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Low-Jitter USB: Dan Lavry, Michael Goodman, Adaptive, Asynchronous

Discussion in 'Computer Audio' started by jude, May 20, 2010.
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  1. khaos974
    That's what the R&D head of Antelope Audio claims, he said this in a youtube interview, I'm a bit skeptical about this claim but here you go http://www.youtube.com/watch?v=-65gN44G9hU , it's either into this  video or the second part.
     
  2. bigshot
    I can see how some kinds of jitter could be worse than others. For instance jitter that affects frequencies outside the range of human hearing obviously are less of a problem than those that affect audible frequencies. However the concept that it could be euphonic makes absolutely no sense at all. The distortion caused wouldn't be harmonically related to the fundamentals like IMD at all. It would only affect particular frequencies. How can that possibly be a good thing?

    Am I missing something? Is the only reason it was mentioned because this guy mentioned it? Are there any listening tests to back it up? I don't see how someone could claim something is euphonic without someone listening to samples and saying " yes, this sounds better". Euphony isn't something that exists in numbers on paper.
     
  3. audioengr
    Simple.  Jitter can mask other system sibilance.  If the sibilance is worse sounding than the jitter that masks it, then the jitter is euphonic.
     
    Lots of systems have high levels of sibilance due to bad silver cables, poor preamps and even some amps.
     
    Steve N.
    Empirical Audio
     
  4. bigshot
    The chances of jitter lining up precisely with an octave below a sibilant frequency is extremely slim. The chances that the masking frequency is raised in volume by the jitter enough to create a masking effect is nil. Masking doesn't correct frequency response imbalances. It's the cause of them.

    I think people lay awake in bed at night thinking stuff like this up.
     
  5. h.rav
    me thinks SN doesn't know what he's talking about.
     
    Here is a good reading about cables, jitter, Steve... : http://recforums.prosoundweb.com/index.php/mv/msg/11678/0/0/0/
     
  6. HeadphoneAddict Contributor
  7. gabrielo


    Quote:

    Mr. Goodman,
     
    Can you please explain what you mean be cleaning up the signal?
    Does it mean you buffer the data as they arrive and  reclock it before feeding it to DAC chip?
     
    Could you also be so kind as to explain what is the meaning of Adaptive and SYNC USB-Sound?
     
    I have seen few people comment that one way can only accept data in fixed 48Khz frequency and that the other can get different frequencies.
    Is that true or that it is a misinformed concept?
     
    Does adaptive mean slave (target/server choose your lingo) and ASYNC mean master (initiator/client) or that it is more than that?
    Are we talking Control-Flow or more than that?
    I don’t see why it should matter much who initiates the transfer if the data are being buffered on the DAC side and being re-clocked before passed to the DAC chip.
     
    You can work in double buffering mode where one buffer receives data from computer (jittered ) and one buffer where you set the data to correct frequency and feed it to DAC chip, and then switch the buffers  and so on ...
     
    In theory it is better to be the one controlling the flow (ASYNC mode ?) as you won’t be sensitive to overflow (will that be a sound drop?), but with big enough local buffers this should not be a problem (one second of red-book audio is less than 200KB)
     
    Regards
    /gabriel
     
  8. nick_charles Contributor


    Quote:


    Quote:


     
  9. bigshot
    I hear dead people! They are between -90dB and -125dB!

    Hoodoo is alive and well and being dished up by the truckload right here on our beloved HeadFi.
     
  10. Bianci1969
    Dear Bmac-
     
    You're completely incorrect. You need to get your facts correct.
     
    Bianci, you are aware that the synchronous USB protocol that Mr. Goodman developed for Centrance is licensed to other manufacturers and is used in some well respected (and measuring) DAC's, right?
     
