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Thread: USB audio

  1. #1
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    USB audio

    Today we have USB 1, 2 and 3.
    Often these specs are mixed up with USB audio specs.
    USB audio class 1 allows for 24 bit / 96 kHz 2 channel PCM audio.
    It will run on USB 1.1. as this is what fits in the 12 MB/s capacity of USB 1.1

    A lot of people thinks USB audio is limited to 16/48.
    Obvious this is not a limitation of the standard but a matter of a cheap hardware solution.

    The USB audio class 2 standard was finalized early 2009.
    The sample rates 176 and 192 are supported.
    This requires USB 2 (high speed = 480 MB/s) to run.

    All major OS (Win, OSX, Linux) has native support for a USB audio class 1.
    OSX and Linux also support USB audio class 2 from mid-2010 on.
    On Win you need a third party driver.

    Transfer modes
    Data is exchanged over USB using one of the four possible modes:

    ē Control Transfers: command and status operations,
    ē Interrupt Transfers: device requires the attention of the host
    ē Bulk Transfers: large volumes of data like print jobs
    ē Isochronous Transfers: time sensitive information, such as an audio or video stream
    o Guaranteed access to USB bandwidth.
    o Bounded latency.
    o Stream Pipe - Unidirectional
    o Error detection via CRC, but no retry or guarantee of delivery.
    o Full & high speed modes only
    Audio uses the isochronous transfer mode, a kind of Ďsoftí real time mode.

    When the computer sends the audio stream to an USB port, if first reads the data from the hard disk and caches blocks of the data in memory.
    It is then spooled from memory to the output port in a continuous stream (Isochronous mode ).
    Data is sent out in frames every millisecond.
    This happens whether there is any data in the frame or not.
    The rate at which the frames go out is determined by a oscillator driving the USB bus.
    This rate is independent of everything else going on in the PC.
    In principle this guarantees a constant flow of the frames.

    In practice the frames might not be filled properly with data because some program simply hogs the CPU or the PCI.
    Anti-virus polling the internet at high priority are a well-known example.

    Synchronization
    Isochronous transfer can be done with three possible types of synchronization in the USB audio device.

    Synchronous
    The clock driving the DAC is directly derived from the 1 kHz frame rate.
    This mode was used by the early USB audio devices.
    They were limited to 48 kHz and pretty jittery.

    Adaptive
    In this mode the clock comes from a separate clock
    A control circuit (sample rate guesser) measures the average rate of the data coming over the bus and adjusts the clock to match that.
    Since the clock is not directly derived from a bus signal it is far less sensitive to bus jitter than synchronous mode, but what is going on the bus still can affect it.
    Itís still generated by a PLL that takes its control from the circuits that see the jitter on the bus.
    This is the mode that most USB audio devices use today.

    Asynchronous
    In this mode an external clock is used to clock the data out of the buffer and a feedback stream is setup to tell the host how fast to send the data. A control circuit monitors the status of the buffer and tells the host to speed up if the buffer is getting too empty or slow down if itís getting too full.
    Since the readout clock is not dependent on anything going on with the bus, it can be fed directly from a low jitter oscillator, no PLL need apply.
    This mode can be made to be very insensitive to bus jitter.

    The warm reception in the audiophile world of asynchronous USB as developed and promoted by Wavelength inspired other brands to offer asynchronous USB DACs .

    Asynchronous mode is not better by design but by implementation because you can implement a top quality (low jitter) clock in the DAC.

    Things to look for
    When buying a USB DAC always check the specs of the USB input.
    A 24/192 DAC is indeed a 24/192 DAC but it might be combined with a 16/48 limited USB input.
    If you canít find the specs of the USB input, assuming it is 16/48 limited is a safe bet.
    At the present I donít see any reason to by a 16 bit USB DAC.
    24 bits can be obtained at the same price.
    Even if you donít have 24 bit sources (might change in the near future) a 24 bit depth is beneficial when using digital volume control.
    A well implemented asynchronous design probably beats an equally well implemented adaptive mode design. But a USB DAC is more than a synchronization mode only!

  2. #2
    Is it true that that it is debatable if one can hear any audiable difference between 16 bit and 24 bit?

