OK, I've been picking up recently that audiophiles are really into USB dacs in preference to DACs hooked up by good ol' toslink using the SPDIF protocol. And apparently they prefer USB to work in asynchronous mode. So I'm wondering, what is supposed to be the big deal with this scheme? I get why USB audio took off - PC connectivity. My first PC speakers/'soundcard' was a set of Microsoft MSS80 Digital Speaker system I bought in '99. No s/card needed, just plug the speakers in and go (after driver install of course).
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But I thought that was a matter of convenience, not 'sound quality', so what's the reason(ing) behind this new audio fad? I've also just read about the nutjob-priced Light Harmony da Vinci dac which is a 'true 384K' USB2.0 dac. Uh-huh, um, so what? An enquiring (if skeptical) mind wants to know. Isochronous Transfer Characteristics The host guarantees bandwidth to isochronous endpoints. This bandwidth guarantee is a feature unique to isochronous endpoints. The amount of bandwidth allocated to a particular isochronous endpoint is determined by information requested by that endpoint's descriptor.
Data transfers take place inside USB data frames. For Full Speed USB devices such as the Silicon Laboratories C8051F32x, a data frame spans 1 millisecond. A device can be configured to transmit as few as 1 byte per frame or as many as 1023 bytes per frame. The USB specification defines IN and OUT designations from the perspective of the host, meaning that IN endpoints transmit data to the host and OUT endpoints receive data from the host. During every 1 millisecond interval during communications operation, the host signals isochronous IN endpoints to transmit bytes to the host, and signals isochronous OUT endpoints to receive bytes from the host.
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Unlike other transfer types, isochronous transfers have no hardware-controlled handshaking or error-checking ability, so errors may occur occasionally. Systems must be able to recover from occasional errors without the help of hardware. Click to expand.The data loss / error mentioned is an intrinsic property of all real-time systems and pervades all broadcast based A/V transfer technologies: analogue, telephone, radio, TV, S/PDIF, HDMI.
What's more important is the fact that the host (your PC) is responsible for the transmission to the endpoint (the USB DAC) - a push. This puts the responsibility for the timing on the host, but PCs are unfortunately so busy doing all sorts of things that have little to do with audio, that the timing tends to be a little wonky.In addiiton, it's derived from that 1ms (1kHz) base clock. USB Audio Class 2.0 addresses this by 'reversing' who's in charge. Here the endpoint pulls the data into its buffer at whatever speed it can maintain, slowing it down when the buffer fills, but clocks it out of its buffer using its own independent clock. There are also some other enhancements, such as support for 24 and 32 bits, additional sample rates and high speed transfers (480 MHz), much of which cater to the answer 1) types above. In any case, 2) is the longer version of Steven's 'jitter' answer.
I think TokyoBlues has made the key point. I don't think anyone would say that a USB Sound Card or USB DAC is the ultimate end-all be-all solution to every problem. There is a time and place where it serves its purpose very well.
In TokyoBlues case it is simply because he/she doesn't have any other digital outputs from his computer. Or for someone who have a PC with a poor sound card, they weigh a new sound card against a USB Sound interface.
Each person makes his own best decision for his own personal situation. There is no one universal right answer; no one size fits all. USB Sound Cards/DACs serve a purpose and fill a market need.
But only for people who have the need and purpose to be filled. Though do keep in mind that a lot of home recording studios, with multi-channel recording capability, are using USB interfaces.
If the equipment is good, then this interface must serve the need nicely. I don't think the preference is for USB over optical/coaxial, it is simply that USB is more universally available.
And for recording, though that is a limited market, you need Digital INPUT, and USB digital In is again common. I think USB 3.0 is just released, so the data transfer rates will be higher (5Gbps). That should be able to stream tremendous amounts of audio both in and out.
Of course, as with all things, the quality of sound you get from a USB Sound Card is related to how much you paid. In a desktop PC, while USB is still an option, it is possible to get fast sound cards that have as many as 10 analog input and 10 output channels with 24b/96k sample rates. One weighs that against a 8-in/8-out USB interface with similar sample rates and a similar price. So, it is not about right or wrong, good or bad, it is about what fills the need of the user. For a use with access to quality optical/coaxial outs from their computer, then probably a standard external DAC is a better choice.
