Asynchronicity: A USB Audio Primer

Thanks for the excellent article Chris. You are doing a great job.

Chris, thanks for the audio primer.<br />

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I have read that USB is capable of 24/192 if the manufacturer develops a special driver. Since the Four Musketeers of superior USB capable DACs (Ayre, Wavelength, dCS, and Empirical Audio) are capable designers and programmers, where are the 24/192 DACs from each?<br />

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I also remember reading than ethernet is also a very excellent interface for computer audio but where are the ethernet DACs and I don't mean wireless?<br />

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Clearly every personal computer built generally has USB and Ethernet connectors compared to the relatively few that have toslink, coaxial digital, or firewire. So is the problem that the DAC manufacturers have not fully embraced computer audio and are keeping one foot in the legacy door of CD transports and other audio products?

Very nice job, Chris. Well done!<br />

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I want to single out this statement you make: <br />

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<em>A DAC with a so-called poor USB implementation may have the best S/PDIF implementation on the market and vice versa. Thus the sound of a DAC may vary widely based on the input used.</em><br />

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This goes to one of the main bones of contention I have with the TAS article. In addition to its failure to discriminate (or even discuss) the different types of USB implementation, the article seems to lump all DACs with a USB input together in the category USB DAC. The way the ARC and Bryston review is positioned in the magazine makes it seem like these products are the ARC and Bryston entries into the USB DAC sweepstakes. I think that's misleading. These are their DACs, which happen to offer USB inputs. It's too late to put the genie back in the bottle, but we'd be better off if "USB DAC" was used only for products like the Ayre, Wavelength, and UltraFi that were designed specifically for USB. <br />

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I know, I'm splitting hairs.

Chris,<br />

Great article.<br />

My dog doesn't like to be wagged by his tail, so I bought the Ayre for him.<br />

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Aaron H

Great article, Chris. But I don't remember seeing or hearing any USB DAC's in your "state of the art" demos at the Symposium!<br />

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Jeff

Thanks<br />

very clear.<br />

Wow - you got a dCS front end. Well done. I hope you have got a transparent system. You are going to need it ha ha ha...<br />

Best<br />

Andrew

HI Guys - Thanks for the kind remarks thus far. <br />

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@ audiozorro - DAC manufacturers have definitely embraced computer audio but many of them are still figuring out how they'll get into the game. Meaning, should they do Firewire, USB, 24/96 or higher etc... Right now the economy has some manufacturers in a holding pattern as well. I'm positive you'll see 24/192 USB eventually. The manufacturers I've talked to are going to do it right instead of rush it out the door just to have a component that says 24/192 on the box. Don't hold your breath for 24/192 USB, it may be a while. <br />

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@ Soundproof - Thanks for the USB information. I wrote an article in April 2008 about problems with USB audio via certain USB ports on computers. It's definitely a concern that everyone should take serious, but steps can be taken to reduce and even eliminate the problem in many instances. Here is a link http://www.computeraudiophile.com/content/USB-Port-Not-USB-Port<br />

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@ DanRubin - That's a hair worth splitting.<br />

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@ jivers - You had to squeeze that in here didn't you. I would have liked to include some state of the art USB implementations in Studio A, but a line has to be drawn somewhere. I don't think the MBL / Magico v3 / Ayre QB-9 system in the lounge was a disappointment :~)<br />

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I dipped my toe into the waters of hirez with a little E-MU 0404 USB DAC, which does up to 24/192 at 4% of the cost of the top CASH picks. It doesn't need a Lynx card, but is able to take hirez files directly. The only issue is that it uses Win XP and not Vista. It also won't do 192 or 176 with a Mac. It can play the RefRec HRx files at 176/24 with no problems. I am using Media Monkey as the software. Larry

Hi Larry - There's no free lunch as I'm sure you know. The EMU DAC does support 24/192 and is only $200, but just because it goes to "11" doesn't mean much. I think there is good reason nobody else is supporting 24/192 via USB. While it's technically possible there are some major compromises. This particular DAC has very high noise related to the oscillators and the USB circuitry. Thus, the jitter is about 15x higher than some of the CASH list products that only support 24/96 via USB.<br />

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That said, this DAC does have a big following and many people are really enjoying the sound.<br />

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Chris, thanks for the explanation. As this was a test for hirez, I've decided to see what the fancier setups are like. I very much liked what I heard at the Computer Audiophile Symposium and today I had Tim Muratani and Mike Romanoski over to my home to scope out setting up a demo of the new Amarra Model 4 DAC and the software package that was demo'ed at the Symposium. It runs on a Mac through firewire, not USB. I am very interested in copying a fair amount of my vinyl onto 24/192 and use the software to remove clicks and pops. It looks like I would be able to generate the RIAA and other EQ curves in the digital domain as part of the A to D conversion process. I have a lot of old records (mostly EMI and Decca classical from the late '50s through the '60's that use non RIAA EQ. I'll let you know how it turns out. So far I have been very impressed by the expertise of Tim and Romo and how to optimize this all in my system. Larry

