Film-Tech Forum ARCHIVE: Surround Delay Time

This is topic Surround Delay Time in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Paul Trimboli (Member # 1509) on 01-21-2003, 08:09 AM:

Would it be acceptable to use a Pro Logic reciver from a home cinema to use in a cinema? What kind of extra delay would need to be added? Would it be Dolby SR? It is actually built up from a kit, was the only kit ever to be Dolby Licensed and uses real Dolby Chip. It has up to 25ms Delay, can I plug the surround component into a digital delay of another 25ms which can be adjusted to give me differnt delay times? Do "real" cinema processors have anything special the would make them better? Noice Reduction?

Posted by Ron Lacheur (Member # 1126) on 01-21-2003, 08:41 AM:

ProLogic is not SR. This has been done before, as I recall a few threads in the past that people were using Receivers to power the sound in their home setups.

In 25ms delay, is this just for the surrounds or all channels?

Posted by Michael Schaffer (Member # 1204) on 01-21-2003, 12:21 PM:

25ms+25ms=50ms. How big is your theater? For a smaller room, 25ms should be enough. Refer to the Dolby CP manuals for the formula. However, as mentioned, home ProLogic lacks SR noise reduction! There are external SR modules, but these would raise the cost of the installation. Better see if you can find a used Dolby processor (e.g. CP45/55).

Posted by Gordon McLeod (Member # 33) on 01-21-2003, 03:36 PM:

By the time you aquire all the gear to give you room equalization, slit loss compensation, the prologic the noise reduction you could buy a good used profesional processor

Posted by Paul Trimboli (Member # 1509) on 01-23-2003, 12:44 AM:

Thanks for your reply, I will look around for an older professional unit to use in a set up!

Posted by Paul G. Thompson (Member # 655) on 01-23-2003, 01:04 AM:

I thought the surround delay was based on 1ms per foot, since sound travels at 1100 feet per second. But what part of the auditorium it is referenced to seems to be up to the engineer, and how close to the screen the first set of surround speakers are placed. I have seen some auditoriums where the first set of surround speakers are about 15 feet from the screen, and the last set are on the rear walls! To add insult to injury, split delays are not employed. Then they set the basic delay to 60 ms for a 60-foot auditorium.

I have usually set mine up referencing the middle of the surround field, and adjust as necessary to compromise the annoying echo that one might experience in sloppy and "loose" surround decoders in some of the older processors. An overall "walk-around" in the auditorium generally yielded very accurate delay settings providing the first set of speakers were placed mid point or up to 2/3 of the way back from the screen.

I have been known to disable surround speakers that are too close to the screen if split delays were not employed.

How do you guys set it up?

Posted by Christos Mitsakis (Member # 143) on 01-23-2003, 09:43 AM:

Some older pro-logic preamps have longer time delay, as far as 100ms, under "movie surround" mode - or so. Standard Dolby pro-logic goes as far as 30ms. This is to give, erroneusly, the sense of large reverberant auditorium during home play back.
BUT.. [thumbsup]

I have used my old Maranz AV500 in an open air cinema with great sucess. Auditorium length = almost 30m, time delay set to 70ms, Soundcraft spirit folio mixer for slit loss compensation and cell preamp. ...Where cost matters. Cinema owner got exellent comments from the patrons.

Christos.

Posted by Michael Schaffer (Member # 1204) on 01-26-2003, 11:51 PM:

I have been to Greece many times (my ex-girlfriend is from Naxos), and I always liked the open air thing very much. Sure, it`s not the optimum way to present movies, but I found it very nice and relaxing. A favourite open air venue of mine was right in the middle of Athens, in Kolonaki. BTW, Athens is one of my favourite cities even though it`s so stressy and dirty - but it`s really alive!

