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USB DAC's have an advantage?

post #1 of 14
Thread Starter 
For equivalent quality parts within a given DAC, will a USB DAC have an advantage due to lower jitter?

I read this page and was unaware that a USB DAC will almost eliminate jitter.

http://www.wavelengthaudio.com/Cosecant.html

I realize this is a very expensive DAC and I'm not considering it, but it has me thinking that the upcoming Perreaux DAC might be very good considering Perreaux is known for good products as well as the USB advantage.
post #2 of 14
Well I know its nothing to do with the original question (which I cant answer, BTW), but Perreaux do seem to be quite keen on their forthcoming DAC. This was their reply to an email I sent them about the DAC.

Quote:
The USB DAC is still a little way off but having spoken to the guys a lot, this is the one they see as being particularly good. I'll let you know as soon as I've given one a go on our iMac.

I suspect it will (cost) be a little more than the headphone amp as they are doing some pretty trick things that no-one else has really attempted in the hifi arena.
post #3 of 14
the one and only advantage is that USB chip itself is a master, computer is slaved to it, so that the clock is cooked inside the DAC, not inside computer, that's it.. short clock signal path and using Texas's USB DAC solutions, there is SpAct PLL which I've been told provides pretty low jittery clock.. I'm gonna try it and experiment with it a bit myself..

don't know what tricks they're talking about at Perraux..
post #4 of 14
Quote:
Originally Posted by Watchdog
For equivalent quality parts within a given DAC, will a USB DAC have an advantage due to lower jitter?

I read this page and was unaware that a USB DAC will almost eliminate jitter.

http://www.wavelengthaudio.com/Cosecant.html

I realize this is a very expensive DAC and I'm not considering it, but it has me thinking that the upcoming Perreaux DAC might be very good considering Perreaux is known for good products as well as the USB advantage.
The information on the Wavelength Audio page is mostly garbage, written by someone who either doesn't know what he's talking about or can't express himself properly. The part where he describes having to "sync up" with the sender has to happen in any case whenever a device needs to support multiple sample rates off a single clock (this process is called a phase-locked loop or PLL). There are also clear factual errors (e.g. the part where he says any sample rate above 48kHz would require Firewire -- that's not even true for USB version 1).

The clock recovery algorithm (SpAct) used by the Burr-Brown USB receivers (the most popular interface ICs) is exactly the same as the algorithm used by their S/PDIF receivers. I'll confess I don't know the details of the algorithm used by competing ICs manufactured by Philips Semiconductor, but I'd be surprised if they were much different. This isn't to say that USB DACs are bad -- not at all -- just that it's not at all clear that they're all that much better at handling jitter than conventional S/PDIF receivers.

Still, if you're looking for a computer-based audio solution, the upcoming Perreaux USB DAC does look tasty.
post #5 of 14
the main difference between SpAct used in S/PDIF receiver and USB chip is that in case of S/PDIF, it has to lock on and follow the incoming signal and it can vary in time so the PLL has to adjust to it all the time, on the other hand, USB chip itself is a master and asks computer for next data when the buffer gets empty, the PLL is there just to be able to support multiple sample rates using just one crystal, it doesn't need to lock on USB bus's clock.. at least that's what I believe is happening.. PCI audio controllers do that the same way except they have usually two oscillators..

do you know something more about it, Wodgy?? I might be wrong of course..
post #6 of 14
where to get info on this Perreaux USB DAC?
post #7 of 14
my apogee mini-dac has a usb 2.0 and optical in as well...i don't notice any different except increased mobility (not every computer has optical out).
post #8 of 14
Quote:
Originally Posted by Glassman
the main difference between SpAct used in S/PDIF receiver and USB chip is that in case of S/PDIF, it has to lock on and follow the incoming signal and it can vary in time so the PLL has to adjust to it all the time, on the other hand, USB chip itself is a master and asks computer for next data when the buffer gets empty, the PLL is there just to be able to support multiple sample rates using just one crystal, it doesn't need to lock on USB bus's clock.. at least that's what I believe is happening.. PCI audio controllers do that the same way except they have usually two oscillators..

do you know something more about it, Wodgy?? I might be wrong of course..
What you're describing is a synchronous protocol. The Burr-Brown devices actually use an isochronous protocol (check the datasheet). The sound device cannot request more data from the main CPU, but this is not strictly an asynchronous protocol because there are some timing guarantees (because the on-chip buffer is not infinite and there will be glitches if the CPU cannot keep up).

