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RE: " USB REGEN " by UpTone Audio ... any opinions ?

Posted by Thorsten (M) on May 25, 2015 at 19:44:51

In Reply to: RE: " USB REGEN " by UpTone Audio ... any opinions ? posted by Serge_S on May 25, 2015 at 12:51:11:

Hi,

The Issue that has been made prominent in the context of the Gizmo being discussed is what I would call "frame noise". In any serial transmission system Data is packed into some form frame with distinct "boundary markers".

In USB 2 the main frame rate is 1mS (1kHz) which for High Speed is subdivided into 8 microframes each 125uS long with space (including frame markers and other overheads) for up to 60kb worth of data.

Usually not the whole 60k is used and often it is shared between multiple devices - each of which may have a "packet" of data. The theoretical limit is 13 bulk packets per microframe. This is not achievable with current host controllers, which can receive 10 bulk
packets/microframe or send 8 bulk packets/microframe.

So we have frame noise at 1kHz and 8kHz and Packet noise at theoretically 13 * 8kHz (104kHz) but in practice 64kHz (8 Microframes with 8 bulk packets) or less if using isochronous streaming (most USB audio).

Now the processing in frames and microframes may cause a cyclic load on the supply and if much error correction etc. is needed the load may increase. Such cyclic load may modulate the power-supply and cause problems if for example it is allowed to modulate audio clocks.

Perhaps surprisingly (or not) the mechanism is almost entirely analogous (or parallel) to the one in CD Transports where despite 100% error free reading of data sonic differences are observed between clean and dirty CD's, with the application of CD-mats etc. et al.

Incidentally, the varying PSU load (and the resultant noise) due to different frame fill factor with data packets and different data in packets is what I would class as the second kind of "packet noise" and it is semi-random but data related in nature, so in some ways more pernicious and harder to treat (again, similar issues apply for example to SPDIF) than the 1kHz and 8kHz frame noise which competent power supply design should control well (the emphasis being competent and should).

Is Ethernet (or WiFi, or MADI or anything else) free from framed/packetised processing? Nope, like any serial system this is how it works so it faces the same issues, just at different frequencies.

Most "coockie cutter" USB (and ethernet and other) Circuitry treats the clock as bad as old style low end CD-Players (powersupplies shared with transport, physical decoding layer and microprocessors, so noisy and modulated by many factors).

Frame noise, packet noise and other noise sources on the USB Power line can impact the clock and thus the audio even if the data is 100% correct. Pretty much any of the "turnkey" USB Solutions is very bad in this respect. I know not one commercial off the shelf module or Chip Manufacturers reference design that is better than "absolutely terrible" (I do not know all of them, only most) in this respect.

Any self respecting high end audio designer of course would look at such designs and fix the inherent problems in the reference designs and not use third part modules of questionable design. Looking inside much commercial gear makes me ask if there is a lack of self respect among high end audio designers or a lack of high end audio designers who understand digital and USB sufficiently well.

What is the bottom line?

As long as the system is correctly designed to avoid cross-contamination of clocks (and grounds/power supplies etc.) and as long as the cable length is kept reasonable within the design limits of the systems and as long as appropriate quality and electrical specification cables are used, non of these problems (degraded Signal Integrity, Frame/Packet noise induced jitter etc.) should be observable.

If these problems are observable they indicate a fundamental design flaw. Which is not to say that they are not observable on much of the gear/cables out there.

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

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