A problem with audio on some USB-ports. Well done with the write up, Chris.<br />

You have to ascertain which of your USB-ports is suited for audio, as not all are.<br />

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The use of USB Audio Devices on the Left-Hand USB Port Is Not Recommended. This applies to ALL MacBook Pro Models (Core Duo and Core 2 Duo).<br />

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• The 15" MacBook Pro models have 1 USB port on the left side, and one USB port on the right side.<br />

• The 17" MacBook Pro models have 2 USB ports on the left side, and one USB port on the right side.<br />

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Due to the current USB configuration of the 15" MacBook Pro under OS X, use of USB audio devices is supported on the right-hand USB port only. Use of such devices on the left-hand USB port(s) is not advised because it may cause audio interrupts and/or dropped samples. However, the use of an iLok on the left-hand port has been qualified and is fully supported.<br />

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Due to the current USB configuration of the 17" MacBook Pro under OS X, use of USB audio devices is only supported on the right-hand USB port, and the left-hand USB port farthest from the screen. Use of such devices on the left-hand USB port closest to the screen is not advised because it may cause audio interrupts and/or dropped samples. However, the use of an iLok on the left-hand port has been qualified and is fully supported.<br />

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These USB port recommendations are specific to USB Audio devices only (such as the FastTrack USB or Audiophile USB). USB Keyboard and Control Surface products do not have a recommended USB port at this time.<br />

Hi Larry - Wow, what a small world. I just got off the phone with Tim about thirty minutes ago! I think Romo is a very knowledgeable and great person to work with as well. Please let me know what you think of the demo once you have it running in your system and once you have a chance to convert some vinyl. <br />

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Sounds like you have some very fun listening ahead of you.

Just some questions: <br />

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* what does it mean Polling Interval 1ms, in both adaptive and asynch mode?<br />

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* can the oscillator of the motherboard influence the SQ of adaptive USB dacs?<br />

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* can a greatly optimized PC audio reduce the difference between adaptive and asynch USB dacs? I'm thinking to a Linux based machine, with no tasks to do but MPD to play music. I think in this scenario the timing should be improved considerably<br />

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* what about the buffer of the TAS1020 chip? Shouldn't it help when dealing with jitter? If data coming from PC is used to feed this buffer, and the DAC clock is used to acquire audio samples from this buffer rather than from USB directly, I see no issues in the adaptive implementation.<br />

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Rider,<br />

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<cite>* what does it mean Polling Interval 1ms, in both adaptive and asynch mode?<br />

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You can see the Polling interval is also set on the adaptive. This is when the computer sends the Start of Frame (SOF) and is programable from 1-32ms<br />

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<cite>* can the oscillator of the motherboard influence the SQ of adaptive USB dacs?<br />

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Not really though if it does change the variation of the SOF frame then yes it would effect adaptive. But I have not seen that happen in my experience.<br />

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<cite>* can a greatly optimized PC audio reduce the difference between adaptive and asynch USB dacs? I'm thinking to a Linux based machine, with no tasks to do but MPD to play music. I think in this scenario the timing should be improved considerably.<br />

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Actually not, see it has to do with the variation in timing between the Adaptive device. Optimizing your computer will only effect the variables inside the computer which makes things sound different. It will not effect the adaptive device any.<br />

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<cite>* what about the buffer of the TAS1020 chip? Shouldn't it help when dealing with jitter? If data coming from PC is used to feed this buffer, and the DAC clock is used to acquire audio samples from this buffer rather than from USB directly, I see no issues in the adaptive implementation.<br />

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Actually the way the code is setup in the TAS1020, NO. See if you look at figure 2-1 on page 25 of the TAS1020B you can see that the the SOF goes into a 16 bit timer and this is output via ACGCAPL/ACGCAPH. These values are used to dertermine the difference between the SOF timing and the internal timer and the difference is applied as the Adaptive difference into the Master Clock (via Frequency Synthesizer).<br />

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See there are two jitter problems with Adaptive:<br />

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1) The inherent change in the Master Clock due to the way Adaptive protocol works.<br />

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2) The Frequency Synthesizer it self has significant jitter even if you set it and left it there. More than 100x that of an external fixed oscillator.<br />

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24/192: Gang this protocol requires Class 2 Audio support which is not available in Windows in Linux. When they get closer to implementing it we will have a plan to support it.<br />

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Thanks<br />

Gordon

thank you for your prompt response and continuous knowledge sharing.

