HDMI vs. Coaxial Digital Interconnects

DonH50

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The framing and coding format relate to the digital data only; the bit error rate is awfully low for a decent CD and playback system due to the error correction etc. Jitter is rarely an issue for recovering the data; the problem is that the clock recovered from the data stream often has relatively high jitter, and is then applied directly to the DAC. We can recover the right bits from a fairly ugly eye, but the sampling clock is not so forgiving.
 

Orb

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The framing and coding format relate to the digital data only; the bit error rate is awfully low for a decent CD and playback system due to the error correction etc. Jitter is rarely an issue for recovering the data; the problem is that the clock recovered from the data stream often has relatively high jitter, and is then applied directly to the DAC. We can recover the right bits from a fairly ugly eye, but the sampling clock is not so forgiving.

Agreed but the clock information is part of the data stream when looking at it from a framing-signal perspective.
But the point is to show its beyond just a simple 1s and 0s that some may feel transmitting audio is all about (where some feel digital is digital and no problems can occur due to using just 1s and 0s).

Cheers
Orb
 

DonH50

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Yes, but all that matters to the clock recovery circuit are the edges, it doesn't know from bits... We are not in disagreement on anything, just focusing on different things. I have an article on the red-book format, someplace...
 

Orb

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Yes, but all that matters to the clock recovery circuit are the edges, it doesn't know from bits... We are not in disagreement on anything, just focusing on different things. I have an article on the red-book format, someplace...

Yeah appreciate we are not disagreeing and hopefully its not coming across that way.
I see it as we are focusing slightly differently on what to present in this discussion, which as you rightly said earlier is freaking complex - even just from the simple S/PDIF digital audio perspective.
I feel the 1st step for those approaching this from digital is just that and so there can be no differences (including by jitter) is to understand that it is not just 1s and 0s making up the audio but also requires clock/sync type information that also is includes additional structure information, so those 1s and 0s are far more than just the audio data when thinking about digital transmission.

Cheers
Orb
 

naturephoto1

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Yeah appreciate we are not disagreeing and hopefully its not coming across that way.
I see it as we are focusing slightly differently on what to present in this discussion, which as you rightly said earlier is freaking complex - even just from the simple S/PDIF digital audio perspective.
I feel the 1st step for those approaching this from digital is just that and so there can be no differences (including by jitter) is to understand that it is not just 1s and 0s making up the audio but also requires clock/sync type information that also is includes additional structure information, so those 1s and 0s are far more than just the audio data when thinking about digital transmission.

Cheers
Orb

It is just so unfortunate that some on other websites, including Blu-ray.com where I spend the most time, that most there insist that there is and should be no difference between coax and HDMI cables in at least relatively short lengths and that it is only 1s and 0s and therefore there can be no differences in sound. Very very frustrating. They insist that jitter, reflection, signal length, etc. have no bearing or affect on the sound.

Rich
 

Orb

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It is just so unfortunate that some on other websites, including Blu-ray.com where I spend the most time, that most there insist that there is and should be no difference between coax and HDMI cables in at least relatively short lengths and that it is only 1s and 0s and therefore there can be no differences in sound. Very very frustrating. They insist that jitter, reflection, signal length, etc. have no bearing or affect on the sound.

Rich

Well thankfully we got Amir and Don here :)

Cheers
Orb
 

DonH50

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Thanks Orb, though I am a tyro at the audio side compared to many here... Jez' a hairy-knuckled engineer what plays trumpet! ;)

It is just so unfortunate that some on other websites, including Blu-ray.com where I spend the most time, that most there insist that there is and should be no difference between coax and HDMI cables in at least relatively short lengths and that it is only 1s and 0s and therefore there can be no differences in sound. Very very frustrating. They insist that jitter, reflection, signal length, etc. have no bearing or affect on the sound.

Rich

Then they do not understand that, at the sampling point (the DAC in playback), it is essentially an analog clock signal derived from the digtial bits that causes problems. The bits could be perfect and it won't matter because the clock is off. There are pictures in my jitter threads, someplace... Those threads show perfect signals with imperfect clocks.
 

fas42

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Don, please explain to me and others why there is such a need to be extremely fussy about cabling, etc, if the DAC is doing the right thing by creating an extremely stable (low jitter) clock by whatever means, and the buffering of the input data is done to a level that guarantees that clean leading edges, or whatever, are delivered to the D/A conversion circuitry.

Thanks,
Frank
 

garylkoh

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Don, thanks for all the hairy-knuckled insights.

Have you had a chance to look at the Black Cat Veloce cable? It's a strange length 1.23m for $123. It's one of the best I've tried money-no-object. But as Frank noted, is it necessary if the DAC is doing the right thing? Between the Burmester phono stage ADC output and the Weiss Minerva DAC input for my listening event, I'm trying to figure how to improve things.
 