  3. #3
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    Going from 16 to 24 bits the dynamic range grows from 96 to 144 dB.
    Sounds impressive but the noise floor of your gear might be somewhere around 110 dB.
    In practice it allows you to hear very tiny details at a very very low level if the noise floor of your gear is low enough
    This is discussed in more detail here: http://www.whatsbestforum.com/showth...igh-resolution
    A bit more on 16/24: http://www.whatsbestforum.com/showth...igh-resolution

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    [WBF Founding Member] Nicholas Bedworth's Avatar
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    And another thing to consider. Once you've got all the bits out of the computer and into the digital audio device, a critical parameter is the phase noise and jitter of the clocks that drive the AES or S/PDIF going to the DAC. Lower levels of jitter are hugely important for truly satisfying sound.
    Nicholas Bedworth, CTO
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    www.digitaldirect.com

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  5. #5
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    Andreas and I were streaming 352.8 and 384k from USB 2.0
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    Even a blind squirrel gets a nut once in a while!

  6. #6
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    A very thorough, clear explanation of USB issues, Vincent.

    You seem to have thought about many of the common misconceptions floating around the internet and provided factual material to address them.

    Many consumer and audiophile USB DACs depend on the generic USB audio support in Windows or MAC OSX. Some pro-audio products work with their own driver. The limitations of the OS provided USB audio driver are not relevant for those DACs. Some audiophile DACs now come with their own driver.

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  7. #7
    Thanks for your imformative replies. What prompted my original question was I recently bought a NOS dac that puts out only 16 bits but apparently allows 96/196 to pass thru. The interesting thing,that dac replaced an previous dac that up samples up to 24/196; the NOS dac sounded to my ears more natural and easy on the ears and with the same amount of detail. I play all lot of hi-Rez so I want to make sure I was not cheating myself out having 24 bits truncate down to 16 bits. It could be that the the NOS dac had better implementation which produce lower jitter. Perhaps I need to find a " true 24/196 dac" that would be better suited for hi- Rez playback. Thanks again for this conversation. Sam

  8. #8
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    Quote Originally Posted by Shum3s View Post
    Thanks for your imformative replies. What prompted my original question was I recently bought a NOS dac that puts out only 16 bits but apparently allows 96/196 to pass thru. The interesting thing,that dac replaced an previous dac that up samples up to 24/196; the NOS dac sounded to my ears more natural and easy on the ears and with the same amount of detail. I play all lot of hi-Rez so I want to make sure I was not cheating myself out having 24 bits truncate down to 16 bits. It could be that the the NOS dac had better implementation which produce lower jitter. Perhaps I need to find a " true 24/196 dac" that would be better suited for hi- Rez playback. Thanks again for this conversation. Sam
    Let me add a bit here. The number of bits a DAC supports is a nominal value. It is not the performance metric. All it means is that the DAC accepts that many bits. It doesn't mean it produces that much resolution.

    Let me expand. If you feed a DAC value 001 and then 002, the latter needs to be precisely double the output voltage. In a 16-bit sample, the lowest order bit, represents an awfully small value. It takes an excellent DAC to reproduce it effectively. Let's look at an example. This is linearity error graph for a low cost DAC (Music Streamer):



    A perfect DAC would have a line at 0. As you see, around -70db, it starts to deviate from ideal. What does this mean in terms of resolution? Simply divide that number by 6 and you get the number of bits at which linearity moves away from perfection: 70/6 = 11.6 bits!!! Even at -80db we have a resolution of 13 bits (with 2db of error).

    Now, this is an extreme case. Good DACs ace the above test and produce essentially no error up to 16 bit when fed 16 bits. Best DACs approach 20 bits of effective resolution (usually with about 2 db of linearity error).

    Net, net, I would happily accept a 16-bit DAC that really did reproduce 16 bits. Most mass market 16-bit systems hover at 14 bits of fidelity. True 16 bits actually sounds great

  9. #9
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    Quote Originally Posted by amirm View Post
    In a 16-bit sample, the lowest order bit, represents an awfully small value.
    What is the magnitude? 2V /65536?

  10. #10
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    Quote Originally Posted by Vincent Kars View Post
    What is the magnitude? 2V /65536?
    There is no standardization there unfortunately and as such, DACs vary on how much they put out. Here is a comparison I have for another reason which I have posted here before:


    As you see, the Mark Levinson has output of ~4 volts before it gets highly distorted whereas the Denon stops before 3 volts. Note that these are RMS values so actual voltage is this * 1.4 (for sinewave). So for ML, it is 5.6 volts/655356 = 0.00009 volts per step. For Denon, it would be 0.00006 per step.

    I looked and the stereophile review unfortunately does not list the output voltage for the Media Streamer above .

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