I understand why you'd naturally think the implementation would be synchronous, but due to the nature of digital audio, like any constant data stream a synchronous protocol doesn't offer any advantages unless you increase the complexity. The receiver never needs to ask "give me more data" because that request would be redundant -- the CPU knows that the receiver will be needing more data in X milliseconds just by the constant nature of digital audio. On a heavily loaded USB bus, a synchronous protocol would make more sense, because the receiver could send feedback back to the sender saying, in effect, my buffer seems to be dangerously low on a regular basis, send more data. However, the USB audio protocol is not that complicated. With the advent of USB 2, the bus is almost never that heavily loaded.

PCI audio cards use DMA so that cards can pull data off the bus themselves. The USB bus doesn't support DMA from devices.

Anyway, this isochronous protocol is why SpAct is essentially implemented the same way on USB receivers as it is implemented on S/PDIF receivers. At first glance it seems that a more sophisticated receiver IC might maintain two clocks, one recovered from the incoming data from the USB bus (to put data in a buffer) and another physical clock to run the internal DAC or output the data via I2S (i.e. taking data out of the buffer), but if you work through this in your head you'll realize that if the two clocks are even slightly off from one another the buffer will always be either empty or full. Hence the need for SpAct.
post #9 of 14
thanks for explanation, Wodgy.. you're right, isochronous is something different than what I thought it is.. that's bad.. the jitter will largely depend on each computer's USB bus wonder if it's any better then DIR1703 even if those two work simillary..
post #10 of 14
Quote:
Originally Posted by Glassman
thanks for explanation, Wodgy.. you're right, isochronous is something different than what I thought it is.. that's bad.. the jitter will largely depend on each computer's USB bus wonder if it's any better then DIR1703 even if those two work simillary..
The one advantage that these chips do have is that data arrives in 1ms packets, so the PLL can smoothly transition to each new estimated sample rate.

I was curious whether the Philips Semiconductor USB receivers worked in a similar way, and indeed they do. Check out the datasheet for UDA1325 and the brief discussion of their bit clock recovery algorithm (which they call SFG).
post #11 of 14
If what you listen to are files that are on the hard drive (ie. lossless rips from CDs), would you have any jitter at all?
post #12 of 14
Quote:
Originally Posted by Zoide
If what you listen to are files that are on the hard drive (ie. lossless rips from CDs), would you have any jitter at all?
Yes, as the primary source of jitter here is the USB bus. How much is hard to say, because neither Glassman nor I has access to measurement equipment suitable for finding precise values. I personally wouldn't worry about it though. SpAct over S/PDIF is supposed to have low levels of jitter (<75ps). The main point to this thread is just to debunk the nonsense on Wavelength's web page, not to cause anxiety in general users. If Wavelength is selling a $3500 USB gadget, you'd think they'd have a competent understanding of the underlying hardware, but alas the high-end market doesn't always work that way. You can probably safely assume that their level of technical knowledge is on par with the level of grammar demonstrated on their web page (sentence fragments, strange word breaks, e.g. "what ever", misspellings, e.g. "allot", etc.).

Head-Fier Sean H did report that he was able to improve the sound from his M-Audio Sonica's digital output (which uses the Burr-Brown TUSB3200) by adding a Monarchy jitter corrector, but I'm not convinced that this effect is more than placebo, and the particular DAC he uses is particularly sensitive to jitter.
post #13 of 14
What DAC does he use?

I'm thinking of buying the Piccolo, but there are two versions of receiver chip. As I said in this thread (http://www5.head-fi.org/forums/showthread.php?t=74216):

I'm thinking of buying a Piccolo DAC, and in the AOS website they say that it's available with either the DIR1703E or the AK4117 receiver chip. The DIR1703E consumes more power than the AK4117 (68mA vs. 45mA) and has some compatibility problems with devices such old Sony PCDPs w/ G protection, but has less jitter (75ps vs. 200ps).

What are people's experiences with the two versions? Are there any significant differences in sound quality?
post #14 of 14
They don't say on the Wavelength website which DAC chip they use, other than that it's a multibit converter used without oversampling or analog filtering.

I liked the Piccolo when I heard it. I'm not sure which receiver it had. Personally I'd ask AOS for advice and a recommendation. It's doubtful that anyone else has had any chance to compare versions with the two different receivers.
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