RBFC

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I just borrowed a 1 & 2 meter (same mfgr. and product line) cable from my dealer, and also a 2 meter Purist Audio digital interconnect. All RCA terminated... I'll be testing these out for any apparent differences and report my impressions.

Great discussion, everyone!

Lee
 

amirm

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Don, please explain to me and others why there is such a need to be extremely fussy about cabling, etc, if the DAC is doing the right thing by creating an extremely stable (low jitter) clock by whatever means, and the buffering of the input data is done to a level that guarantees that clean leading edges, or whatever, are delivered to the D/A conversion circuitry.

Thanks,
Frank
I will take a shot while Don is out :).

Our audio systems are synchronous. This means that they must stay in lock-step with the source. The source is the master of timing. The DAC is not allowed to get ahead (if it does, it will run out of samples to play) or behind (in which case, over time it will overflow its buffer). The situation is even more severe in video applications where audio is slave to that and if you make the audio late or early due to buffering, your lose sync.

So while probably every DAC out there buffers the data, its DAC clock must, in some way, track the source. The trick then is how to track the source, but not minor variations in it (jitter). Imagine if you had to walk next to me even though I slowed or sped up from time to time. If you tracked my every move, then you also have the same variation. If you did not track my every move, then you may get ahead of me or fall behind.

A circuit called Phased Locked Loop (PLL) is used when we have a clock and we want it to track another clock. Alas, PLLs are good at getting rid of variations at certain frequency range but not all. More sophisticated dual-stage PLLs can attempt to solve that problem but they are complex to design.
 

fas42

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Alas, PLLs are good at getting rid of variations at certain frequency range but not all. More sophisticated dual-stage PLLs can attempt to solve that problem but they are complex to design.
Firstly, as far as I am aware they may have trouble in adjusting for variations at very low frequencies, below 100Hz, but that shouldn't be an audible problem. Secondly, I'm pretty sure some devices go up to a triple stage PLL to get the job done.

The point really being, it's up to the designer of the DAC to ensure that it can handle jittery data. Anything else, and it's a failure of engineering; the user shouldn't be forced to play with cabling to make it work correctly ...

Frank
 

amirm

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It is up to the amp designer to build a perfect amp too. Is it a failing of said amp if it can't put out 2,000 watts with zero percent distortion?
 

RBFC

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If a 2 meter cable helps the DAC and transport to communicate more effectively, despite good engineering practices that reduce instability to admittedly low levels, then I'll use a 2 meter cable. As stated earlier in this thread, there are solid scientific reasons that the longer cable MAY improve data transfer.

Lee
 

fas42

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Is it a failing of said amp if it can't put out 2,000 watts with zero percent distortion?
Sorry, bad analogy, amirm :)

If the designer CLAIMED that he had designed such a amp, that, 2000 watts and zero distortion, then it would be a failure. But the process of creating a jitter free clock from jittery data in is a straightforward design exercise, executable with standard design techniques, and in fact the purveyors of many DACS claim the ability to to be able to deal with high levels of jitter. To repeat, Naim have solved the problem very nicely by sidestepping it: the DAC watches the data coming in and selects among a number of internal extremely stable clocks the one that best matches the incoming signal, the jitter is as low as how expensive a quartz clock the manufacturer wants to use ...

Frank
 

fas42

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If a 2 meter cable helps the DAC and transport to communicate more effectively, despite good engineering practices that reduce instability to admittedly low levels, then I'll use a 2 meter cable. As stated earlier in this thread, there are solid scientific reasons that the longer cable MAY improve data transfer.
The trouble is that it is a band aid solution, which means that every tiny, tiny thing you do makes a difference. Bend the cable by just so much will change the sound, vibrate it, fiddle with the end, everything will have an effect: it's a nightmare!

Frank
 

DonH50

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Amir explained my position quite well (thanks Amir!) However, we're talking about at least two different things here, and this is going to get deeper than most might appreciate (though barely entering the engineering realm)...

1. Impedance mismatches that cause signal variations that move the edges and zero-crossings used to extract the clock from the data stream. This is what cables can impact; the length of the cable affects when those mismatches arrive at the source or load. If a "bounce" happens in the middle of the "1" or "0", and assuming it is not too large, then there is negligible impact. If the bounce hits during a transition (edge, change in state), then it changes the effective bit period and thus the clock frequency. This effectively causes deterministic jitter.

You can adjust where the bounce hits by adjusting the cable length, at a given frequency. Thus, these "magic" cable lengths. Catch is, it's not just the length that matters; there are other parameters of importance even in the cable, and of course we (consumers) do not know the actual source and load impedances nor length of internal wiring from connector to the internal buffers, let alone the detailed characteristics of the waveforms, so in reality I find such claims somewhat dubious. How's that for waffling? :) That means cable length might have an effect, but the "best" length is likely to be a little different for every system.