But for those who don't have that option, and who want better sound then the internal sound card, then USB is the better choice. Yeah, I've done a little reading and the 'reasoning' (NOT backed up with any published data or studies that I've found so far) is that isochronous transfer is clocked by the PC, and the PC's data clock is.apparently.
sufficiently unreliable that data packets get dropped/timed-out. Which would lead to whatever problems arise when a DAC's buffer empties (no idea how DACs are supposed to handle 'gaps' in buffered data though.). USB2.0 allows asynchronous data transfer which gives the DAC/receiving circuit control of the clock and bus. I can't help wondering though how bad jitter.actually.
is on a USB1.1 bus - the data rate is still much higher that an audio DAC needs to feed it (1.5Mbit or 12Mbit - Red Book audio is 1.38Mbit/s). Anyone know how large a typical DAC buffer is? And don't ALL DACs have internal clocks that clock data out of the buffer?
(ie, as long as the buffer never empties then how the data gets there doesn't matter as long as it's not corrupted/lost altogether.) I read somewhere that jitter on a USB1.1 bus is measured in nanoseconds, though I can't confirm this yet. Anyway, at a 'full speed' of 12Mbit/s a 1.5Mbit/s (rounding up) data stream would have to have a huge cumulative jitter before a dataword (16bit I assume) became too late and the DAC had to skip a word - and remember, each word is just 1/44,100th of a second. IOW, any glitch - even without any interpolation - is hardly likely to be audible.
So, I have to wonder how important it really is for a DAC to clock data off the PC. Because Red Book audio a) works at quite a low data-rate, and b) it's by virtue of its analogue output rather error tolerant (with interpolation, anyway). I'm not sure about USB, but for standard RS-232 Serial communication, every byte is 11 bits, there are start and stop bit ahead of and behind the data.
So, if we are reading 44,100 bytes per second, that is, 485,100 bits per second. Doubling that for stereo we have 970,200 bits per second. Stepping that up to 5.1 sound, we then have 2,910,600 bits per second. But, that does not account for error correction, retransmission, and other similar delays.
It is not all about data transfer rate, far more it is about the consistent unbroken even flow of data. I don't necessarily think it is about the advantage or one over the other, rather a case of maximizing what you have.
If the option of optical/coaxial output is available, I think it is preferred over USB, but it is not always available. It also depends on the application. I've never seen home studio recording hardware with an optical/coaxial interface, it is always USB or Firewire. Or in the other example I gave, direct analog connections to a mutli-channel high bit rate sound card.
The implication was that people prefer USB. I think people use USB because that is the best option for them all things considered. It is not a preference, it is a matter of practicality. But I think an optical/coaxial interface for normal music playback is preferred if it is available. Click to expand.It's 44,100 16-bit samples per channel, per second. So 44,100. 32 bits of raw data = 1.411200Mbit/s raw data rate.
Frame packing and control signals will add overhead, but not much. Call it 1.5Mbit/s then. This implies a time period per bit of 666 nanoseconds per bit at 1.5Mbit/s data transfer rate. For a timing error to result in a bit error, would require a cumulative jitter of, say 100ns or for an entire word to be missed 1600ns or 1.6us (micro-seconds)! This isn't even close to a 1% error within the USB1.1 1ms control period.
But lets suppose a bit-error forces a word-error (otherwise all the bits are out by one place.), then anyway the size of the buffer required merely has to hold enough words to cover for the max deviation over time between the PC clock and the DAC clock. Given that USB1.1 works normally at 12MHz then one would assume reasonably that every 1ms any variation in data supplied is corrected. Cumulative jitter would vary across each 1ms control period but within that period it's not going to get that high cumulatively, is it? What I'm getting at is that in reality isochronous 12MHz data transfer over USB1.1 is more than capable of keeping a reasonably designed external DAC working happily without data loss. So then what we're left with is why external DACs EVER cost a lot of money when the ONLY thing they need to completely resolve jitter issues would be a (literally, ha!) cheap-as-chips FIFO buffer clocked off the masterclock. Now, sure, you still have the output stage design to consider, but again for a 2Vref output into a nominally high-impedance input down-circuit we're not talking about some great engineering challenge here. And if USB2.0 asynchronous transfer solves jitter issues without even requiring a separate buffer, then I'm buggered if I can even begin to imagine what that da Vinci DAC does to justify even a 100th of its cost.
Click to expand.Well, this is what I had picked up. That async USB2.0 was where it was at for serious external DAC design; that (some!) audiophiles apparently believed it made the best sound, even over toslink/spdif. Anyway, I've done a bit of reading and I'm fairly certain now that they are just fooling themselves and, as happens so often in this hobby, allowing companies to prey on their stupidity with super-expensive items designed to relieve these audiophools of the heavy burden of carrying their money. I agree with Mark, and I think its a very big deal, one that manufacturers are (thankfully) waking up to very quickly. Whats exciting about USB DACs is that you have the opportunity to turn any music source into a perfect transport, that adds zero jitter to the replay system - where bits really are bits.
