P4C800e and 5.1 suround sound question

Discussion in 'Asus' started by Jeff, Oct 10, 2005.

  1. Jeff

    Jeff Guest

    For the experts,

    I know the board supports 5.1 because of the spdif out, but when outputted
    to my amplifier, the rear two channels don't light up as SL and SR on the
    amp's display though that same amp with a feed from my dvd player will.
    make SL and SR glow

    Any ideas ?


    Jeff, Oct 10, 2005
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  2. Jeff

    Paul Guest

    SPDIF carries two channels. If a 48KHz sampling rate is
    used, stereo channels, 32 bits per channel, the result is a
    3.072Mbit/sec stream. After biphase mark encoding (which carries
    a lot of clock content, so the clock rate can be recovered at
    the receiver), the stream is 6.144Mb/sec. SPDIF can be carried
    on a coaxial cable, or on TOSLINK optical. The 32 bits is
    not all used for the sample data, and the sample size will be
    smaller than that.


    Now, the thing is, your receiver has 5.1 channels, and a
    stereo signal is going to be pretty useless. An encoding
    format called AC-3, allows stuffing 5.1 channels of information
    into the 2 channels of the SPDIF cable. (It is a compression

    On a DVD disk, the information necessary is already there. On
    a computer, it is a simple (?) matter of moving the AC-3
    stream from the DVD, to the computer audio hardware, and right
    down the cable to your receiver. That is called "passthru".
    At the receiving end, the Dolby AC-3 decoder in your receiver,
    analyses the stream, and if the AC-3 stream is detected, a LED
    probably lights up on the receiver.

    Dolby licenses all sorts of technologies for sound. A fee is
    charged for the right to encode in AC-3 and the right to decode
    AC-3. When you bought your receiver, you paid for the right to
    decode an AC-3 stream. There are likely stickers glued to the
    receiver, attesting to the Dolby licensing.

    On the encoding side, somebody had to pay Dolby for the right
    to put an AC-3 stream on a DVD. To just "passthru" that stream,
    there is no licensing issue on the computer.

    On the other hand, if you had a 5.1 analog source on the
    computer, and wanted to stuff it down the 2 channel SPDIF
    coax cable, you need an encoder. You can purchase software
    tools to do that encoding in real time (but there is quite
    a significant processing delay of 0.5 seconds or so). On
    an Nforce2 motherboard, Nvidia put a hardware encoder on
    there, so you can do that same encoding process in real
    time with much lower latency. Nvidia would have had to pay
    Dolby for each and every chipset shipped, even though the
    vast majority of consumers would never use the capability
    (and that lousy business model, is likely why it hasn't
    been done since).

    To summarize, if you want 5.1 down your SPDIF cable, you can
    get it by playing a DVD or other media where the content
    is already available as an AC-3 stream. If you have 5.1
    analog on the computer, want to encode to AC-3, ship it
    to your receiver, which decodes back to 5.1 again, you'll
    need an application to do that.

    In researching this, I get the impression the driver for the
    AD1985 is more anal than your average driver. It doesn't
    seem to support the passthru modes available on other sound
    devices. (Driver writers are acutely aware of DRM, and
    don't want to pass exact digital samples that would allow
    exact copies to be made. I don't see an issue here, with
    passthru, so stopping passthru doesn't make a lot of
    sense. Certainly many other sound devices do passthru.)

    There are several threads here, where you can read about
    other people's experiences. I don't know if this AC3filter
    tool is going to help you or not - I don't even
    understand how or why Dolby Labs allows it ?

    "Looking to update AD1985 AC97 try this" - newer drivers
    I've tried 5150 and like it because the amount of reverb
    in the driver was reduced for analog output at least.

    "SoundMax and SPDIF Problem" - more relevant to you

    "(Mentions AC3filter)"

    Description of Nvidia's hardware AC-3 encoder for Nforce2
    Encoder is called DICE (Dolby Interactive Content Encoder).

    Good luck,
    Paul, Oct 10, 2005
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