Does S/PDIF out have any latency before DAC?

Discussion in 'Converters / Interfaces' started by digdug, Mar 10, 2010.

  1. digdug

    digdug Guest

    Does S/PDIF out disregard the sample buffer settings of an interface, and only have sample rate latency?
    (e.g. 48kHz = 48sample buffer / 44.1kHz = 44.1 sample buffer)

    In other words, say you have on-board sound with a S/PDIF out; If you send out a 44.1kHz audio stream to an external DAC, will the only latency be 1ms + External DAC latency?
  2. Boswell

    Boswell Moderator Distinguished Member

    Home Page:
    This is a function of the software driver, not the hardware. Most drivers are written to have a FIFO buffer from which either an interrupt routine or DMA hardware pulls a single new data word when it is needed and writes it to the device input buffer. The FIFO is re-filled by a burst of data from a background task in the computer when it has time. If, due to other processor activity such as operations on the system disk, this burst of data does not happen often enough, the FIFO empties and drop-outs occur in the audio output.

    With this model of data output, it doesn't matter whether the hardware device serialises the data for S/PDIF output or writes it to a PCI device for immediate conversion to analog - the latency will be similar for both cases.

    USB, FireWire and other serial bus-orientated output devices use a different model, as it is the bus itself that has low-level drivers, and the device drivers sit on a layer above them. Latency effects for output devices attached to one of these buses are distributed between the device driver, the USB/FireWire driver and the buffers in the external device itself.
  3. digdug

    digdug Guest

    Thanks for the reply!

    If I understand you correctly, the better the driver the lower the latency... The better the hardware (whole PC), the less dropouts occur?

    My whole concern is that, I've been considering going the modular route with an external stereo DAC, and eventually ADC for some input. For right now I just need a DAC, and I didn't know whether going from my on-board S/PDIF to a DAC, OR buying a "decent" consumer pci sound card with s/pdif and better drivers to a DAC, OR just getting a USB DAC.

    I've coming from using a firebox, but it caught fire... So I'm looking for an upgrade, and needless to say I'm irritated with firewire, even if that may be irrational. I've been looking at the DAC1/DAC1USB, the Stereo96DAC, and the MiniDAC, but if I go the sound card to ext DAC route, it looks like I need to consider the sound cards latency, regardless if I'm using the analog outs. Kind of defeats the purpose I suppose...

    There are some consumer cards that give 2ms latency with ASIO, so a round trip with a DAC1 @ 48kHz would be 4ms just for the output. Sounds like I'd be better off just getting the USB DAC1, else when I get an ADC I'd already be worse off than I was with the firebox for round trip latency.

    I need a stereo DAC with the lowest latency possible with future low latency ADC expansion in mind, what route should I take?
  4. TheJackAttack

    TheJackAttack Distinguished Member

    currently Billings
    USB is much higher latency than firewire so that more than doubles your irrational thoughts on firewire. The lowest latency interfaces will always be PCI/PCIe based. If you want to plan to work in 192k then you need a PCIe card. RME makes cards like this in their HDSP line. A consumer card is just not appropriate for recording or mixing. The Benchmark unit is no doubt an excellent unit and does superlative conversion provided your USB bus is not utilized by other devices.
  5. digdug

    digdug Guest

    I was only considering the consumer card for the s/pdif out going to an external DAC, but now that seems out of the question.
    The only USB unit I was considering the DAC1USB, due to their "AdvancedUSB" tech.

    I don't need 192kHz support, 24bit/96kHz is fine as a brickwall for me. I have looked at the RME multiface II + HDSP, and although it's an older unit it would probably fit my needs as far as latency and future expansion goes. I've read the AD converters are not as great as the fireface 800, but I'd still rather stay away from FW...
  6. TheJackAttack

    TheJackAttack Distinguished Member

    currently Billings
    FYI, the "advanced usb" of the DAC1 has to do with word length and syncing so it isn't directly related to latency. 2ms of latency is probably not achievable via USB or Firewire. I think 5.8ms (a buffer of about 256 samples) is as good as you can hope for with a tremendously tweaked machine and no dual use of the computer. If you want to see what your computer is running at just sitting there, download DPC Latency checker and run it for ten or fifteen minutes. Then you'll know what you can expect from USB or Firewire. It won't tell you why you might get spikes or might or might not have high average latency. It is just a snapshot of what is running.
  7. Link555

    Link555 Well-Known Member

    North Vancouver
    Might want to look at the Lynx cards too. I have Lynx L22, it has 2 analog ins + 16 digital I/O for about $700CAD
  8. digdug

    digdug Guest

    Max of 79 microseconds, average around 49-55 idling for about 10 minutes...
    Oops, you're correct about the "Advanced USB", I was under the impression it helped with latency as well.

    Either way, their unit does boast impressive conversion latency... After you add buffer latency and ADC latency it would be nice to be around 6ms round trip. :)
    Thanks for the replies!
  9. digdug

    digdug Guest

    I've looked into them as well, and they are one of the few companies that make professional equipment, that supports down to 8bit/8kHz audio.
    It would be handy on the rare occasion when I have to work with 8bit/22,050kHz audio, so I don't have to use a consumer card to play those back.

    Decisions, Decisions...
  10. o0Julia0o

    o0Julia0o Active Member

    so what is the latency-best method to connect a DAC to the PC?
    PCIe ist the best, Onboardsoundchip the same? And optical has besser latency then Firewire?
  11. audiokid

    audiokid Chris Well-Known Member

    BC, Canada
    Home Page:
    Welcome: (y)

    To date, yes. This could be because the PCIe is in its own "dedicated" port. Closets to the motherboard.

    To my experience, anytime you can "dedicate" a port just for audio, the better results. So a FW interface is best on its own port too. Same for Madi, etc.

    When it comes to multi-tracking, AES EBU or MADI interfacing is my choice.
  12. o0Julia0o

    o0Julia0o Active Member

    thank you, audiokid :) i heard onbaord-DAC latency is bader in latency then USB-DAC. It is even more clothe to motherboad then PCIe.. ist burned into the motherboard ;)

    "to make music is not good on-board sound. And for several reasons: First, the sound quality is relatively modest. Secondly, the speed, switched to the sound and can be output is relatively lame. The Fachchinese speaks of "latency"."

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