Film-Tech Forum ARCHIVE: 5.1 Surrounds in 7.1 Set-up

This is topic 5.1 Surrounds in 7.1 Set-up in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Robert John Jeromson (Member # 2559) on 07-20-2019, 02:45 PM:

We have an new install configured for 7.1 set-up using a Dolby CP750. We noticed last week that in 5.1 there didn't appear to be any surround effects coming from the rear speakers. I understand that in 7.1 the rear channels split out as separate channels but in 5.1 should we be getting left/right surround audio from the rear speaker? I hope this makes sense.

Posted by Marco Giustini (Member # 4544) on 07-20-2019, 02:52 PM:

Hi Robert,

On the 750 you need to manually enable 5.1 or 7.1 depending on the content you are playing. This can be done from the front panel.
You can also automate this via server cue, either via GPI or via ethernet cue.

If you keep your sound processor in 7.1 all the time, you will experience what you reported when playing 5.1.

I hope this helps.

Posted by Robert John Jeromson (Member # 2559) on 07-20-2019, 03:12 PM:

Thanks Marco, so just so I understand if we set the processor for 5.1 all the time and the use a server cue to switch to 7.1 when necessary we will get surround information from the rear channels for 5.1 films?

Posted by Steve Guttag (Member # 268) on 07-20-2019, 03:31 PM:

It isn't a matter of "leaving the processor" in a format. If you are cuing the CP750 from the server, you have to cue it to EVERY format. That is, if you switch it to 7.1 mode from the server, it will remain there indefinitely until you cue it back to 5.1 (or some other decode mode). Note, the CP750 doesn't care whether you manually change it via the front panel or via an Ethernet (or RS232) command...they are all equivalent.

cp750.sys.pcm_2_channel_decode_mode_1 lr_discrete

Is 5.1 mode.

cp750.sys.pcm_2_channel_decode_mode_1 4_discrete_sur

Is 7.1 mode.

Posted by Robert John Jeromson (Member # 2559) on 07-20-2019, 03:36 PM:

Thanks Steve, that makes complete sense, appreciate your help!

Posted by Marco Giustini (Member # 4544) on 07-20-2019, 03:54 PM:

What Steve said!

Just out of curiosity: didn’t your installer explain you this critical step? Just curious as I have seen many sites running everything in 7.1. (As back in the days I’ve seen sites running everything in EX, but at least with that you wouldn’t miss a channel!)