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Author
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Topic: DCI Server audio output question
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Dave Macaulay
Film God
Posts: 2321
From: Toronto, Canada
Registered: Apr 2001
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posted 02-16-2017 09:18 AM
Digital cinema audio is output as 8 AES digital stereo signals for 16 channels total. The signal specs are close enough that a D-A designed for S/PDIF will usually work but an adapter circuit is advisable if you need to go that way.
Output channels are assigned under INTEROP standards but there's some oddity in channels past the six required for 5.1 audio:
AES| Audio
Pair| Channel
1 ... 1= Left, 2= Right
2 ... 3= Centre, 4= Subwoofer
3 ... 5= L surr, 6= R surr
4 ... 7= HI, 8= VI (not sure this is actually standardized but it's consistent in North America at least)
Then it gets a bit weird. Although not specified in the standard, 7.1 surround channels BSL and BSR are on signal 6, channels 11 & 12... but these can also be screen channels LE and RE. Channels 9-16 also need to carry DBox and other sync tracks, plus the HI & VI tracks are usually repeated.
For 7.1 you use 11 and 12 for BSL and BSR, but might get LE and RE sound if you use the wrong CPL or VF.
The SMPTE DCP package format allows the audio channel vs track assignments to be specified in metadata. How that plays out remains to be seen, systems will need to accommodate INTEROP packages for some time.
Note that some servers have a way to reassign output channels, usually used for 7.1 to move 11/12 to 7/8 for cinema processors with and 8 channel digital input. I've found that this reassignment may be ignored when a SMPTE CPL is played - it's safer to do signal swaps in cabling to avoid HI and VI unexpectedly playing out of the rear surrounds.
Server output connectors for the AES signals varied. Originally most were on a DB25 without a standard pinout, custom cables were usually needed to connect to the processor or D-A converter. When IMB systems came out to allow 4K and HFR playback, they all (as far as I know) put the audio on two RJ45 connectors using a couple of CAT5/6 cables to the audio system, only one is required for 5.1 systems. The RJ45 pinout is consistent in my experience. Using shielded network cable and connectors is advised, the IMB/IMS/ICMP RJ45 sockets I've seen are shielded cable compatible. Even for servers with DB25 outputs, it's much easier to run CAT5/6 to the audio rack for digital audio than to use a long bulky multichannel AES cable with large DB25 connectors. Odyssey makes DB25-2xRJ45 adapter cards that make this simple.
Some servers did have analog audio outputs, I think all were on a DB25, again various pinouts were used.
Even now, connector issues are common. The cinema audio world seems to have settled on two DB25 analog multichannel pinout standards, commonly called "THX" and TASCAM". These are not compatible... so make sure you have the correct cables or adapters before heading out on a remote installation! (Doremi has their own non-standard analog pinout, used on DCP2000 servers with an analog output (rare) and on the AUD-2A 6 channel basic D-A unit - both also use a non-standard 8xAES DB25 pinout)
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 02-17-2017 07:08 AM
quote: Dave Macaulay
4 ... 7= HI, 8= VI (not sure this is actually standardized but it's consistent in North America at least)
Then it gets a bit weird. Although not specified in the standard, 7.1 surround channels BSL and BSR are on signal 6, channels 11 & 12... but these can also be screen channels LE and RE. Channels 9-16 also need to carry DBox and other sync tracks, plus the HI & VI tracks are usually repeated.
For 7.1 you use 11 and 12 for BSL and BSR, but might get LE and RE sound if you use the wrong CPL or VF.
Dave, as originally conceived, 7/8 were LC/RC (Section 3.3.3 of DCI spec). HI/VI were supposed to be on channels 15/16. Some early players only handled 8-channels (and most, if not all were only wired for 8-channels) so when HI/VI were gaining speed, 7/8 became the HI/VI tracks (how many DCPs have LC/RC?) and LC/RC moved to 9/10 as their "official" home. 11/12 are now the back surround channels. I have never heard of LC/RC being on channels 11/12. Dbox seems to have camped out on channel 13. DCI has punted the channel assignment from their spec (as they have to many other sections) as SMPTE standardizes things. So the DCI spec merely refers to SMPTE 428-3 for channel assignments. That said, I believe 428-3 was withdrawn because other documents covered channel assignments. 428-3 was an early document back in the early days of DCinema.
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