The S/PDIF link does two jobs, it carries the data, and it also carries the sample clock. The data is robust, the sample clock is fragile. For those keen enough, google 'manchester biphase mark encoding'.

Jitter in the sample clock really matters. The regular tick of the clock is essential to the D2A process. The best place to have the clock is on the circuit board next to the D2A chip, but in the case of S/PDIF there is no flow control, so potentially data will arrive too fast or too slow for the local clock. This was why early DAC implementations locked themselves using PLL techniques to the S/PDIF clock.

More recently, jitter mitigation techniques have reduced the sensitivity of DACs to S/PDIF jitter, and async USB solves the problem by disconnecting the sample clock completely from the clock in the data source (CDP, streamer or whatever).

 

[matt49]

Exactly. If you were designing a digital music system from the ground up today, you wouldn't use SPDIF and consequently two clocks. You'd deliver the data to the DAC asynchronously via USB or Ethernet or wireless.

 

[matt49]

Duplicate thing

 

[davedotco]

I think there are two issues here, both of which can lead to audible issues.

There is the jitter that is inherent in Spdif data transmission which in a compedently designed system should not be audible, though I find the fact that some dacs that reclock the incoming data, actually lose lock on some transports due to high levels of jitter, to be troubling. This may have an effect on sound quality but really should not given the current state of the art.

Secondly their are variations in the ability of the transport to read the disc with total accuracy. These are mostly simple issues of electromagnetic engineering such as vibration control, mech allignment etc. They can cause some data to be missed at a level that is beyond the redundancy of the system, causing irrevocable damage to the data stream. If you corrupt the data stream sufficiantly you may well get audible effects.

And, in true Monty Python style, the third issue is the effect of transport noise on the dac in question. In the past I have heard some awful results from some dacs in this respect, though not so much in recent times.

I understand from the published liturature that such issues should not cause the kind of quality related variations that we talk of when discussing the sound quality of such components, but I have heard such differences far too often to dismiss them out of hand. If I still had acess to the range of kit that I had as a dealer, I would love to check this out in a more rigorous manner.

 

[nopiano]

Hello, Tom, and welcome to the madness!

I'd suggest you get your dealer to demonstrate his point with a listening test. I am inclined to believe that a cheap BDP will sound less good than a dedicated CDP, but then I've not experiemented with an external DAC. Jitter exists and can be measured (see Hi Fi News' tests) but I cannot speak for the audibility. That said, having grown up with analogue, I tend to intuitively prefer anything that measures better as long as it still sounds good. But the endless curiosity with sound is the things we think we hear that cannot be accounted for by measurement, e.g the cable debates. And these days, I'd choose listenability over 'accuracy' any day.

 

[TomLondon]

Thanks everybody for your input.

It all looks like if the designer of the DAC does not derive the clock of the DAC from the SP/DIF signal - something which would sound fairly stupid to me - there should not be any difference in what comes out ofthe DAC. Curious listening to an actual device this weekend - but sounds I can spend the 600 or 700 pounds more productively between the amplifier and the speakers.

All the Best

Tom

 

[TomLondon]

Thanks everybody for your input.

It all looks like if the designer of the DAC does not derive the clock of the DAC from the SP/DIF signal - something which would sound fairly stupid to me - there should not be any difference in what comes out ofthe DAC. Curious listening to an actual device this weekend - but sounds I can spend the 600 or 700 pounds more productively between the amplifier and the speakers.

All the Best

Tom

 

[steve_1979]

Curious listening to an actual device this weekend - but sounds I can spend the 600 or 700 pounds more productively between the amplifier and the speakers.

That would be my advice. Spend the bulk of your money on the amp and speakers.

 

[Overdose]

Curious listening to an actual device this weekend - but sounds I can spend the 600 or 700 pounds more productively between the amplifier and the speakers.

That would be my advice. Spend the bulk of your money on the amp and speakers.

unless he meant the things between the amp and speakers.

 

[ellisdj]

Why not do a demo of your blu ray vs a decent.digital streamer i.e cd player recommended and see if you can hear a difference

The shop is local you are very lucky.

Unless i missed it what blu ray player do you plan to use?

 

[steve_1979]

Curious listening to an actual device this weekend - but sounds I can spend the 600 or 700 pounds more productively between the amplifier and the speakers.

That would be my advice. Spend the bulk of your money on the amp and speakers.

unless he meant the things between the amp and speakers.

:rofl: