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Ralf11

Assertions on DACs & Digital Audio

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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.

 

 

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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

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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.

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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...

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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.

 

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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.

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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.

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@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)...

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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.

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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.

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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.

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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 . 

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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?

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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 😀,

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