Posted by Mark Gulbrandsen (Member # 72) on 01-27-2003, 08:43 AM:

By the time you aquire all the gear to give you room equalization, slit loss compensation, the prologic the noise reduction you could buy a good used profesional processor
_______________________________________________________________
Hey, don't tell Keith that...you'll ruin his day...... [Wink]

Mark

Posted by Christos Mitsakis (Member # 143) on 01-27-2003, 08:55 AM:

Michael, next time you're here, make shure to be in touch.
Many open air theatres, especially in summer retreats are operating in an almost two months period. So for most of them it IS to much to invest in a full dolby sound system (even analog - Mark I agree with you but second hand market for older processors here is almost non existable). The movie rental prices of the distributors every year go up, and the audience is going "down". Consider, except in large cities, that all summer theatres are second run.
Most of them are still runing in mono, or plain 2 channel stereo, using DJ's mixers [puke]

. In the best cases you can find home processors (I've seen Lexicon CP2 and CP3)with mixed results of sound quality.
Where I am involved I use a decent two to four channel mixer (like the soundcraft) always a preamp-processor (it gives me much flexibility) and if budget is very low, custom made, or used in good condition PA speakers. The results are excelent for the specific time and place. The extra high freq boost of the unprocessed SR is an asset. Sound becomes clear in VERY low volume settings. In most cases you have to turn down the volume in night hours so no complaints are made from the neibouring houses. I hate this but it is an unavoidable reality for open air theatres.

Christos.

Posted by Ray Derrick (Member # 1001) on 01-29-2003, 03:32 AM:

It is not commonly realised that delay is used on the surround channel for TWO reasons. Firstly to 'synchronise' the screen and surround channels. This is because the surround speakers are much closer to the audience than the screen speakers, so audients will hear them much sooner. So the first objective is to delay the surround channel so that in the worst positions the room, (the rear corners) the surround sound does not preceed the screen sound. This technique is also commonly used in public address systems where additional speaker clusters may be placed say, half way down the auditorium. Without delaying these "rear" speakers, all the sound seems to come from these rear speakers rather than from the stage. It can be really annoying.

Secondly, for optical soundtracks, an extra 10 to 20 ms is added to that which is required for synchronisation, to make any front channel leakage into the surrounds less audible. (Haas Effect).

So in setting up the delay you should take at look at the rear corners of the theatre and determine which seats will have the biggest time difference between screen and surround and calculate the synchronisation delay for those seats, then add 10-20mS for optical formats. For digital, the extra 10-20mS is not necessary because of the high separation between channels.

Of course large auditoriums pose a special problem in that audients sitting towards the front may hear an echo from the surrounds. The best solution is to delay the front surrounds less than the rear, but since cinema processors don't provide multiple delays this must be achieved using external delay units. If that is outside the budget then you just have to reduce the delay and make the people up in the back corners really suffer!

Well after all, if you're prepared to pay out good money and then sit in the back corner of a cinema, you deserve to suffer! [Smile]

Posted by Steve Guttag (Member # 268) on 01-29-2003, 09:10 AM:

Once again I'll pull out my handy RANE RPM-26(V, I or Z)...you get two channels in and 6 channels out...all with their own delay and EQ.

Another key to doing deep theatres or multiple surround zones...is to balance it out right. That is, when you adjust the EQ, and level of each zone...you will need to move your microphone(s) into each zone...remeasure the loudness of the stage channels and adjust accordingly. Thus if you placed your mics in the "prime" seating area to set the 85dBc level...then move them back into the "underbalcony" or balcony areas (less prime), remeasure your stage channels...they will be lower as you move back in the auditorium. Re normalize your rear surround zones based on how loud the stage channels are...this way everything is well balanced and rear echo is eliminated.

Steve

Posted by Jim Spohn (Member # 1547) on 01-29-2003, 10:27 AM:

Paul...I use a Pioneer Audio/Video 5.1 Ch 100 Watt per channel amplifier in my auditorium when using a digital projector down front. The front three speakers are Altec Voice of the Theatre A-4s and the surrounds are Warfdales. You can adjust the distance the speakers are away from you in feet with the remote control (I know that it's really milli-seconds of delay)The maximum setting is 30 feet. By adjusting the front speakers 30 feet away and the surround 30 feet away with a little trial and error you will find that the setup works VERY well. My auditorium is 110 feet long and 50 feet wide and the surround speakers are 60 feet from the screen. Granted that the back rows don't get good surround, but then again I've never had 650 people in the auditorium for a film. I have also feed audio from the booth to that setup and it sounds fine.

It's nice to sit in row J and fine-tune the gain individualy for each of the 5 channels and also the surround distance. Since all the house lights, stage lights and curtains are operated by a hand-held remote control, I can run an entire digital show while sitting in the audiance....Jim