USB Myths and misconceptions

Posted by John Swenson (A) on January 27, 2006 at 00:18:11

I've been hearing a lot of stuff around here and other forums about USB audio interfaces and reactions to those statements. A lot of it is untrue or at least half truths, I thought I'd clean some of this up and hope to give a little dose of reality to the discussions here.

To start off I have quite a bit of experience with this, I have been building my own DACs for the last several years including S/PDIF interface, USB to S/PDIF and USB to I2S direct to DAC chips. I have built many combinations and both listened and done measurements.

First a quick overview of USB modes, synchronous, adaptive and asynchronous. All of these are "isochronous" whih just means the bus has reserved bandwidth for the data. Synchronous is the really cheap, very high jitter interface, its not seen all that often today, when you see reaaly bad numbers for a USB interface its probably running synchronous. Adaptive, much better, it still has a PLL but its referenced to a local crystal oscilator, the jitter can be quite low if done right. Asynchronous, the receiver is in complete control, output clock comes directly from low jitter clock, no PLL involved. Very low jitter if done right. If done not so right can be about the same as adaptive.

Almost all my experimantation has been with adaptive mode. There are only a couple chips that can implement asynchronous mode and they take reprogramming to run in that mode, that programming is turning out to be very difficult, some day I hope to get it worked out.

The up shot of the tests shows that if you use asaptive mode direct to I2S and feed that into the DAC chips you can get in the 65 to 70ps range, very good but not perfect by any means. Note I did NOT say its jitterless, just quite low jitter. Yes there ARE 1KHz sidebands but they are very low if the interface is done correctly.

Any attempt to convert USB to S/PDIF and then to a DAC results in much greater jitter than this, in the 150 to 250ps range.

Thus the big advantage to using USB is if you do NOT have S/PDIF in the stream in any way. Thus using a USB to S/PDIF converter running into your super duper DAC is NOT going to see most of the advantage that's possible to achieve with USB. Many companies that are coming out with "USB DACs" are just sticking a USB to S/PDIF converter in front of their existing S/PDIF input DAC, people then buy these because USB is supposed to be better, then find it doesn't sound that much better, so they conclude that USB interface is a bunch of hype! The truth is that the USB interface was implemented such as to negate the advantage it DOES have.

The other thing I hear is that someone buys am inexpensive USB DAC and somehow expects it to sound way better than their current much more expensive "high end DAC". The interface is an important part of the package but by no means the only part. Even using a USB interface directly to DAC chips does not mean its going to sound fantastic. You still need to use the right DAC chips, output stage, power supply, decent quality components where they do the most good etc.

If attention to detail is applied to all stages of the device, and direct USB to DAC chips is used, the results can be spectacular, unfortunately at this stage of things very few such products exist. Most of what you buy today is going to be a compromise somewhere along the way. There is no magic genie such that the inclusion of this one thing automatically makes a device the best in the world. A correctly implemented USB interface is ONE of the parts that can be used to achieve very good digital sound reproduction, the rest have to be there as well.

I hope this clears up a few things, if anyone still has confusion about the subject I'll be glad to try and shed some light.

John S.

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Topic - USB Myths and misconceptions - John Swenson 00:18:11 01/27/06 (33)

John Swenson - ? - krisgel 16:57:28 01/28/06 (6)

Re: John Swenson - ? - John Swenson 20:11:48 01/28/06 (5)

DDDAC modules now come pre assembled - scruffy_ 05:52:22 01/29/06 (3)

Re: DDDAC modules now come pre assembled - pburke 08:33:47 01/30/06 (2)

Oh, wow!!! - scruffy_ 09:49:39 01/30/06 (1)

Re: Oh, wow!!! - pburke 11:51:14 01/30/06 (0)

Re: John Swenson - ? - TVeye 04:05:20 01/29/06 (0)

Re: USB Myths and misconceptions - ASB 03:47:23 01/28/06 (1)

Re: USB Myths and misconceptions - John Swenson 20:18:29 01/28/06 (0)

Re: USB Myths and misconceptions - DannyR 02:37:32 01/28/06 (0)

Thanks for the explanation - Scrith 11:01:59 01/27/06 (5)

Re: Thanks for the explanation - fmak 22:21:06 01/27/06 (0)

Re: Thanks for the explanation - audioengr 11:25:01 01/27/06 (0)

Re: Thanks for the explanation - 00940 11:09:44 01/27/06 (2)

Re: Thanks for the explanation - Scrith 11:22:00 01/27/06 (1)

Re: Thanks for the explanation - 00940 11:27:39 01/27/06 (0)

Re: USB Myths and misconceptions - 00940 10:50:47 01/27/06 (6)

Re: USB Myths and misconceptions - John Swenson 12:10:07 01/27/06 (5)

USB modes and chips - Kenn 17:05:22 01/27/06 (2)

Re: USB modes and chips - John Swenson 19:58:32 01/27/06 (1)

Thanks. Good info. [nt] - Kenn 22:44:37 01/27/06 (0)

Re: USB Myths and misconceptions - BFitz 14:22:08 01/27/06 (1)

Re: USB Myths and misconceptions - 00940 14:50:26 01/27/06 (0)

Nice Post! - EL Cheapo 08:45:27 01/27/06 (5)

Direct I2S - John Swenson 12:16:26 01/27/06 (3)

Empirical - EL Cheapo 14:33:22 01/27/06 (0)

Re: Direct I2S - laznos 12:46:39 01/27/06 (1)

Re: Direct I2S - 00940 13:10:36 01/27/06 (0)

None! - Jon L 10:07:06 01/27/06 (0)

Re: USB Myths and misconceptions - Gus2 07:18:05 01/27/06 (0)

USB to I2S options? - demian 04:43:31 01/27/06 (1)

Re: USB to I2S options? - pburke 09:58:12 01/27/06 (0)

Re: USB Myths and misconceptions-thank you - fmak 01:09:01 01/27/06 (0)

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