What's the most sensible way to set up sound in a 35mm drive in theater?

[Justin Roman]

I'm a sound tech that works mostly with live sound. Another tech and I have become friendly with the operator of a 35mm drive-in theater that uses an FM transmitter. They've been having some sound issues and we volunteered to come take a look and see what we could do.

Their system is two Simplex projectors with what I think is a Christie changeover box between the two. The film audio from there splits and goes to two pieces of gear: a DTS XD10P and an Eprad Starscope. My understanding is that these have preamps to pull the proejctor output to line level, and then perform surround sound processing on the audio signal. I'm somewhat confused by this arrangement, though, since the signal gets turned back into stereo when it goes out over FM. Right now the surround outs from the DTS go into 6 different channels on a Yamaha mixer, which then are routed to a switch just before the FM transmitter. Meanwhile, another copy of the audio signal is sent to the Starscope, and the output from that is sent to the switch, and then again on the the transmitter. The switch allows you to select either the Eprad of the DTS processor.

Can anyone explain to me what the surround decoders are doing, besides the pre-amp function? I believe the projectors have both magnetic and optical heads, but the XD10P doesn't seem to have an accompanying CD player for surround playback. I would think that all you would need is a pre-amp on the stereo feed from the projectors, and then stereo line out from the mixer (which has a few other inputs, like an announcement mic and music playback) to the transmitter. There's also a lot of unbalanced signals in the system which contradicts live sound best practice but maybe that works for theater sound. I don't know. Keeping things simple seems the best way to avoid any potential issues. I intend to also email the projectionist who configured the system the way it is. I just don't want to come across as combative since I'm the new guy on the block.

We managed to clean up some other issues and the projection team seemed happy with how it was sounding, so I also don't want to "fix what ain't broke". I guess I'd just like to hear some thoughts on how film based drive ins typically handle sound.

[Josh Jones]

As far as film audio is concerned, The right way to do it is to capture the Lt/Rt (Left total, Right total) after the noise reduction process, but before matrix decoding. The Starscope was a competing processor to Dolby which used a clone of Dolby A type noise reduction. Most films after 1995 or so used SR noise reduction, but was more or less backwards compatible with A type NR. The Starscope preamps should also handle projector changeover for you, provided it is wired correctly and both preamps work.

You could then downmix the output of the XD10 to stereo, and feed your mixer with that for voiceovers and background music. It would also be a good idea to provide some level of processing for the FM. Many of the newer transmitters have some sort of dynamics processing and stereo generation onboard.

I agree, unbalanced signals in a power related area are generally a recipe for hums, buzzes, and noise, but all the old film gear was unbalanced all the way up to the amplifiers. It was only with the last generations of processor that balanced audio became the norm.

Josh

[Justin Roman]

Thanks for the fast reply! Is there a particular processor besides these two that you would recommend for this use case?

[Josh Jones]

Everyone has their favorites, but for that application I'd say stick with what you have.

Josh

[Ryan Gallagher]

Firstly, how are they describing the sound "issues" that they are having? There are lots of potential places to look depending on the symptoms. Are the issues present when switched to either processor, or restricted to just one of them? Or are they more on the FM transmission side of the chain?

Do they exclusively provide Stereo FM to the audience, or is there anywhere in the park people can sit like on bleachers/picnic tables... IE are they also providing venue sound to certain areas, concession stand, etc, Possibly even 5.1 sound to some areas when the print contains Dolby SR?

The XD10p output was balanced. Starscope appears to have been unbalanced output? (though I if you have a paper manual there that would be a great thing to scan so Brad can host it in the warehouse), I could not find one online.

We would need a bit more concrete info on their film sound intended capabilities, the rest is mostly speculating trying to fill in the blanks....


What they have might seem a bit redundant and weird on the surface from an outsider. But I'm guessing the reason for the two processors and switching was that perhaps they in fact had a "Super Starscope"? Or one of the generations that could handle magnetic sound formats? The XD10P would have been a later addition/upgrade to handle dolby SR optical decoding (if desired), but it did not provide mag pre-amps, so it was wired in parallel as switchable. If this conclusion is true, they are almost always using the XD10P, except on mag print bookings, which are extremely rare these days. It's worth asking them if they EVER intend to screen mag prints again? Their mag reader heads, if of an earlier generation, may also be worn and suspect.

