Film-Tech Forum ARCHIVE: Voices in the surround channel

This is topic Voices in the surround channel in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Michael Brown (Member # 890) on 10-13-2001, 12:43 PM:

We just installed Dolby. Its either a Cp 45 or 55. We run in Dolby A and I noticed a lot of the vocals also come from the surround as well as the front channel. Is this normal or is something wrong. It seemed to be the worst during Dude where's mt car. I suspect that the levels of the surrounds have been set too high anyway.

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Michael Brown
Bradford Student Cinema
www.Bradfordstudentcinema.co.uk

Posted by Brad Miller (Member # 2) on 10-13-2001, 02:19 PM:

It's either the azimuth of the optics in your soundhead, the Dolby level in the processor is off, or you have a defective matrix card (although very rare).

Out of curiosity, when there is straight dialogue in the track and everything else is dead quiet, do the leds on the noise reduction card bounce up and down "equally", or is one always higher than the other? Bear in mind the dialogue may be too low for this to register. It may take a scene with someone yelling.

Posted by John Walsh (Member # 168) on 10-13-2001, 02:19 PM:

For a quick test, you'll need to run a Dolby tone test film and verify the two channels are of the same level.

Posted by Greg Pauley (Member # 503) on 10-13-2001, 03:05 PM:

Since this is a new install, you might want to check the delay setting on the surround delay. Although, this sounds like you need to have the tech come back and check your a-chain again.

Posted by Darryl Spicer (Member # 711) on 10-13-2001, 05:31 PM:

depending on how high the surround levels have been set, you can get crosstalk in the surrounds.....have a feeling that the focus on the lens slit is out of adjustment causing loss of frequencies that will cause the matrix not to perform correctly.....the led's on the matrix should be at a minimum when setting the dolby tones correctly....if you can not get a minimume reading on the matrix then there could be a problem at the soundhead and A-chain.....

Posted by Gordon McLeod (Member # 33) on 10-13-2001, 08:14 PM:

It sounds like the azimuth of teh sound head is incorrect. This causes a phase difference between the two channels and that is how the surround channel is derived.
On the cat 150 the centre led should be bright the others should be effectively out when a cat 69t is running. Only UltraStereo has a led null bargraph on the matrix
Poor focus will actually degrade the surround channel not create false decoding since it will effect both channels equally.

Posted by Paul G. Thompson (Member # 655) on 10-13-2001, 09:48 PM:

I fully agree with Gordon.

I think a complete front end optic alignment should be performed.

Paul

Posted by Randy Stankey (Member # 64) on 10-14-2001, 01:19 AM:

I second the motion as well. But since there were already three people ahead of me that means I'm fourth.... So, I FOURTH that motion.

Get a complete A-Chain done on that projector.

Posted by Ray Derrick (Member # 1001) on 10-16-2001, 09:23 AM:

I fifth the motion! However there is one other factor to consider.

Most features these days are recorded in Dolby SR not Dolby A. The dynamic action of these two systems are quite different and incompatible. Also the left and right NR cards operate independently (they are not stereo coupled). So if you chose the wrong format, you will often experience an increase in surround dialogue leakage whenever music or effects are present at the the same time as dialogue.

------------------
President
Panalogic Corporation Pty Limited
44 Carrington Road
Castle Hill NSW 2154
Australia
Phone: 61 (0)2 9894 6655
Fax: 61 (0)2 9894 6935

Posted by Paul G. Thompson (Member # 655) on 10-16-2001, 03:15 PM:

Very good point, Ray.

Posted by Steve Scott (Member # 630) on 10-17-2001, 12:07 AM:

I agree, the optics & a-chain need to be aligned. I've found that even on our CP500, when it runs coke trailers that were printed in SR in A-type, the dialoge gets flashed into the surrounds everyso often, it also seems to be a bit wavering between the front L & R channels.
I've read that while it is somewhat backwards compatible, SR alinged systems will only take advantage of the increased noise reduction & equalization if run with SR features. A-type just doesn't work well in these, as has been my experience.

Posted by Michael Brown (Member # 890) on 10-17-2001, 10:09 AM:

We do run in dolby A so it could be a SR/A incompatability problem. We are having the projector serviced on friday and they will look at the sound equiptment as well.

