Why does SPDIF basically suck?

[Audiophile Neuroscience]

I remember when Dunn and Hawksford first published Is_The_AESEBU_SPDIF_Digital_Audio_Interface_Flawed

about the jitter problems with spdif.

 

Here and in other threads it is oft quoted that the unidirectional nature of spdif is a jitter trouble maker and the bidirectional nature of usb allows data flow controlled and verified by the DAC.

 

For me at least computer usb audio surpassed transport delivered spdif/ aes-ebu some years ago. I suspect this was all about improvements with implementation and ancillary gear to address noise etc.All I really hope for is that usb continues to be researched and allowing reasonably inexpensive upgrade paths without having to invest in a whole new digital wiz bang interface and hardware. Just my 2c.

[Audiophile Neuroscience]

On 5/12/2018 at 8:36 PM, mansr said:

You can do something more elaborate than the standard PLL and achieve much lower jitter, but you always must somehow avoid drift between your local clock and the incoming signal. USB does this by providing a feedback channel whereby the receiver can instruct the sender to slow down or speed up as needed to keep the fifo at a suitable fill level. S/PDIF is unidirectional, so nothing like this is possible there.

 

8 minutes ago, One and a half said:

S/PDIF can be reclocked and the results improve quite well

 

Are the above statements compatible?

 

 

 

[Audiophile Neuroscience]

7 hours ago, mansr said:

The problem for the receiver is recovering the clock without excessive jitter.

 

What is confusing for me, as far as I can understand the concepts, is the notion that the clock doesn't have to be *recovered* so long as it is *replaced* by a better one. I recall the fad of external super clocks for this purpose. My naive understanding is that the supposition would be that you can have as much jitter in the incoming signal provided the buffer/FIFO is just accurately reclocked before conversion to analog- problem solved?? This kind of 'argument' was often presented by people in the past who advocated that jitter was a non issue for "properly" designed DACs.

 

2 hours ago, Summit said:

 

I agree a well implemented coax can sound better than most USB. To say that SPDIF suck sucks 

 

 

Depends what you mean by "most USB". Comparing apples with apples, for me, well implemented USB sounds better than well implemented coax spdif or aes/ebu or glass ST fibre (the bayonet connection)

[Audiophile Neuroscience]

18 minutes ago, mansr said:

Absent a feedback channel for flow control, the playback rate must be slaved to the sender in order to avoid clock drift.

 

So if spdif lacks the feedback channel it must be slaved to the sender, with its jitter? In the alternative, why can't you just discard the sending clock, forget about drift, just use a new clock at the receiving end?

[Audiophile Neuroscience]

5 hours ago, mansr said:

If you ignore drift, you'll need to either drop or insert samples whenever the clocks slip by more than a sample period. The S/PDIF spec requires a frequency accuracy of 1000 ppm for the sender. Suppose your local clock is running at a perfect 48 kHz while the sender is at the upper end of the permitted range, that is 48048 Hz. Every second, you'll be receiving 48 samples more than you know what to do with. You have no choice but to discard them, and this causes distortion. Similarly, if the sender is slow, you'll have to somehow pull 48 samples per second out of thin air, again distorting the signal.

 Thanks for the explanation Mans. I think my gross misunderstanding is/was assuming a clock is a clock is a clock ie so long as the receiving clock is accurate you need no reference to the source clock for timing.Each sample with its corresponding bit depth would be clocked out, in the order that it arrived (FIFO). Certainly in the example when the sender is slow I get now that you cant clock out samples that have not yet arrived.

[Audiophile Neuroscience]

So, no doubt naively, I am conceptualizing that if the buffer is large enough to hold *all* samples then controlling/synchronizing their flow becomes moot......

 

11 hours ago, mansr said:

 So yes, in practice, you can get away for longer with a smaller buffer.

 

 

 

Don't you mean a larger buffer??? (confused)

 

11 hours ago, adamdea said:

Anyway the major advantage of trying out a really long buffer

 

That's "long" as in large right?

 

.

11 hours ago, adamdea said:

 

a really long buffer is to discover that it doesn't make any difference and stop angsting  about jitter.