2. The clock derived from the data stream is fed as the clock to the DAC. Note that I have caused some confusion; as a designer of ADC and DAC chips, the clock recovery circuit is not in my purview. When I say "DAC" I am talking about the actual DAC core; not the clocking circuits, not the input data buffers, nor even the output buffer circuit. I will generally (always, hopefully) state explicitly when I include those circuits. Critical as they are, they are not, to me, "the DAC". Sorry about that! Now, there is some sort of clock recovery circuit, and most often that includes a phase-locked loop (PLL) of some sort. As the name implies, it seeks to match a oscillator it controls to an external input signal by aligning their phase. In practice, that means looking at the edges. The data bits can be taken off part of the PLL, and the clock from another. The input has very wide bandwidth, but the output (control) bandwidth to the oscillator is very low.

PLL design for clock and data recovery is a compromise among fast signal capture (wide bandwidth), low sensitivity to incoming noise (low bandwidth), and all the internal parameters (myriads). Broadband noise will impact the phase detector at the front end; very low frequency noise will fall within the PLL's bandwidth and cause low-frequency pumping (this is the 100 Hz that was mentioned earlier). The main problem, IMO after ten second's thought, is not random jitter. That the PLL will reject pretty durn well, and besides just raises the noise floor a little. We will not notice that (at least most of us won't and for reasonable random jitter levels).

The real problem is deterministic jitter. That type is related (correlated) to the signal and/or clock, and that puts "beat patterns" into the clock that the PLL will not reject. Any transmission system with finite bandwidth (that means anything we can make) will have some amount of deterministic jitter and inter-symbol interference (ISI). The latter is when one bit affects the next, and the one after that, because in the real world there is a little memory in every system. That moves the clock around a little with the signal, and that causes spurs (distortion signals) to pop up out of the noise. This is shown in those jitter threads I keep referring to, off in the technical forum here on WBF.

Sorry for going off the deep end, hope it helps - Don
 

RBFC

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Frank,

I never said the 2 meter cable will be necessary to make the CD system work. I stated that I got 3 different cables that I'm going to try out and see if I can perceive any effects. I must admit that I am fatiguing of your posts that look down upon all of us "who don't yet understand how audio works". Yes, Naim and others claim to handle jitter, but that does not mean that there can be no further reductions in jitter-induced artifacts. Have you personally measured the jitter between the Naim and various transport mechanisms, and with cables of different manufacturer and length?

I also stated that if a 2 meter cable improved the data transfer, by virtue of cable/connector interface reflection reduction, then I would use a 2 meter cable. That's certainly a less expensive solution than replacing the components at both ends of the cable! Since I haven't even begun this experiment yet, I cannot say that any specific length of cable will make a difference in my system. I guess I'll just have to muddle along with my substandard system, and patch it occasionally in hopes it will someday be worthy.

Lee
 

DonH50

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But the process of creating a jitter free clock from jittery data in is a straightforward design exercise, executable with standard design techniques, and in fact the purveyors of many DACS claim the ability to to be able to deal with high levels of jitter. To repeat, Naim have solved the problem very nicely by sidestepping it: the DAC watches the data coming in and selects among a number of internal extremely stable clocks the one that best matches the incoming signal, the jitter is as low as how expensive a quartz clock the manufacturer wants to use ...

Frank

It is not that easy due to the data alignment problem Amir pointed out. You need a big enough cache to insure you never over run your buffer. It does not matter how stable the internal clock is if the input data rate is a little off; you'll eventually run out of space in your FIFO. Furthermore, as stated above, the real problem is deterministic jitter; random jitter is "easy" to handle, but deterministic is not, especially when it's handed to you by the incoming source. Deterministic jitter is the dirty little secret in these systems, and the reason for big buffers and quasi-isolated clocks for the DAC.
 

fas42

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The real problem is deterministic jitter. That type is related (correlated) to the signal and/or clock, and that puts "beat patterns" into the clock that the PLL will not reject.
That is the crux of the problem in terms of my understanding your very comprehensive description :). I don't quite follow there; would you be able to give a simple graphical or otherwise explanatory view of that. A picture tells a thousand words ...:):)

You need a big enough cache to insure you never over run your buffer. It does not matter how stable the internal clock is if the input data rate is a little off; you'll eventually run out of space in your FIFO
But that IS the point of the Naim; it does have a big enough cache; the FIFO is not a weak link. Even if the DAC switched internal clocks every 5 minutes or so in an extreme situation, you won't hear that level of jitter ...:D:D:D

Frank
 

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