Film-Tech Forum ARCHIVE: weird channel setup

This is topic weird channel setup in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=16;t=003876

Posted by Armand Daiguillon (Member # 10334) on 12-10-2019, 03:42 AM:

Hi all,

I'm trying to make my sight and sound better and am focusing on the sound right now - but I ran into something weird.
I have 4 amps yet for some reason they are only using 6 of the 8 channels available. And when i look at my sound processor I see that there is signal output on the LeftRear and RightRear channels. So I wired the unused LeftRear and RightRear outputs on the processor to the 2 unused channels on the amps. I did not get what I expected.
I played a gdc 7.1 pink noise test dcp and got nothing on either channel, nor did the I see anything being output from the processor's indicator on those channels, which I thought was odd because I usually do see output from the processor (it has little bars that go up graphically representing each channel) So I thought, maybe the gdc test dcp was bad (not likely) so I played the beginning of Frozen 2 and I got odd things ...out of Leftrear I got a weird "echo" of center channel and on the Rightrear I got a woman's voice describing what was happening on screen.
Now before I make a fool of myself and call my local servicer (which I try to avoid since we need to save every penny right now) Is there any logical reason why it was set up this way ? I know that the mapping of L/R/S/LS/LS/LF, etc can be done on the server (not something I can do) but is there any reason why it would have been mapped that way ?

Any ideas are appreciated.

Thanks !!!

Posted by Mike Babb (Member # 1379) on 12-10-2019, 05:41 AM:

You are hearing HI (ch7) and VI (ch8). Back surrounds are on channels 11 and 12 by default.

Posted by Marcel Birgelen (Member # 6801) on 12-10-2019, 08:53 AM:

Like Mike already indicated, your channel mapping is wrong and you're hearing the Hearing Impaired and Visually Impaired tracks playing through your speakers.

Can you describe your setup in some more detail? What projector/IMS/Server and audio processor are you using?

Since you seem to have working back surrounds connected to the amplifiers already, it would be a shame not to use them and to correct this setup so you can play proper 7.1 soundtracks. Meanwhile, you should play the 5.1 version instead.

Posted by Gordon McLeod (Member # 33) on 12-10-2019, 09:22 AM:

What make of processor is involved?
The Lb and Rb on the JSD 80 are not what they seem out of the box

Posted by Harold Hallikainen (Member # 5405) on 12-10-2019, 10:23 AM:

Agreeing with others, it would be good to know what sound processor you are using. A lot of processors (JSD-80, JSD-60, JSD-100, CP750) were originally 8 channel processors. The track file arrangement in a DCP is normally as defined at https://isdcf.com/papers/ISDCF-Doc4-Audio-channel-recommendations.pdf which puts HI/VI on channels 7 and 8. If the server does one to one channel mapping putting track 8 on AES channel 8, and the processor has an 8 channel AES/EBU input, it gets track 8 (VI) on channel 8. This is fine for a 5.1 auditorium. In a 7.1 auditorium, the last 2 channels of the processor input need to get tracks 11 and 12. This can be done with routing in the server or by swapping pairs 4 and 6 on the server AES/EBU output. But that leaves HI/VI (channels 7/8) with nowhere to go. Processors can typically generate HI from an LCR mix, but cannot generate VI (description of what's going on on the screen).

The JSD-100 can have a DI-80 board installed to run 7.1 with VI.

The JSD-60 requires an external D/A like the DAX-202 to run 7.1 with VI.

The CP750 has a firmware update that allows the HI/VI AES/EBU pair to drive the digital 2 input with the resulting analog coming out the CH7 and CH8 connectors. A transformer or other means to convert the balanced AES/EBU to drive the digital 2 input is required.

This is all summarized at http://ftp.uslinc.com/MultiProduct/Accessibility/Cinema%20Sound%20Processor%20to%20IRC%20Interface%20161025.pdf .

Harold

Posted by Armand Daiguillon (Member # 10334) on 12-10-2019, 06:43 PM:

The media server is a gdc sx 3000 and the sound processor is a usl jsd 60.
With that there should be no problem mapping properly to 7.1?

