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some questions about Digidesign's white paper

Discussion in 'Recording' started by compusic, Jan 9, 2008.

  1. compusic

    compusic Active Member

    has anyone read the Digidesign's white paper " 48 bit mixer"? the link is below:

    as for some points, I'm arguing with a local Pro Tools HD user, and we can not persuade each other. I have asked these questions in Digi's forum, but official response is very slow.

    to make things simple, I have these question below:

    1. what's the resolution of HD'S internal bus? I mean audio bus.

    someone thinks that as Pro Tools' MASTER FADER can be connected with internal bus and receive data from internal bus, and MASTER FADER is a 48-bit mixer, then the internal bus is a 48-bit bus. He thinks that the MASTER FADER of Pro Tools HD functions like a master fader on a analog console, then the bus the MASTER FADER connected with is a master bus, and this master bus shoule have the same resolution as the MASTER FADER.

    2. if say, set a audio track's output to internal bus 1, a aux track's input is bus 1, and set a MASTER FADER track's output to bus 1. then which one of the descriptions of the signal flow is correct?

    A: audio track outputs to bus 1 first, MASTER FADER receives data from bus 1, and processes the signal as needed, including level adjusting, then output to aux track.

    B: MASTER FADER receives data from audio track, and processes the signal as needed, including level adjusting, then outputs to bus 1, the aux track receives data from bus 1.
  2. compusic

    compusic Active Member

    one more information. I just figure out how the other guy thinks about the relationship between Pro Tools HD's MASTER FADER and internal bus.

    He thinks the MASTER FADER shoule behave like a fader on analog console, that's fader's input must be connected to a bus.

    But I think in HD, the situation is quite different. the HD'S MASTER FADER's input, in fact, is not connected to any HD'S internal bus, the MASTER FADER just receives data from different sources, if there's any bus here on the MASTER FADER'S input side, we should say MASTER FADER creates a virtual bus which is different from HD'S internal bus. for example, track 1, 2 and 3 all output to bus 1, and when MASTER FADER's output is set to bus 1, then MASTER FADER breaks the direct link between the tracks and bus 1. MASTER FADER receives all 3 tracks' data on its input side where we could say there's a virtual bus created, then MASTER FADER output the summed data to bus 1.

    whose opinion is correct?
  3. compusic

    compusic Active Member

    well, it seems difficult to express my thoughts in words, maybe some drawings are helpful.

    first suppose a situation, audiotrack 1 and 2 both output to internal bus number 1, and an aux track's input is the internal bus number 1, create a master fader track and set its output port to internal bus number 1. below are 2 pics which represent the other guy's and my understanding of the signal flow. which one is correct? the signal flows from left to right.

    first image:

    and second image:
  4. taxman

    taxman Active Member

    I don't have an answer for you, but would like to know how you drew the diagram.
  5. natural

    natural Active Member

    neither diagram makes sense to me.
    My understanding is that audio input is converted at the converters to 24bit.
    Once inside PT all processing is at 48bits.
    Master faders don't output into AUXes. They receive from auxes.
    Master faders convert the audio back to 24 bits and send them back to the converters.

    FIG2 in the white paper seems to be clear.
  6. Boswell

    Boswell Moderator Distinguished Member

    It's standard fixed-point design, and is all pretty clearly explained in the original paper. The 24-bit inputs are summed into a 56-bit accumulator, positioned so that 128 channels at +6dB (not +12dB as in the paper) can be summed without risk of overflow. The low 24 bits allow channel attenuations after panning of 90dB without losing bits. The resulting accumulation is dithered at the 24-bit level and truncated before feeding the mixer output.

    I think you are getting side-tracked by bringing auxes into it. They operate at the 24-bit level just like other channels.
  7. natural

    natural Active Member

    Boswell, When you say that auxes operate at 24 bit, do you mean the input, output, or both?
    I thought that input from the outside world came in at 24 bit (whether it be Audio Tracks or AUX tracks) but any processing, (or internal busing to the aux) would be at 48bit.
    Then that is sent to the Master fader which dithers it back down to 24bit and spits it back to the converter.

    The white paper doesn't seem to mention auxes, And I think this is where the OP is having an issue.
    Plus, am I reading his diagram correctly where it looks like the signal is going from the Master to an AUX? This would be incorrect no?
    Now I'm confused.
  8. Boswell

    Boswell Moderator Distinguished Member

    Auxes are like other outputs. They get fed 24-bit inputs and are mixed at double-precision (48/56 bit) and then truncated and dithered to 24-bit just like the main output.

    I think the confusion is because of bringing auxes into it at all. They are simply more outputs with their own accumulators just like the master output. It's less work to make them the same than to make them different.

    Of the diagrams, the first one is the nearest to what happens, but there is no 48-bit "internal bus". There should instead be an "accumulator" box with 128 x24-bit inputs and a 48-bit output going to the fader for that output.

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