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Thread: 20db audio loss over NDI

  1. #1
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    20db audio loss over NDI

    I'm still working through my new TC1 and testing out everything. My team and I have noticed that when we use a LiveU over a LU2000 server we lose 20db of audio. We're using NDI to put the LiveU in our TC1. Anyone else have this problem? Thanks for the help!

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    We did a test: Took 2 cameras on two separate LiveUs and put them on the TC1 over NDI. They each had the same source audio (tone). Then we took the SDI output from the LiveU server. Here's a picture of the results:
    https://imgur.com/a/bZaqI

    Both NDIs were 20db lower than the SDI feed.

  3. #3
    I don't have a LiveU to test. But could they have separate adjustment of audio levels on the NDI output? As far as I'm aware, what is shown in the audio mixer is what is being received. Have you spoken with LiveU to make sure it isn't something on the sending end?
    Kane Peterson
    Solutions Architect
    NewTek, Inc.

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    I'll check on my side when I can put my hands on a LU2000 server.

    However, the 20dB problem make me think about a tricaster related problem. I alreadya got that 20dB difference with Dante recently : http://forums.newtek.com/showthread....=1#post1530935

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    Quote Originally Posted by kanep View Post
    I don't have a LiveU to test. But could they have separate adjustment of audio levels on the NDI output? As far as I'm aware, what is shown in the audio mixer is what is being received. Have you spoken with LiveU to make sure it isn't something on the sending end?
    We have spoken to LiveU. They're checking things on their end and have escalated the problem to their Tier 2 staff.

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    Quote Originally Posted by MatthewBLive View Post
    We did a test: Took 2 cameras on two separate LiveUs and put them on the TC1 over NDI. They each had the same source audio (tone). Then we took the SDI output from the LiveU server. Here's a picture of the results:
    https://imgur.com/a/bZaqI

    Both NDIs were 20db lower than the SDI feed.
    A likely explanation is that LiveU's implementation of NDI is using a different reference (max) floating point level compared to the TC1. All audio in NDI is floating point, and whilst the 'normal' range is -1.0 to +1.0 - you could go up to +/-10.0 or more if you wanted. Floating point allows for infinite audio headroom and no quantisation within the NDI domain, but it doesn't solve problems when coming into or out of a restricted headroom system.

    The challenge arises when converting between floating point and integer audio, and what you convert +1.0 into, or the reverse. The NDI Documentation does cover this in detail but its an area where developers very often meander. If LiveU have converted their 16-bit peak level into +10.0 in NDI, but TC1 is using +4.0 as peak 16-bit then you will get a big offset between the effective levels at each end. Audio in SDI is kept as integers, so the same conversion / variation doesn't take place there.

    Ultimately it really depends on how much headroom the devices at each end are maintaining. If one is up to +4 and another is up to +20 then this sort of mismatch is possible.

    We have made a recommendation to NewTek to add a metadata field to NDI audio to communicate the senders level-scaling assumption to the receiver, which would allow the receiver to match level appropriately.

    Meanwhile, either LiveU or NewTek could provide a mechanism to manually adjust the Floating point to Integer conversion.

    Alternatively something like Sienna's NDI Smart Processing Router could insert a processing module to 'scale' the floating point audio to match (without compromising it).

  7. #7
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    Great info, thnaks
    Jose Burgos
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