Assertions on DACs & Digital Audio

[Ralf11]

I just finished reading through a big book (whose name & author I'll reveal later).  I am going to post some quotes (or close paraphrases) for discussion (and/or derision).

 

Please don't reveal the title or author esp. if you think you don't know it - let's not bias things.

 

 

[Ralf11]

ok, here we go:

 

on p. 186 -  "the best-sounding USB cables are 1.5 meters long."

[mav52]

38 minutes ago, Ralf11 said:

ok, here we go:

 

on p. 186 -  "the best-sounding USB cables are 1.5 meters long." 

Not sure the author ( yep I know who it is)  provided any supporting evidence to back up his claim

[The Computer Audiophile]

2 hours ago, Ralf11 said:

ok, here we go:

 

on p. 186 -  "the best-sounding USB cables are 1.5 meters long."

I’ve been told by a few engineers that I should use 1.5 meter AES cables, but these are far different from USB cables / data. 

[Ralf11]

No, no supporting evidence for that claim.

 

The only thing I can think of would be jitter induced by reflections.

 

but....

[The Computer Audiophile]

Per @Ralf11's request, he has been granted Moderator privileges for this topic as the OP.

[davide256]

Does it have a good investigation  of  noise effects on digital circuitry for audio/ Ethernet and D/A conversion? If not, it probably doesn't get  past the limitations of digital music before 2010.

[Ralf11]

David, I would say it is not good on that topic but there is some info & I'll post other claims later on.  BTW, this the 5th ed. of the tome, with a renewed (c) of 2015...

[ThenewGearPPK]

22 hours ago, Ralf11 said:

ok, here we go:

 

on p. 186 -  "the best-sounding USB cables are 1.5 meters long."

 

A few company state the same, with with slight variations.

 

Something along the lines of 1.3 meters being ideal, but almost no one makes it in that exact length.

 

[Ralf11]

Any idea why that might be so (assuming it is so...)??

[plissken]

Just now, Ralf11 said:

Any idea why that might be so (assuming it is so...)??

 

It could have something to due with the termination value and signal reflection.

[mav52]

I'm sure the answer or maybe some tech cable make up information is located in the USB 2.0 spec http://sdphca.ucsd.edu/lab_equip_manuals/usb_20.pdf and the USB 3 Spec/ https://www.usb3.com/whitepapers/USB 3 0 (11132008)-final.pdf at the moment I just don't the time to read the docs.

[mansr]

9 minutes ago, mav52 said:

I'm sure the answer is located in the USB 2.0 spec http://sdphca.ucsd.edu/lab_equip_manuals/usb_20.pdf and the USB 3 Spec/ https://www.usb3.com/whitepapers/USB 3 0 (11132008)-final.pdf at the moment I just don't the time to read the docs.

Why don't you link to the official documents?

USB 2.0: https://usb.org/document-library/usb-20-specification

USB 3.2: https://usb.org/document-library/usb-32-specification-released-september-22-2017-and-ecns

 

I have read, or at least skimmed, most of those specs. Nowhere is there anything whatsoever to support the notion of a minimum cable length. There is a maximum length due to constraints on signal latency. That's it. There are of course signal quality requirements, but how they are met is outside the scope of the spec.

[Ralf11]

@mansr @jabbr   any thoughts on signal reflection increasing jitter?

 

- seems far-fetched, but I cannot come up with any mechanism for this claim (plausible or not)...

[mansr]

3 minutes ago, Ralf11 said:

@mansr @jabbr   any thoughts on signal reflection increasing jitter?

 

- seems far-fetched, but I cannot come up with any mechanism for this claim (plausible or not)...

If the cable is within spec, it won't matter. Excessive jitter can make the receiver lose sync. This will cause either drop-outs or a device disconnect, either of which will be blatantly obvious.

[jabbr]

23 minutes ago, Ralf11 said:

@mansr @jabbr   any thoughts on signal reflection increasing jitter?

 

- seems far-fetched, but I cannot come up with any mechanism for this claim (plausible or not)...

 

A digital signal should have a constant rise time and within spec to allow the receiver to trigger at the same point/phase. A variable rise time will cause jitter in the receiver. A reflection can alter the signal/rise time between samples and thus cause jitter in the receiver. In a similar fashion crosstalk causes correlated jitter.

[Ralf11]

what is the likelihood of that degree of reflection as a f() of cable length...??

[mansr]

14 minutes ago, Ralf11 said:

what is the likelihood of that degree of reflection as a f() of cable length...??

If the cable is within spec, zero.

[fas42]

Some interesting comments, here ... http://jplay.eu/forum/index.php?/topic/326-length-of-usb-cable/

[mav52]

3 hours ago, mansr said:

Why don't you link to the official documents?