All USB interfaces, unlike toslink, spdif, HDMI or most implementations of I2S, have an asynchronous mode that allows the DAC to control the data flow. This is the ideal architecture, one that hifi has been missing out on for decades - ever since spdif was invented. And the best thing is that its ubiquitous - its cheap and everything support it. I can't wait for AVRs & processors to support it.
It was to all intents and purposes invented by Gordon Rankin of Wavelength Audio only a few years ago for his high-end USB DACs, and he called it Streamlength, which is now licensed to other manufacturers. Here's a good explanation from Ayre and Wavelength Audio: Nick. Innovations usually start off at the high-end, then work their way down-market to the rest of us. If nobody was ever allowed to sell expensive things, we wouldn't be where we are today. I believe the entry price has come down below £200 now. Here's a list of DACs and USB-spdif convertors that have an asynchronous mode (may need updating).
There are lots of USB DACs, but its only the asynchronous mode that minimises jitter in the system. Wavelength Audio Cosecant & Crimson DAC Ayre QB-9 Ayre DX-5 Arcam Solo rDAC Analog Research Technology Legato USB-SPDIF Asynchronous Converter Audiophilio 1 & 2 Audiolab 8200CD and 8200CDQ M2TECH hiFace, Evo & Young dCs Debussy & Puccini Berkeley Audio Design Alpha USB Halide Bridge Musical Fidelity V-link & V-DAC Aesthetix Pandora Grace Design M903 Bladelius USB DAC Firestone ILoveTW Onkyo DAC-1000 Teac UD-H01 DAC Nick. What the hell? I could have sworn I'd posted another post in here on Ayre's marketing bumpf, etc.! Anyway, if an SPDIF-USB convertor with asynchronous mode works, then clearly so does SPDIF!
I must be missing something? My main point I'd like to make then is that absent published research showing actual measured jitter, including cumulative jitter, etc we really have nothing to go on in assessing whether there was ever a problem with isochronous USB transfers.
Is it too much to ask for? There is a lot of discussion about Asynchronous mode, however I feel that this has been a tad overused.
Many people now think that Asynchronous mode is the best, however it is just generally easier to implement than a very high quality adaptive mode solution. A very good quality adaptive mode solution can outperform an async if done correctly. Another reason for USB DAC usage is high definition. Toslink is limited and most USB DACs will go to 24/192 now, something that Toslink struggles with from most PCs/Macs. Some Toslink hardware will do 24/192 but this is few and far between.
Kind regards Phil. There is a lot of discussion about Asynchronous mode, however I feel that this has been a tad overused. Many people now think that Asynchronous mode is the best, however it is just generally easier to implement than a very high quality adaptive mode solution. A very good quality adaptive mode solution can outperform an async if done correctly. Another reason for USB DAC usage is high definition. Toslink is limited and most USB DACs will go to 24/192 now, something that Toslink struggles with from most PCs/Macs. Some Toslink hardware will do 24/192 but this is few and far between.
Kind regards Phil. Click to expand.Check out CEntrance's AdaptiWave technology, its explained in detail by them.
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Leema Acoustics also partially choose their own method for their recent DAC too, which is why it's been stated as Asynchronous / Adaptive 24/192 implementation. Some Asynchronous implementations are not better than an adaptive solution and can even offer higher jitter as some implementations use frequency synths to generate the local clock, this at certain frequencies adds jitter and alot of jitter at that. Regards Phil.
Recommended USB DACs and USB Audio Interfaces for HighEnd Audio PCs Both within the context of HiFi / high-end audio as well as for recording studios there is now a wide range of high quality DACs available. As long as you reduce the requirements for a USB audio interface to those that are really essential (high quality conversion of the USB signal up to 192 kHz / 24 bit or more, very good analogue output stage, a high quality converter chip, such as ESS Sabre, Burr Brown, XMOS, Wolfson or the like, and maybe playback of DSD / DXD audio files), you can get a very cheap high-end DAC with a top sound.
When choosing the right device, let your ears decide and not the technical specifications: You don’t need an asynchronous mode. Adaptive converters provide great sound-neutral results for little money. Up-sampling can be expensive as well, and is worth it only when consistently implemented.
Since HD audio files with more than 192 kHz / 24 bit are still quite rare and the improvement in sound negligible, a 24/96 converter will do just fine Below you can find a selection of devices for high-end and studio technology, all of which can be easily purchased at Amazon.