Posted by Mat Overton (Member # 1022) on 10-21-2001, 08:15 AM:

Can I just point out that the views represented above are those of our projectionist, and not those of the Bradford Student Cinema.

I am not aware of any problems with our sound system, and Mike has not been to see me regarding any issues with the surround channels.
I also know for a fact that the system was installed to the highest possible standard by our service engineer. The surround channels are operating at the correct level - which I saw for myself during the install. Also our service this week, was arranged specifically to sort out a problem with our projector, NOT our sound system. The optics are prefectly aligned, and an a-chain was completed which proved that the system is still running to the specified Dolby levels.
There is the correct delay between the front speakers and surrounds.
We are experiencing NO problems running SR films in Dolby A, and I have no doubts about the professional nature of our fitters.

Mat Overton,
Chairman
Bradford Student Cinema

Posted by Gordon McLeod (Member # 33) on 10-21-2001, 12:41 PM:

Well I would highly recomend that the Dolby Jiffy test be played through the system both the older "A" type version and the new SR version and verify that all is well. I doubt that a projectionist would post that they belive there is a possible problem unless they firmly believed there is one

Posted by Steve Guttag (Member # 268) on 10-21-2001, 12:48 PM:

Mat,

I was following along with your statements until you got to the part about having absolutely NO problems running Dolby SR films in A.

For starters...that is quite simply impossible. While both use a complementary compander system, they do NOT use the same filters nor dynamics. As such, your system has no choice but to mistrack both Lt and Rt channels in both level and frequency. Whereas the 2:4 decoder uses level and phase relationships to determine channel placement, it will also HAVE to mistrack from the orginal mix. Another problem is that SR uses what Dolby calls "spectural skewing" which to the layperson will mean, without genuine SR circuits, the signal will sound a bit bright without deep bass.

The effects of improper SR decoding cannot be overemphasised. Most, if not all, SR emulators don't track it well. Furthermore, your soundhead has to be in tip top shape for the SR circuits to not whack off the high-end. That is, with reverse scan soundheads, you should be fine, with forward scan, there will be some artificial HF attenuation, much worse so for 1 mil slits and taller.

If you don't have SR noise reduction, you should obtain it as soon as possible. If you do have it, you should use it.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Gordon McLeod (Member # 33) on 10-21-2001, 01:10 PM:

I agree with most of steves comments except that the emulators are far better than no SR. Also Dolby's own 222SR/A card is an emulation
The panastereo and the Ultrastereo SR is very good. And face it a lot of people would not complain about the copycat smart cards. In fact there 2 band system address's the problem of high end effectively in a brute force fashion. Bottom line some for of SR playback is manditory

Posted by Steve Guttag (Member # 268) on 10-21-2001, 01:37 PM:

Gord,

I never stated that some form of SR emulation isn't better than no SR. Heck, take to the absurd extreme...Dolby-A is an SR emulation! I personally have heard very few good SR emulations. Dolby's CP-650 hasn't thown up any red-flags on my listening tests and I was prepaired to be more skeptical on that one (digital emulation and all). I have not given it as much listening as I would like to make a definative statement though. The same goes for the Pana CM-353...I have heard good results but the total time I have listened to an optical track with it can be measured in minutes, not even hours so I will reserve judgement.

As to the others you have mentioned...while better than nothing, I guess, entirely not good enough in my opinion. To the point, I feel, the patron is being ripped off. This industry doesn't need low-end systems out there. Home theatres already beat them.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Gordon McLeod (Member # 33) on 10-21-2001, 03:33 PM:

I have a feeling that budget is a major concern in this location.

Posted by Mat Overton (Member # 1022) on 10-21-2001, 03:53 PM:

Very true. And SR is next on the upgrade list.... along with decent surrounds...... I think that half the issue here is that we don't have the best surround speakers in the world, but we have to make do with what we do have. Our venue also has poor acoustics, and there's not a lot we can do about that. We don't run in a purpose built theatre, it's not even our venue..... it belongs to the university. I wouldn't be surprised if the main issue was sound reflecting off the back wall of the auditorium.

I know that SR soundtracks will never sound brilliant in Dolby A, but what I meant was that they sound as good as they realistically can do!

What concerns me is that this posting has been brought to the attention of our installers by a reader of this forum, and suggests that they did not install the system properly. My posting here is to reassure readers that they carried out the Dolby install as professionally as possible, despite dealing with our equipment that doesn't quite live up to a professional standard.

