Film-Tech Forum ARCHIVE: Dolby cat745 and HDMI audio

This is topic Dolby cat745 and HDMI audio in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Leo Enticknap (Member # 534) on 11-05-2014, 06:52 PM:

We've got an event coming up at which someone potentially wants to connect a computer via HDMI to our NEC projector, and take 5.1 audio from the same HDMI into the theater's sound system.

Our NEC has a Dolby cat745 IMB in it. Question: does anyone know whether, if I connect their computer to the HDMI input of the cat745 and select that on Show Manager (we have 4.8.1.19 on that server, which can select the HDMI input on the 745), will it pass the audio from the HDMI input through to the CP650 in the same way that it would the audio from a DCP? Or will it purely display the picture and ignore the audio?

Many thanks in advance.

Posted by Mattias Mattsson (Member # 4308) on 11-05-2014, 07:56 PM:

Yes it will pass the audio through the Audio Out cables to the sound processor.

Although, it will only handle 2-channel PCM, DD or DD+ (no DTS formats), if I remember correctly.

Posted by Steve Guttag (Member # 268) on 11-05-2014, 09:56 PM:

Incorrect. It handles multichannel just fine. The key is...it does NOT decode...it will handle just PCM audio so it will play whatever you have on the wire...if you have 5.1 audio...it will have 5.1 audio on it.

Posted by Leo Enticknap (Member # 534) on 11-05-2014, 10:12 PM:

So just as long as the HDMI audio is LPCM, not Dolby or DTS/Datasat, it should work. Many thanks, folks.

Posted by Carsten Kurz (Member # 5396) on 11-05-2014, 10:15 PM:

Yep. If it's e.g. a DVD or BluRay with an AC3 stream - you will hear nothing. If the file is linear PCM 5.1, or the player software is able to convert into that format, it will come out multichannel. Windows has options to configure various channel assignments for the HDMI audio device. Per default, it is 2ch, but can be set to up to 8ch.

- Carsten

Posted by Sam D. Chavez (Member # 1841) on 11-05-2014, 10:58 PM:

I would advise avoiding passing audio thru the HDMI. In fact, if possible use a HDMI to DVI cable and run toslink or coax direct to the processor for audio. Just went thru a bunch of trouble with pops and clicks and lines in the video using Blu Ray if everything is not just right.

Better safe than sorry.

Posted by Mattias Mattsson (Member # 4308) on 11-06-2014, 06:02 AM:

I stand corrected then.

Does anyone know what the "Specify capabilities of your sound processor" in the Dolby config does then? There you can choose if your sound processor supports Dolby Digital and/or Dolby Digital Plus. Does it do a mix-down if you unselect any of these formats and they are encountered in the HDMI input?

Posted by Carsten Kurz (Member # 5396) on 11-06-2014, 06:09 AM:

I don't think so, I think it only enables control cues to use these format options in SPLs.

Most player devices should request the Dolby IMB HDMI audio capabilities, some would revert to LPCM multichannel output. Most current BluRay Players e.g. would do that, and notebooks as well. But only if the 745' current software is able to answer such a request properly.

- Carsten

Posted by Leo Enticknap (Member # 534) on 11-06-2014, 12:01 PM:

Thanks. This is for the launch event for a video game, and so I'm guessing that audio compression/proprietary formats won't be an issue. This will be the easiest and most straightforward way of doing it if it all works - we have a backup plan that is slightly less simple and straightforward if it doesn't.

Posted by Marcel Birgelen (Member # 6801) on 11-06-2014, 12:59 PM:

What platform are they using? Windows? If so, make sure the default audio device is the HDMI output.

If they're demoing a game, then the sound channels are most likely generated on the machine themselves. So, the output will be Multi-LPCM, which should be fine. If they also want to play video which contains e.g. AC3, DTS, etc, make sure the audio gets decoded to LPCM on the machine and not passed trough to the IMB to avoid unnecessary headaches.

Make sure you choose the correct layout (5.1, 7.1, etc.), you best also disable all kinds of enhancements, you don't want your PC to do the equalization for you. Make sure you choose a bit-/ sample rate that's compatible with your decoder/ audio processor, if possible, keep it at 24-bit / 48KHz, as that matches most DCPs.

If they're using DirectX for their game, it should usually follow the Windows setting, but many games allow in-game adjustments. I guess this is something they will be able to figure out themselves.

If this all fails, go get a multi-channel sound card (if not already present) and connect those to the analog multi-channel input on your audio processor. Hopefully you have a suitable cable lying around [Wink]

quote: Sam D. Chavez
I would advise avoiding passing audio thru the HDMI. In fact, if possible use a HDMI to DVI cable and run toslink or coax direct to the processor for audio. Just went thru a bunch of trouble with pops and clicks and lines in the video using Blu Ray if everything is not just right.

S/PDIF will give you two PCM channels at most. Some sound-cards are capable of outputting AC3 on-the fly, but their performance is often a bit flaky. Uncompressed multi-channel formats like Dolby TrueHD, etc. will definitely not work via a single toslink or coax S/PDIF.

Posted by Leo Enticknap (Member # 534) on 11-19-2014, 11:15 AM:

Worked like a charm. It was a little bit of a faff to get their Mac to output audio through HDMI, but otherwise all good. Thanks everyone.

Posted by Carsten Kurz (Member # 5396) on 11-19-2014, 12:06 PM:

Mac OS X has an excellent and very transparent audio config utility to assign all imaginable channel configurations to the various audio I/Os. The trouble is that most people have never been there before and need some pinpointing.

- Carsten

Posted by Leo Enticknap (Member # 534) on 11-19-2014, 03:25 PM:

The owner of this particular Mac was a game developer, and thus knew what she was doing to the point of knowing roughly where to look in the control panel.