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Can the XT1143 pass 5.1 LPCM audio over HDMI?

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Gerry Lawson
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Hi all,

We are trying to play 6 channels of audio (synchronized to 5 video monitors.) We are currently able to play 5 channels of ac3 audio successfully but would like to use lpcm to play all 6 channels.

 

We currently are using 6 XT1143 units authored as a Brightwall with the master unit supplying the audio with an embedded ac3 5.1 file. The master unit is not connected to a display device (just serving the audio.) The ac3 is passed through to a SmartAvi U200S over TOSLINK (optical.) The ac3 encoding LFE channel contains only bass information so isn't useful for our sixth channel.

The video codec used is H264. The file serving the audio is Mpeg2 with ac3 embedded. This doesn't need frame-accurate synchronization.

We have an Atlona at-hd570 unit that I thought would decode multichannel audio. It seems to be more of an audio routing device which extracts audio streams from HDMI and send them to different outputs. We used this device to mix the audio channels outputting LPCM over HDMI from a laptop (then 6 channels of analog from the Atlona.) It was assumed that this would be easy to embed and pass through from a brightsign. As of yet we can't get a brightsign unit to passthrough multi-channel LPCM in a way that the Atlona device can process.

The Brightsign xt1143 unit has either frozen or the audio has been a stereo clicking in my tests. So far I've tried:

  • combining h264 video with lpcm audio using the quicktime 7 technique.
  • Encoding lpcm into quicktime video using Adobe Media Encoder (CS4 and CS6.)
  • My old Compressor app is crashing when I try to encode mpeg-2 with LPCM.
  • I am able to encode a matrox h264 with lpcm but the Brightsign hangs when playing it.

As LPCM is capable of 6 uncompressed channels we would like to use this as our form of audio delivery.

(TLDR)

Is it possible for a brightsign unit to send (pass-through) 5.1 LPCM audio over HDMI?

Is there another way to get 6 channels of audio out of a brightsign?

Thanks for any help,

Gerry

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9 comments

  • 0
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    Bright Scripters

    Although the master is there just for the audio, you must encode a video track with your surround audio.

    This page may provide some helpful information, and has specific info about the Adobe workflow.

    https://brightsign.zendesk.com/hc/en-us/articles/218065147-Supported-audio-formats

     

    Also, ffmpeg could help in creating mpeg2 files. ffmpeg has a steep learning curve, but the reward is well worth the effort.

     

     

  • 0
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    Gerry Lawson

    Thanks for the reply.

    I have looked (extensively) at the link you provided. LPCM is only mentioned specifically for the HD2000 (which looks like it was a great unit.) It says the following for the XT-x43:

    Dolby Digital (AC3) 5.1 audio in an MPEG2 or MOV video (pass-through over HDMI or TOSLINK w/ expander). Other multichannel audio formats may work as pass-through, but they are not officially supported.

    I'm not certain if LPCM can pass-through over hdmi in a similar was that ac3 does. It seems different from the research that I've done, but I'm not sure how it's different. Also the Altona unit that I have has a switch specifically for (bitstream / LPCM5.1 / LPCM stereo) - implying that there is a technical difference between bitstream and however LPCM is carried over HDMI.

    I think that the Adobe information is specifically related to ac3 encoding (which is working fine for me.)

    Some of this is addressed in the comment by Brandon about bitstreaming vs. decoding (referencing ac3 on a laptop.) I just don't know if LPCM 5.1 is a format that requires decoding in order to stream over HDMI. I had thought that it was a passthrough operation but now am not sure.

    I've used ffmpeg (a long time ago) and appreciate its functionality. I've been spoiled by gui applications for a long time but plan to dive back into the power of ffmeg command line processes. I just didn't want to go too far down that road in this project if the goal wasn't possible.

    Sorry for the wall of text. Just trying to be specific.

  • 0
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    Bright Scripters

    How accurately do you need the channels to be synchronized?

    Would you consider using 3 players, each playing two channels?

    BrightWall may not play audio through all if its members, so you may need to synchronize using Advanced Synchronization instead.

    That can be found in Presentation Properties -> Interactive

  • 0
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    Brandon

    So a few things in play here.

