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Topic: USB audio, the dark side (Read 2352 times) previous topic - next topic
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USB audio, the dark side

Hello everyone.
I've been investigating USB audio for a bit, and I've found out that it
a) doesn't have error correction
    a1) nor does optical or coax
b) does have error detection
    b1) not sure about those two
As I understand, optical and coax is transmitted using S/PDIF transmission standard, which is based on AES3. Also, receiver is synced to source, as in source device dictates data rate. As I understand, modern DACs at receiving end have memory buffers which cache some data, so decoding goes without problems.
Is my reasoning correct?

On the other side, we have USB audio, which, by my understanding of how stuff works, works in simmilar fashion as coax or optical. So... is it any better, for send/receive part, than coax or optical? What would be the advantages?

I am not an audiophool, rather, I am enjoying music more than technology, but I like to know how things work.
The reason I ask is, I've encountered this site, and this article,
https://darko.audio/2020/02/period-pain/
Which, by my understanding, overblows some aspects of USB transmission, but... where lies the truth?

I am well aware that, if I have glitchless audio from my computer to DAC with generic brand of USB cable, nothing would be gained if I buy some expensive cable, except, maybe, at some measuring level, better signal transmission because of cleaner copper, or better shielding... but at audio level, nothing would be gained.
Or would it? By the writing, it seems that there are people who believe that it would be better to have more expensive and better constructed USB cables.
TAPE LOADING ERROR

Re: USB audio, the dark side

Reply #1 –
Both USB and SPDIF (be it electrical or optical) do have error detection.
However, the USB Audio Class 1 or 2 audio standard works pretty much like SPDIF.
It is set to isochronous mode hence a quasi real time stream so no retry in case of errors.

In case of SPDIF, the sender generates the clock hence the receiver has to follow.
USB in isochronous mode with   asynchronous synchronization allows the DAC to control the amount of data hence the DAC has a free  running clock.

As most today’s DACs have their own clock and apply asynchronous sample rate conversion anyway, input jitter is probably a very moot point.
A bit more detail on my website:
https://www.thewelltemperedcomputer.com/KB/SPDIF.html
https://www.thewelltemperedcomputer.com/KB/USB.html

Audiophile USB cables are like analog audiophile audio cables.  They have proven to be excellent in transferring often huge amounts of money from one bank account to another.
Stay away from it.
If you want to be sure, buy certified digital cables, not voodoo one.
TheWellTemperedComputer.com

Re: USB audio, the dark side

Reply #2 –
I think  S/PDIF is "more pure".  

If you are playing a DVD using S/PDIF on your computer and it's got Dolby Digital or DTS, that encoded audio is going-out directly to your home theater receiver where it's decoded.   If you have "basic" S/PDIF DAC without the decoders it won't work.

With USB, the drivers & software will decode it and just about anything will work.    Any  "cheap USB soundcard" play 24/192 audio and you'll never know it's getting downsampled.   Or, you could be playing a surround file and the drivers will automatically downmix for a stereo soundcard (USB or internal).

Re: USB audio, the dark side

Reply #3 –
It seems more pure when you don't understand basic comouter architecture. S/PDIF (be it electrica or optic) is part of a soundcard (included in the main board or as an add-on card) and it is all down to drivers. If the drivers are poorly written or the settings are messed-up, it could output garbage just as easily as a USB sound card.

On a more generic note, no matter the output interface (USB, S/PDIF electric or S/PDIF optic) we live in a world where these interfaces are robust enough to work flawlessly in 99.9% of the cases. And the chance of the user noticing the 0.1% cases are even slimmer because of sounds masking each other, the user not paying enough attention and so on. So unless your copper cable was chewed by the cat and now it is only hanging on a fee strands you shouldn't loose any sleep over the subject.

Re: USB audio, the dark side

Reply #4 –
In the text, one argues that it is not possible to transfer USB audio without ANY errors because of WiFi equipment radiating around, and that shielded cable, which, AFAIK, USB cable is, acts like an antena, so there is slight difference in voltages transmitted in case of interference, and signal is changed to the point it... I don't know, glitches the playback? I know that, if the cable can, in such environment, transfer data at full speed (be it around 40 MB/s for USB2 or whatever USB3 devices you have, 120 MB/s for normal HDDs), it can certainly transfer that what, 4 MB/s stream from 24/96 audio.
Can anyone decipher the reasoning behind the text?
TAPE LOADING ERROR
 

Re: USB audio, the dark side

Reply #5 –
Quote
In the text, one argues that it is not possible to transfer USB audio without ANY errors because of WiFi equipment radiating around, and that shielded cable, which, AFAIK, USB cable is, acts like an antena, so there is slight difference in voltages transmitted in case of interference, and signal is changed to the point it... I don't know, glitches the playback?
Somebody doesn't understand digital at all!    A bit is either a 0 or 1.*   It takes a LOT of noise/corruption to "flip" a bit.   And, that almost never happens.    These posts here on the forum go through all kinds of connections all over the world and if there's a spelling error you can be pretty sure it's a human error rather than data corruption. 

And when you do get a digital errors things can get very bad quickly.   A single random flipped bit in your bank account can make a million dollar error just as easily as it can make a 1-cent error.

...To be fair, there is error-checking and error correction over the Internet that doesn't happen with "regular" USB audio but USB is still very robust.  If there's a problem with USB audio it usually originates with some kind of multitasking/interruption, etc.**



* I don't know the details of USB, but there are defined voltage levels.   For example, with 5 Volt logic "nominal" 5V is a 1 and "nominal" zero Volts is a zero.  One system/technology that I am familiar with will read anything greater than 3V as a 1, and anything less than 1.5V is read as a zero.   Anything in-between is undefined and might read one or zero.  Those are input specs.   The output specs are tighter so if you write (output) a 1 it will always be read (input) as a 1, and if you write a zero it will be read as a zero.    So, noise up to 1.5V has no effect on the digital signals. 

With analog, 1.5V of noise on a 5V signal puts the noise at about -10dB which would be pretty horrible.    And with a more-typical ~1V line level signal, the noise could exceed the signal!    -60dB is 1/000th of the signal and "typical" shielded analog cables don't even pick-up that much noise!   

** WI-Fi has been known to "interfere" with recording, but it's a interrupt problem, not an interference problem.