USB 2.0: https://usb.org/document-library/usb-20-specification

USB 3.2: https://usb.org/document-library/usb-32-specification-released-september-22-2017-and-ecns

 

I have read, or at least skimmed, most of those specs. Nowhere is there anything whatsoever to support the notion of a minimum cable length. There is a maximum length due to constraints on signal latency. That's it. There are of course signal quality requirements, but how they are met is outside the scope of the spec.

I ve got those as well, like I noted I didn’t have the time .  Nor have had the time to read either.  Glad you read them and got nothing . 

[mansr]

11 hours ago, jabbr said:

A digital signal should have a constant rise time and within spec to allow the receiver to trigger at the same point/phase. A variable rise time will cause jitter in the receiver. A reflection can alter the signal/rise time between samples and thus cause jitter in the receiver. In a similar fashion crosstalk causes correlated jitter.

USB uses NRZI encoding, so both clock and data are carried in edges arriving at a rate of 480 MHz. If there are reflections, they will have the greatest effect when coinciding with the primary edges. Reflected edges arriving midway between them will only cause a slight distortion in the open part of the eye, so that's what we want. This happens if the round-trip time through the cable is a multiple of the edge interval plus one half interval. With a signal propagation speed of 0.2 m/ns, the edge interval becomes 0.42 m. The round-trip distance should thus be 0.21 + n * 0.42 m. Dividing by 2 to get the cable length, this gives us a sequence of optimal lengths (in m): 0.31, 0.52, 0.73, 0.94, 1.15, 1.35, 1.56, 1.77, etc. One of these is close to 1.5 m. Could this be what the author had in mind?

 

Of course, this doesn't actually matter. USB 2.0 specifies an impedance of 90 Ω with a tolerance of 10% for transceivers and 15% for cables. This gives a worst-case reflection coefficient of 0.13. Since two reflections are involved, the final ratio is the square of this number, 0.016 or 1.6%. That's small enough to be of no concern, and the typical case is likely to be much better still.

[jabbr]

40 minutes ago, mansr said:

Since two reflections are involved, the final ratio is the square of this number, 0.016 or 1.6%. That's small enough to be of no concern, and the typical case is likely to be much better still.

 

Hard to know what level of jitter is of concern, but the point should be ala the meme de  jure , is that while 1.6% does seem like a small number, such a level of correlated jitter vastly outweighs the types of clock jitter that are being talked about. ie -120 dB is about 1e-6. and that’s for on spec cables not audiophile cable’s! Again, that + crosstalk etc more important than TCXO vs OCXO. Of course some people’s golden ears appear to be able to resolve better than most of our measurement equipment. Perhaps NASA should hire some audiophiles to assist with deep space exploration?

[mav52]

1 hour ago, jabbr said:

Perhaps NASA should hire some audiophiles to assist with deep space exploration? 

I sure hope not, the space program would be doomed. Audiophiles gave us MQA and Brilliant Pebbles 😀,

[The Computer Audiophile]

8 minutes ago, mav52 said:

I sure hope not, the space program would be doomed. Audiophiles gave us MQA and Brilliant Pebbles 😀,

Ha!

[mansr]

3 minutes ago, jabbr said:

Hard to know what level of jitter is of concern, but the point should be ala the meme de  jure , is that while 1.6% does seem like a small number, such a level of correlated jitter vastly outweighs the types of clock jitter that are being talked about. ie -120 dB is about 1e-6. and that’s for on spec cables not audiophile cable’s! Again, that + crosstalk etc more important than TCXO vs OCXO. Of course some people’s golden ears appear to be able to resolve better than most of our measurement equipment. Perhaps NASA should hire some audiophiles to assist with deep space exploration?

1.6% isn't the amount of jitter, it's the amount of reflected signal. Consider the receiver sensitivity eye diagram from the USB spec:

 

 

The timing skew cause by added noise is greatest for a long rise/fall time. The longest permissible rise/fall time occurs when the signal just barely stays clear of the keep-out area. Here's a figure that illustrates what this might look like:

 

 

Notice that the peaks can only reach about 0.2 V. With a reflection ratio of 1.6%, the worst-case resultant shift in the zero crossing would be about 0.75% UI, which is well within the allocated margin. A higher peak level must also have a shorter rise/fall time, and the maximum skew caused by a 1.6% reflection remains around 0.8% UI.

 

This amount of jitter in the actual DAC clock would of course be unacceptable. However, even the clock recovered from this signal will be much better. Moreover, this clock is only used within the USB PHY. Once a frame of data has been received, it is stored in a buffer and other (locally generated) clocks take over. This whole thing is a non-issue.