Your question about the CDs for surrounds. That XD10"P" was not a XD10 DTS processor (for films that shipped the CDs for DTS digital playback), just a competitor for dolby's line of optical analog processors. From your descriptions it doesn't sound like their booth was ever set up for DolbyDigital or DTS playback. Although if they own the readers that is cause to consider seeking more appropriate processors too.

The Starscope is a rare bird (and not highly spoken of). Here are a couple prior threads:
https://www.film-tech.com/ubb/f1/t005311.html
https://www.film-tech.com/ubb/ultima...c;f=1;t=005837

The most common optical film processors to see in booths now days tend to be relegated to the dolby line, or the USLInc stuff (now owned by QSC/QSYS). I wouldn't consider the X10P a bad one necessarily, it was just not as widely installed, unlike their other gear which you see everywhere that added DTS playback. But booths that can still do Mag sound are gonna have a interesting alternative set of gear lying around, and this sounds like one of those booths perhaps.

Here is a little gem from Brad's youtube archive:

[Dave Macaulay]

First, impressive that any cinema business is using film projectors. Where do they get prints? But anyway:
Not sure why you have a Starscope and XD10P. The XD10P is an analog cinema processor, so is the Starscope but more primitive. Unless it's for redundancy it doesn't make sense, and the projector's analog sound track reader output isn't normally connected to two devices. I won't assume whoever put it together did anything wrong but it is unusual. Old "solar cell" analog readers can't be connected to two inputs but later LED light modular ones with preamps could be.
Basically all 35mm sound tracks since the 70s have Dolby SR noise reduction, and your DTS at least has an SR button so it will handle that. I've never used one so I don't know how good their SR decoder is. Definitely some non-Dolby processors had really poor SR decoders.
The "accompanying" CD player would be a separate DTS unit that does play a special movie CD and uses separate film readers to get the timecode to sync up the soundtrack on the CD. However: the CDs are pretty much not available (yes there are sources for old movie disc iso files but except for rare 70mm releases, current movies do not have them at all).
Using a mixer from the 5.1/7/1 signals to get a stereo mix for the transmitter is not unusual at DI sites. You want the centre channel panned on centre, L & R on L & R at a slightly lower level, surrounds on L & R but even lower level, and sub panned centre at a pretty low level. Dialog is mostly on centre (within the limits of the decoding matrix) so for legibility C should be loudest. Digital audio with discrete (ie not matrix decoded like analog multichannel) sound has (normally) dialog 100% only on the centre channel.
I prefer using a fixed special purpose "mixer" that passively combines the outputs into a 2ch L/R stereo signal. It isn't going to fail, and avoids the "fiddly fingers" operator who knows better how the mix should be adjusted.
Sound issues with analog might be at the processor but are almost always with the projector soundhead readers.
First, dirt and oil in the optics is bad. There's a light source that shines through the film's soundtrack and an optical sensor that "reads" the track through a thin slit to a photosensitive device or with a line CCD sensor. Make sure all optics are clean and not oiled up outside or inside.
The mechanical adjustments in the soundhead are critical. You must have special setup test films to properly adjust the soundhead and reader... but with just a loop of music on film you can adjust the focus and azimuth for the clearest highs. Theose test films are not so easy to get if not at hand.

So I hope you can help get better sound. Describing what the issues are would help.

[Ryan Gallagher]

First, impressive that any cinema business is using film projectors. Where do they get prints?

I was very tempted to ask that same question. LOL We get prints here, but certainly not enough of them to be full time 35mm. A well connected film programmer could maybe pull it off, but I feel like archives probably shy away from loaning to drive-ins for temp/dirt/weather reasons?

A weekend only drive in might be able to pull it off with a selection of repertory prints they have access to, and the occasional special booking? But it still feels like a passion project or non-profit.

[Brad Miller]

I'm guessing this is at the Mahoning Drive-In given your location + the use of Simplex projectors.