 

Do you mean the buffer doesn't alter jitter problems or solves jitter problems?

 

 

[Audiophile Neuroscience]

5 hours ago, Summit said:

 

I mean all the DACs I have own have sounded better with I2S or coax. All of the DACs I have been able to test with different digital inputs have sounded better or equal to USB.

 

 

Fair enough! My experience is the opposite with respect to coax vs usb. I guess that's why they say YMMV

 

[Audiophile Neuroscience]

4 minutes ago, mansr said:

How many samples is that? A week's worth? A month? A year?

 

Hi Mans. I am not challenging you (this time haha) just trying to learn. I gather a bigger buffer is required for a year vs a week. IF my thinking is correct then, I would say that I would be 'happy' if the buffer held enough samples on average to last a full song. Now that may mean a variable pause before each playback but that would be my preference IF it solved jitter (and SQ improved) when using spdif. OTOH IMO just go with USB which doesn't have the same problem and to my ears sounds better anyway.

[Audiophile Neuroscience]

3 minutes ago, mansr said:

How would the DAC know how much to pre-buffer? Nothing in the S/PDIF signal indicates the duration, nor is there any kind of start/stop command.

 

okay I see, thanks.

[Audiophile Neuroscience]

37 minutes ago, adamdea said:

I guess you can call a buffer long or large depending whether you are meansurign it in bits or in the length of time that those bits represent. 

 

The buffer basically solves the "problem" of jitter. To be fair (which I generally avoid) some people would term this the "first order" effects of jitter only ie the problem of the conversion clock having to match anything in the sending clock. People like John Swenson claim that there is a second order jitter effect- all the little bits marching into the buffer have such heavy footsteps that they make the conversion clock wobble  even though they tiptoe out perfectly in time.

 

I have tried a dac with a long buffer and I have tried a proper cd trasnport slaved to the dacs conversion clock. Both of them are reassuring but they mainly convinced me that there wasn;t much to be worried about in the first place. Jitter is largely a bogeyman-the actual evidence of audibility is slight and after it is solved nothing much changes (at least for me).

 

It should be borne in mind that mansr is basically setting out why from an engineering point of view S/PDIF is not the best way of doing things. A properly asynchronous way of sending data would be better (eg ethernet or maybe bulk usb). But that isn't the same as saying there is a real problem with SPDIF which limits the listener. I notice incidentally that some designers and some listeners still prefer spdif over audio usb,.

 

 

Thanks for your thoughtful reply. I'm guessing your views on jitter will be controversial.

[Audiophile Neuroscience]

32 minutes ago, adamdea said:

All views on jitter are controversial on a hifi forum I'd say..

But there was a lot of literature in the early 90s. It is exciting when a problem is "discovered" which can be solved as this gives a justification for newer and better products. Julian Dunn wrote some interesting stuff and proposed the j test. Sterophile uses it to measure jitter and has done for years. Pretty much all proper dacs over £100 (and probably loads of them under it) now pass the j test with flying colours, and they have done for years That much is pretty much a fact.

If you doubt my scepticism try finding some actual evidence of audibility of jitter since Benjamin and Gannon. And look at the levels of jitter found in the output of modern dacs.

 

I quoted the seminal 1992 Dunn and hawksford paper earlier in the thread. Jitter was a concern then and DAC designers since have gone to great lengths to reduce it to ever more vanishingly small values.Evidence for audibility is also always going to be controversial on an audio forum. Interestingly, I noticed that the Red Pill/Blue Pill thread seems to have stalled.

[Audiophile Neuroscience]

10 hours ago, PeterG said:

Moral of the story--if you think your SPDIF sucks, it might be the cable.

 

In your case I would suggest there may have been other variables in play apart from the cable change. People often report a subjective difference in their sound system at different listening sessions. attributability is often difficult to nail down. Whether legend or fact, noisy power grids are sometimes blamed. 

[Audiophile Neuroscience]

8 hours ago, Summit said:

 

Okay am curious, which DACs and Music players that have both coax and USB have you used?