At least we are making the effort to upgrade, I can only do it one step at a time.

Posted by Gordon McLeod (Member # 33) on 10-21-2001, 04:28 PM:

For our curiosity what is the processor involved and would it be possible for you to list the booths equipment

Posted by Steve Guttag (Member # 268) on 10-21-2001, 07:46 PM:

Mat,

Believe it or not, I got that impression from your post (doing as well as can be done). My only real point was that A can not do well with SR soundtracks...it, by design, has to fail in the decoding department which will cause 2:4 mistracking as well.

Gord, since the processor choices that started this thread were the CP-45 and CP-55...one has to presume it is the CP-55 since the CP-45 has Dolby SR out of the box (albiet the dreaded Cat. 222 SR/A).

Steve

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"Old projectionists never die, they just changeover!"

Posted by Neil Chue Hong (Member # 685) on 10-22-2001, 05:14 AM:

Hi Mat!

is your sound system the same one you had at the Sheffield Golden Anniversary? Or have you been upgrading since then?

As you say, your venue isn't brilliant for acoustics - from what I remember, it's great for stage presence, but manages to completely overegg the surround unless you're sitting exactly in the right place.

That was a great get together you organised though - hope to see you again at some point!

Posted by Mat Overton (Member # 1022) on 10-22-2001, 07:07 AM:

Yes it is a 55, running with a Westar projector and forward scan laser reader.

No Neil it's fortunatly not the same sound system! But I do thinkl you're confused, it's Bradford not sheffield, we didn't have surround when we held the BFFS student group conference. But anyway, hope to see you at the Warick conference!

Posted by Neil Chue Hong (Member # 685) on 10-22-2001, 07:14 AM:

Ack - you're right!

The melon on top o' me head is causing my brain to melt...
Is it a Westar 7000? If so, do you have any manuals for it?

Posted by Mat Overton (Member # 1022) on 10-22-2001, 07:18 AM:

Nope.... 2001

Posted by Pete Naples (Member # 761) on 10-22-2001, 01:10 PM:

Gordon,

There is a kit list on the Bradford Uni Film Soc's site, the link is in Michael's original post.

I deal with a lot of University Film Societies, and budget is always a major constraint. Also problems with not having a purpose built venue, which leads to poor sight lines, acoustics, University Buildings Dept not allowing surrounds to be fixed to the wall, etc, etc. The film socs have a pretty tough time of it!

Posted by Pete Naples (Member # 761) on 10-22-2001, 03:25 PM:

Regarding the comment made about HF response on forward scan soundheads:

The Audio Cine Laser mentioned earlier has a beneficial effect on the HF response of most sound heads, this is due to the simple fact that the slit of laser light emitted by the device is actually narrower than most slit masks, resulting in better HF response, for obvious reason. I've also observed that L-R cross talk is dramatically reduced, usually to the point of being un-measureable using L-R test film and my oscilloscope!

Posted by Gordon McLeod (Member # 33) on 10-22-2001, 06:53 PM:

Steve the original post said it was either a cp45 or a cp55 so I was a bit confused which since the 45 has sr (sort of)
On the cine laser don't use it with a narrow slit as I have heard that it can be microphonic then

Posted by Pete Naples (Member # 761) on 10-24-2001, 02:50 AM:

Gordon you are mis-informed.

I have installed countless ACL's now, with a vast array of slit lenses and rep-sets, and only once encountered microphonics. This was on a Westrex 7000 machine, where the rep-set should be mounted on rubber bushes, only someone genius had seen fit to remove said rubber bushes, leaving the rep-set screwed securely to the body of the machine. I simply replaced the rubber bushes and cured the problem.

Posted by Gordon McLeod (Member # 33) on 10-24-2001, 07:42 AM:

I know that one was tested at Dolby in SanFransisco and they note microphonics when used with slits narrower than .5mil

Posted by Pete Naples (Member # 761) on 10-24-2001, 08:05 AM:

Interesting! We've never heard of or encountered this, even on narrow slits.

That said, we are doing things differently on some machines. We are physically clamping the laser mount to the back of the optic tube, thus avoiding any chance of the laser moving relative to the optic, which can be a problem when fitting the laser in place of the exciter lamp.