Tie Line Gain Staging with USB

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This topic contains 4 replies, has 3 voices, and was last updated by Profile photo of SteffenR SteffenR 1 month, 1 week ago.

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  • #102044
    Profile photo of iamkingtriton
    iamkingtriton
    Participant

    Hello all, I need some insight. This is a two parter.

    Part 1:
    Running two SQ’s at church. One for FoH, the other for livestream. We are using Logic X to process audio for our livestream.

    I did a test the other day to better understand how everything interacts.

    I ran a tone from the FoH SigGen, patched it to a Group, tie lined that to the broadcast SQ, tie-lined that to my USB input. When the tone in the FoH sits at 0.0 db, it registers in Logic as -18. When the tone runs at about +6, Logic registers -12. And so on and so forth with about an -18db difference between preamp and what registers in Logic.

    Why is this the case?

    Part 2:
    Once the band gets processed by Logic, we USB tie-line the Main LR of Logic into the Main LR channel of the Broadcast SQ. In Logic, I have my mastering chain with the limiter set at -0.3. This is where I need understanding – When the mix is pumping and the limiter is doing is job, and I’m outputting at -0.3, the input of the Main LR on the SQ is clipping. In order to work around this, I have found that lowering the Main Output in Logic to -5, so the loudest we output from Logic is -5.3 — that seems to be the “ceiling” of the SQ.

    What’s happening?

    #102047
    Profile photo of KeithJ A&H
    KeithJ A&H
    Moderator

    Hi @iamkingtriton –

    The differences here are due to 0dB on the SQ meters equalling -18dBFS in the digital realm.
    You can find more details on this here – http://support.allen-heath.com/Knowledgebase/Article/View/levels-and-metering-in-qu-and-sq

    In brief – with digital signal there is an absolute maximum when all bits = 1 (e.g. a 16bit sample would be 1111 1111 1111 1111), this is 0dBFS (Full Scale).
    To avoid clipping the signal, our digital consoles have 18dB of headroom, so 0dB on the meters is the optimum level through the desk. Removing this headroom when sending the digital signal offers no benefit when recording and only increases the risk of digital clipping, but if you do want to trade headroom for level (as is common with a streaming setup) you can use direct out trims, push a mix output by up to 10dB with the fader or use the 18dB of make-up gain available in the compressor section of the mix.

    It’s the same in reverse when signal’s coming back in to the desk, in order that any recording comes back in at the same level.

    Cheers!
    Keith.

    #102048
    Profile photo of SteffenR
    SteffenR
    Participant

    to part 1:

    the metering reference point of the console is +4dBu then there is a headroom of 18dB to full scale signals (+22dBu on analog outputs)
    Logic shows the 0dBFS as it’s 0 dB

    to part 2:
    again the metering is that what confuses you
    the SQ is a live sound console and needs to avoid levels over 0dBFS(+22dBu)
    the clip meters warn you that you get close to the max without really reaching the max
    that’s what you see…

    #102058
    Profile photo of iamkingtriton
    iamkingtriton
    Participant

    @KeithJA&H, and @SteffenR –
    Thanks. I found another thread right after I posted that explained the -18db difference… should’ve done more research 🙂

    But, good. Thank you. I suppose Metering is something I know a lot less about than I realize.

    So I understand – Once A to D has occurred and the “bits are there”, headroom is not about getting good signal to noise ratio (like at the pre-amp stage), but space before digital distortion (0dBFS)? So, getting a signal at the preamp that is 0dBU RMS, then pushing the direct out +10 like you suggested doesn’t make the now -8 signal in Logic cleaner?

    Here is a metering question in general then – you are saying that the reference point of the console is +4dBu. Can you explain that more, and what you mean by +22? How I understand it –

    -0.3dBFS = +17.7dBu. But because the reference point of the SQ is +4, its actually +21.7dBu, which is why I am finding that setting my output to about -5dbFS is a sweet spot? Or is that wrong?

    Additionally, after the Main LR of the board. We go into a splitter, which goes into the facility and other location. And one of those locations is a blackmagic SDI injector which embeds the audio into the video from the switcher. One of the SDI outs goes to an SDI to Thunderbolt converter, into the streaming computer. Do you see a better or cleaner way to get the audio to the streaming computer, or from the Broadcast SQ to our SDI injector?

    #102061
    Profile photo of SteffenR
    SteffenR
    Participant

    The dBFS is not an absolute value in the analog world
    it depends on the reference point of the converters

    A&H converters have there max possible output level at +22dBu (analog XLR outputs on the back of the desk and all stageboxes)
    the point for 0dB meter read out is at +4dBu
    if you have a different converter that has it’s max level at +26dBu
    you will get a 4dB louder signal on the analog output
    sending -3 dBFS is +23dBu on this converter and +19dBu on the +22dBu converters

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