G'day Gorden<br />

great to see your dacs have found their way down under at long last.<br />

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re: "Actually not, see it has to do with the variation in timing between the Adaptive device. Optimizing your computer will only effect the variables inside the computer which makes things sound different. It will not effect the adaptive device any."<br />

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A fortiori are you saying that the only thing effecting jitter is the usb implementation?<br />

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Cheers<br />

A<br />

A and the rest of the Gang,<br />

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Correct... a computer cannot add jitter to the system what so ever. It is only when the PCM data is converted to I2S (or L/R justified) that jitter will occur.<br />

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The implementation of this will be critical to the overall jitter of the system.<br />

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Thanks<br />

Gordon

I'm not sure I understand this jitter problem. <br />

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Firstly, if the PC sends samples from its memory buffer at 1ms intervals (as per USB specification) to the DAC's own buffer, and the DAC, using its crystal oscillator as a clock, then processes those samples, where do the variations in timing arise?<br />

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Secondly, and more important, does it even matter? According to the AES, jitter below 20ns is inaudible, and even something as dependable, common and cheap (and therefore inherently non-audiophile) as the M-Audio Transit measures a mere 2ns.<br />

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All answers gratefully received.<br />

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Sid

Hi Sid - Welcome to Computer Audiophile. Please post your second question about jitter in a new thread on the forum. I don't want to derail the discussion covering a USB Audio Primer with very technical jitter talk. We all know jitter conversations carry on forever and nobody comes out a winner. <br />

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In the new thread can you also post the AES document that states jitter below 20ns is inaudible? There are several types of jitter, some matter, some don't. Measuring jitter is very difficult as well. I am really interested in the AES testing methodology in how they determined the 20ns inaudibility.<br />

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Thanks a ton Sid!

Oh dear.<br />

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I've stumbled into The Computer Audiophile Jitter Wars, with my very first comment. I'm sorry; I should have spent more time in the forums first to get the lay of the land. <br />

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So, very briefly, here's the AES paper:<br />

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<a href="http://www.aes.org/e-lib/browse.cfm?elib=8354">E. Benjamin, E. Gannon, Theoretical and audible effects of jitter on digital audio quality (1998)</a><br />

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And here's a more recent study published in "Acoustic Science & Technology":<br />

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<a href="http://www.jstage.jst.go.jp/article/ast/26/1/50/_pdf">K. Ashihara et al., Detection threshold for distortions due to jitter on digital audio (2005)</a><br />

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Phew.<br />

Sid,<br />

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Adaptive USB 101:<br />

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Every 1ms a frame called Start of Frame is sent. The SOF frame is a packet all by it self. It does not really mean data is coming down, just that it is another start of a time period as set in the enumeration, which in this case is 1ms.<br />

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The USB receiver also has a 1ms timer going on. The SOF time stamp is subtracted from the USB receivers time stamp and the difference is applied to the Master Clock output.<br />

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Changing the Master Clock on the fly like this adds jitter to the out going I2S (or R/L justified) link.<br />

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The computer and or operating system can effect this. But with newer USB controllers the processor need not actually evoke sending the SOF, instead the controller does this on it's own.<br />

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Therefore it is less likely that the computer will effect the jitter but really the difference in clock speeds between the two devices will determine how much jitter is added to the I2S stream.<br />

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Thanks<br />

Gordon

Good job on the USB primer 101! - even though it sounds a bit like an advertisement for asynchronous USB products - ex. Wavelength - who happens to advertise on your site! - <br />

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Another perspective on why manufacturers may be taking the easy way out by using "quick and dirty" adaptive techniques, is that wired computer based audio may be a passing fad! Lets face it. Getting rid of the wire is MUCH more flexible and convenient. Even companies like HSU research is now using wireless for their subwoofers! If you want to run two or more subs, not having wires running all over your listening area is a key feature! The opinion that wired is always better than wireless is as debatable as the superiority of USB over SPDIF! Like you said - its a matter of implementation.

Bada bing bada boom! - Gotta love the research - not much of that in audiophillia land!

Hi Eric - Can I ask what you are "bada-binging" about? Did you read the research papers or did you not do much research like audiophilia land? <br />

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It seems like you have something against high end audio and are seeking statements to cling on to that purport to discredit high end audio. There is actually a lot of research that goes on in audiophilia land. Extreme engineering is what brings us some remarkable products. Without research we would still be listening to perfect sound forever, also known as CDs. I also think consumers do as much research as they want. The readers of CA are doing the research right here.<br />

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Again, I'd like to keep this article on topic. Talk about USB Audio is what this should be all about. We can certainly take up jitter in a new thread in the forum, or we can continue the plethora of jitter threads already played out.

Hi Eric - Thanks for bringing up the fact that I spoke about Wavelength products and used Gordon Rankin as one source of data about USB Audio. It's critically important to keep this in the forefront. This is why I made it clear in the article who I used as sources of data during my research. Also, it's great that all the readers keep me honest by leaving comments on anything that may seem improper. It is almost impossible to research Asynchronous USB thoroughly without talking to Wavelength Audio. <br />

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Again, I get your point 100% and it's always good to discuss it when reading any article.