 

Some years back I started to wonder about changing to computer usb over transport/spdif when I was getting sound quality from a laptop into an RME Babyface DAC that rivaled my Reference Mark Levinson 31.5 > aes/ebu (or coax spdif or AT&T ST fibre) >30.6. I listened extensively to many combos & dacs at various dealers, too many to list, some of which i also auditioned at home including MSB Platinum stack and a UMT transport, tried and purchased Bricasti Mi Dac and then Gryphon Kalliope.

[Audiophile Neuroscience]

8 hours ago, Summit said:

 

I see that you totally dismiss my experience and my theory regarding conversions between different digital interfaces. But okay let’s say that you are correct and I like some added coloration. If that’s true it must also be the reason I have a JCAT LAN, don’t you think? I mean my preference for some extra coloration is probably constant. 

 

Nice jab !

[Audiophile Neuroscience]

1 hour ago, Superdad said:

 

You mean one that turns itself off at sunset and Friday and won’t turn on until Saturday night? 

 

...or supports the new graphical user interface with laser technology, GUILT

[Audiophile Neuroscience]

1 hour ago, mansr said:

Overgeneralisation of the day.

 

I gather you don't like JPlay Mans? If so, we share something in common. I wondered whether you have some association with JRiver (I do not apart from using their software). In fact, who are you? care to share?

 

[Audiophile Neuroscience]

8 minutes ago, Summit said:

 

Is it really too much to ask for that the ones that claim SQ superiority (or call things crap) of one design over another also state which audio gear the result was made. General statements that DDC or S/PDIF etc are inferior is very hard to take serious if no reference to which gear that been used are made.

 

I do not claim anything apart from telling you what I heard on what gear.As I said YMMV.

[Audiophile Neuroscience]

20 minutes ago, adamdea said:

2 hours ago, mansr said:

Overgeneralisation of the day.

I read that statement as "when I think I am getting something better than I can be predicted to enjoy the thing I think is better in line with my expectations" or "the effect of expectation bias on my enjoyment is reassuringly linear" .

I wish I could say the same as then it would be fairly easy to predict whether it was worth spending money on things I quite fancy buying.

 

As much as I am not a fan of JPlay or the man in question, I think you guys are reading a tad too much into his statement.

[Audiophile Neuroscience]

Mans, I looked you up. It's official, I am old

[Audiophile Neuroscience]

9 hours ago, adamdea said:

11 hours ago, Audiophile Neuroscience said:

Mans, I looked you up. It's official, I am old

...and I feel like a dumb monoglot

I have to admit to being slightly disappointed (and impressed as hell) that Måns' English does not have the heavy Swedish (?) accent I had imagined in my head when I read his posts.

 

Yeh, in my mind's eye, @mansr was a German middle aged ( or older) man. All I can say now is this very bright young man needs to learn to agree with me more often purely out of respect to his elders .

[Audiophile Neuroscience]

8 hours ago, Summit said:

 

You can favor whatever digital interface you want. All am asking for is for you and everyone else that says that S/PDIF is inferior or crap to tell me with which audio gear you made this apple to apple comparison. It’s good practice that one declare the reference if ask for.   

 

I listed MSB, Bricasti and Gryphon,,,,,Levinson but not direct comparison - and various others I cannot recall all the details well enough to list except I know I compared the dig outputs as well as dig filters and upsampling  (if offered). I wasn't doing it for a scientific comparison, just my own interest, general impression.

[Audiophile Neuroscience]

6 minutes ago, mansr said:

Young? I don't even get carded in the States any more.

 

Haha, I saw (what I thought was ) you in a 2012 video about optimizing Linux on ARM. You looked about 12 (kidding). when I was 30 I still looked about 17 which annoyed me no end.Then one day I looked in the mirror and wondered why I saw my father looking back! That annoyed me even more!

[Audiophile Neuroscience]

4 hours ago, Ralf11 said:

oops - I meant optical

 

Seems those Coriolis forces are just as active in the Northern hemisphere at pulling out the wrong words

 

Hey and watch those transmittal vs transmission different meaning nuances - noise transmittal sounds like the noise arrived in a memo