Posted by Dave Macaulay (Member # 813) on 12-10-2019, 07:21 PM:

You can change the audio channel mapping in the SX3000 - check the user manual, it's too much for me to explain. You want to map ch11 to ch08 and ch12 to ch09. If you want HI and VI, you can map 7 & 8 to 11 &1 2 and use an external D-A but you will need to split out the AES signal for chs 11 & 12. This is fairly easy, run A TOP to your sound processor, and for A BOT use a CAT5/6 cable to your D/A and just connect pair 2 to the D-A input.
Easier is to use an Odyssey card (sorry can't give you a part number, it depends on the DB25 pinout) at the JSD-60 input - this converts the DB25F connector to two RJ-45 jacks: connect two CAT5/6 cables (shielded preferred) between the A TOP and 1-8 card jack, A BOT to 9-16. The Odyssey card routes server output chs 11 & 12 to inputs 7 & 8 so you hear 7.1, and puts the HI/VI 7/8 AES signal on a terminal strip to connect to an external D-A if you need HI and VI.
The server method is good if you don't need those channels and the card is better if you do, in my opinion.

Posted by Gordon McLeod (Member # 33) on 12-10-2019, 09:53 PM:

On the JSD 80 you must set the formatssetup with 5 screen channels then map the LB and Rb to Le and RE if left if not Lb and Rb are a matrixed output from LS and RS

Posted by Armand Daiguillon (Member # 10334) on 12-22-2019, 01:08 AM:

So, we got everything working - nice 7.1 sound in all the auditoriums now. (now just need to find a nice trailer to showcase the sound)
Unfortunately we did find 3 speakers that need replacing in the process.

One seemingly odd thing I noticed though, while rewiring the surrounds (took 2 and re positioned to become the new Leftrear and RightRear) most of the surrounds were wired in series rather than parallel. (a few were wired parallel, but most were series.)
Now, the people who wired this originally know much more than I do ... but I can't imagine why wiring them in series would be a good idea. And if series is better ... why not be consistent with it ?

Anybody know why they were wired that way ? Should they have been ? Should I not have put them all to parallel.....

Thanks !!

Posted by Marcel Birgelen (Member # 6801) on 12-22-2019, 01:22 AM:

It depends on the impedance of both your speakers and what kind of loads your amplifiers are designed for.

It's not per definition wrong to wire speakers in series, as long as all speakers are the same. You multiply the impedance this way. The biggest disadvantage is: When one speaker fails, the others will fail with it and it becomes harder to troubleshoot which speaker is causing the problems.

Personally, I prefer to wire all speakers in parallel, with dedicated wiring to the location where the amps are, simply because that gives the most flexibility and usually also the best end-result.

Posted by Dave Macaulay (Member # 813) on 12-22-2019, 09:14 AM:

Modern cinema amps are generally OK with 2 Ohm impedance loads but that's not true of older ones where 4 Ohms is the usual minimum for single channel and 8 Ohms for bridged.
With multiple surrounds and one or two amp channels per side you have to keep the load at the amp above that minimum impedance. Series/parallel connection was used to achieve that. It brings problems though. An odd speaker count either needs a dummy load resistor wasting amp power or a scheme that does not power each speaker the same. A failed speaker can mean several go out or are much louder or quieter than expected.
Current practice should avoid series/parallel connections where parallel strings are connected in series. Putting several series pairs in parallel is ok if necessary as a failed speaker will only affect one other.
But amps are cheap and digital audio can now put a lot of signal into the surrounds - 35 analog surround sound had rather dubious separation so surrounds were basically a bit of ambient sound or loud only when screen channels were loud as well.
Best is to use parallel only with enough channels to not go below the minimum rated impedance.

Posted by Armand Daiguillon (Member # 10334) on 12-22-2019, 04:57 PM:

Thanks all !!

Posted by Stephan Shelley (Member # 8587) on 12-26-2019, 01:24 PM:

Are you setup for descriptive audio and closed captions as well as some form of assistive listening? It is federal law to have it with digital cinema these days. Beware of ADA suits. Having the JSD60 the way it was you can put the DA on the aux output. With the JSD60 setup for 7.1 you have to have some sort of external 2 channel D to A converter. There are several on the market and AES channel 7/8 will have to be broken out.