    1. S/PDIF does not have enough bandwidth to carry more than 2 channels of LPCM, so S/PDIF is out for 5.1 or 7.1 LPCM.
      https://en.wikipedia.org/wiki/S/PDIF
    2. HDMI, on the other hand, can carry 5.1 LPCM
    3. LPCM tracks are essentially uncompressed already, so there isn't decoding necessary to pass it out unchanged (bitstream aka pass through mode) via HDMI but there are some considerations with regard to unpackaging it from the container it's in.
    4. The above said, it should pass over HDMI, but it's not something we've tried.

    The "stereo clicking" you encountered might've been the BrightSign player passing audio through, but the receiving device not knowing how to decode it and playing it as if it was stereo PCM, which would be a clicky-clacky mess.

    If you don't need super tight audio sync, you could use multiple players, or even play different audio out of the HDMI, S/PDIF and analog simultaneously.  The problem on the Series 3 players is that the analog and S/PDIF share the same physical mini-TOSlink jack, and as far as I have searched I have not yet found a 3.5mm analog+optical breakout cable.

    So you'd need at least two players going that route unless you're using a pre-Series 3 player that still has separate analog and TOSlink output.

    Encoding-wise generally you're going to end up using a MOV as the container as it's the most flexible with respect to what it can hold.

  • 0
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    Gerry Lawson

    Bright Scripters,

    The audio doesn't need to be tightly synchronized to the video (the content is more conceptual) but it does need to be synched to the other audio channels as some content is panned across. Synch issues would cause echo effects. I certainly have considered using multiple brightsign units for this. It wouldn't be as efficient as playing from a single player (and file.) Multi-channel sound-scapes are going to be increasingly used in museums so I would like to find a decent solution while I have the time and budget.

    I have used the advanced synch in the past and the Brightwall is sooooo easy! I can't tell you how much O love the Brightwall authoring. Having said that I do want to learn more about programming brightsigns outside of the brightauthor app. Just not immediately.

  • 0
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    Gerry Lawson

    Brandon,

    That's a really interesting thought (playing different content through the different outputs.) I do have some pre-series 3 units around. How would I program that functionality?

    Your other comments are in line with what I've known or suspected. Your 4th note does give me some hope.

    The above said, it should pass over HDMI, but it's not something we've tried.

    What would you say is a good method for trying this pass-through function? It's been hard to know whether playback issues are related to video codec, the way the streams are muxed or if the unit just chokes on lpcm. I can get access to other encoding tools through my professional network (and can try muxing streams through ffmpeg.)There really aren't many encoders that seem to like putting lpcm into a video.) How would you go about testing this passthrough to feel confident that it doesn't work? (a successful test is easier to understand.)

    Thank you very much for your help (both of the responders.)

     

     

     

  • 0
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    Brandon

    Hi Gerry,

    The player has three audio decoders it can use.  Video/VideoOrImages zones take one.  Audio-only zones take one, and Enhanced Audio zones take two.

    So you could have one Video zone and two Audio-only zones.  In the Zone Properties, map the output for each one differently.

    For example,
    Video - HDMI
    Audio-only 1 - SPDIF
    Audio-only 2 - Analog

     

    In terms of testing pass-through, if you have a display that either blindly tries to play the audio input (like my Planar PXL-2430MW), a compressed audio stream that's being passed through will sound like digital garbage- a series of pops/clicks.
    Smarter devices may give an on-screen message.

    If you hear no audio at all, then either the receiving device is ignoring the stream and not telling you, or it's not getting the stream at all.
    Checking the system log in the player's DWS should provide a clue if that's the case.  For example, the probe information may not match the file's actual properties.

  • 0
    Avatar
    Gerry Lawson

    I'm going to try to repeat my tests with the encoded 5.1 lpcm audio in a variety of video streams. I'll see what monitors I have that provide audio information and check the DWS log.

     

    I'm also going to play with multiple audio streams from different Brightsign outputs. That might be a good way to serve serveral streams for  future projects.

    I'll post an update when I have one.

    Thanks for the information and advice.

  • 0
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    Bright Scripters

    A quick conversion with ffmpeg to AC3 moved things forward for us:

    ffmpeg -i OurLPCM.mov -vcodec copy -acodec ac3  lpcm2ac3.mov

     

    Here is some more information to make your head hurt :)

    https://lists.ffmpeg.org/pipermail/ffmpeg-user/2012-May/006750.html

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