    CEntrance offers an Adaptive USB package. They have not been able to figure out Asynchronous USB, yet. That's not to say that they won't in the future.
     
    http://centrance.com/products/dacmini/design.shtml
     
    “Another less common adaptive USB implementation is done using a TAS1020 chip. Manufacturers then have a choice of implementing the chip exactly like the PCM270x without additional programming or possibly using the example code provided by TI, or the manufacturer can purchase code from CEntrance, Inc. to use with the TAS1020. Popular devices using the CEntrance code are the Benchmark DAC1 variants, Bel Canto USB Link, and the PS Audio Perfect Wave DAC. Using the TAS1020 and CEntrance code greatly enhances the USB interface and allows native 24/96 playback without the need for additional device drivers or special software.”  This is directly from the PS Audio forum- a customer of CEntrance.
     
    http://www.audaud.com/article.php?ArticleID=7559  Read the first feature: Adaptive!
     
     
     
    Mr. Goodman could also clarify if you need.
     
    Bianci
     
  11. Bmac


    Quote:
     
    Bianci, you are aware that synchronous and adaptive both refer to the same thing, right? And that asynchronous and synchronous are not the same thing, as you implied in your quote? No? Well thanks for coming out.
     
    The point I was trying to make in the previous post was clearly over your head, so I'll slow things down a little bit. If Mr. Goodman's (synchronous, AKA adaptive) technology is good enough to be used in world class DAC's like Benchmark, Bel Canto, Lavry and PS Audio, and he is able to license his technology for money to those companies, what makes you think he would have any interest or desire to pay a competitor to license their technology to use in his own product, thereby undermining his own product? Does that idea make any sense to you at all?
     
  12. Bmac
    Double post.
     
  13. Bianci1969
    Bmac-
     
    condescension is always the best give away that a nerve has been struck.
     
    Synchronous and Adaptive do not refer to the same thing. Are you confusing the terms Centrance currently sells an adaptive kit. When they started out they sold a synchronous kit. See the below definitions from the Well Tempered computer website.
     
    what makes you think he would have any interest or desire to pay a competitor to license their technology to use in his own product, thereby undermining his own product? Does that idea make any sense to you at all?
     
    It is used in those "world class" dacs because they have to purchase some interface because they can't write the codes required themselves. Since nobody is selling an OEM asynchronous kit that leaves a lot of business for Centrance.  "The best thing is the thing you have until something better comes along."
     
    From the Well Tempered computer
     
    Isochronous transfer can be done with three possible types of synchronization in the USB audio device.

    Synchronous

    The clock is directly derived from the 1 kHz frame rate. There is a PLL that takes in the start of frame signal and generates a clock. Using this scheme its rather difficult to generate 44.1, but very easy to generate 48 kHz. This is a primary reason why many early USB audio devices only supports 48 kHz, they used this mode.
    This mode is very susceptible to jitter on the bus, pretty much anything that causes the output from the host to be jittered (PS noise, vibrations, interference etc) AND things that can cause jitter on the interconnect (interference, reflections, ground noise etc) will wind up with jitter on the readout clock. This is a VERY poor mode to use for decent quality audio.

    Adaptive

    in this mode the clock comes from a separate clock generator (usually implemented as a PLL referenced by a crystal oscillator) that can have its frequency adjusted in small increments over a wide range. A control circuit (either hardware or firmware running on an embedded processor) 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. Its still generated by a PLL that takes its control from the circuits that see the jitter on the bus. Its a lot better than synchronous mode, but still not perfect by a long shot. 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 its getting too full. Note this is still isochronous, the host is continuously sending samples, there is no "per packet handshake" going on. Since the readout clock is not dependant 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
     
    Take it easy.
     
    Bianci
     
  14. mgoodman
    Guys, it appears like the argument is becoming more personal than technical. We are starting to compare dictionary definitions when this forum is about people who are passionate about sound quality first an foremost. CEntrance now has a very successful adaptive USB implementation. CEntrance has an Asynchronous implementation as well. Both offer a great sound. In the near future our customers will be able to compare them. My own listening tests do not reveal a difference, but we will make it possible for you to make your own decision. I'm a little frustrated that this discussion is becoming personal and would not like it to be this way. My time is best spent making great audio equipment, not engaging in argument. Let's focus on listening, because brand allegiance aside, music is our true passion.
     
  15. bigshot
    If we focus on listening it doesn't matter because jitter in inaudible at the levels found in even modest modern CD players. The whole point of discussing jitter is to split hairs into ever smaller fractions. I'm assuming that's the fun of it.
     
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