Basically all 35mm sound tracks since the 70s have Dolby SR noise reduction, and your DTS at least has an SR button so it will handle that. I've never used one so I don't know how good their SR decoder is. Definitely some non-Dolby processors had really poor SR decoders.

The "accompanying" CD player would be a separate DTS unit that does play a special movie CD and uses separate film readers to get the timecode to sync up the soundtrack on the CD. However: the CDs are pretty much not available (yes there are sources for old movie disc iso files but except for rare 70mm releases, current movies do not have them at all).

Two corrections here:

1. Dolby SR did not happen until 1987, and even then it was several years before it became standard on most prints. Dolby A is what you are thinking about and it was used well into the late 90s.

2. The DTS XD10P is TOTALLY DIFFERENT than the DTS XD10. The "P" means sound processor. The "P" models have NO digital audio of any kind. It was and still is a terrible choice of model numbers because it is unnecessarily confusing for people.

= = = = =

If you want good sound, that Eprad Starscope needs to be removed and launched into space. (You would be doing the world a favor.) I would put in a Dolby CP650 or Dolby CP65+DA20 and a pair of SRD penthouses. Run optical and digital feeds into it, flatten and balance the output levels on all channels to 300mv and inject the 6 output channels into a QSYS to do a proper 5.1 fold-down to LtRt 2 channel stereo for the FM transmitter. From there take readings from a bunch of different car radios and apply a gentle parametric EQ average from your readings within the QSYS processing. (Be sure to turn off any detrimental "sound sweetening" filters might be in the FM transmitter.)

Assuming the theater is on a tight budget, you could replace the QSYS with a 6 channel mixer and perform any averaged EQ within the Dolby processor, but if the knobs on the mixer are bumped or messed with in the slightest after calibration, the 5.1 fold-down is going to be destroyed whereas the QSYS would protect it.

[Sascha Roll]

I would probably keep the XD10P (aka JSD80) for analog preamp and Dolby A/SR Decoding and Feed the 5.1 Output into a JSD-60 which has a built-in downmixer to 2.0.

[Jesse Crooks]

There was a lot going on when I got the rundown of that sound system, but I can clarify a couple things. The lefthand sound rack that includes the DTS XD10P is the main rack, and the righthand rack that includes the Eprad Starscope is largely obsolete. The Starscope itself is only still in place as a backup unit. It doesn't provide any additional functionality. And they only have analog optical, no mag.

My understanding is that the Christie AM-3 automation was used when they had a single projector and a platter system, so I would suspect that it does not have anything to do with the audio changeover. These days the projectionists only touch it to start the motor on Projector 1. The audio changeover should be managed by the sound processors.

I had largely ignored the Yamaha mixer because I assumed it was for their non-projection audio, but if the DTS outputs to the Yamaha it sounds like prior commenters are correct that they are downmixing to Left/Right only for broadcast. It may seem weird that they take a 2-channel signal, decode to provide additional channels, and then downmix back to 2-channel, but they have to use the decoded output that the DTS provides.

[Ryan Gallagher]

Starscope left in as a backup/original would have been my other guess if he had not mentioned possible mag audio. That clarifies things Jesse! Cheers.

[Justin Roman]

This is all great info. While there's some great resources online (like manuals etc.) there is no substitute for the knowledge of people who have experience with the gear.

It is at Mahoning. Print sources vary, and they do run some things digital. But they're committed to keeping 35mm alive as a drive in experience. It's a great place with a lot of really dedicated people pitching in to make it happen.

I guess the thing I was most confused about was why decode the surround when it's just going back to stereo. But I see now that the decoders do a lot more than just decoding. I read a bunch about A chain and B chain processing. I'm curious about that QSYS down mixer or the JSD 60 and will look more into it.

There were a variety of issues, one of which was that the sound levels between the two projectors were uneven. The overall signal was also fairly noisy. I think these were addressed by recalibrating the projectors. The only concern I have going forward is that they've been branching out into events with live components, and as of yet I still don't think there's a clean reliable way to get an outside signal into the transmitter feed. I provided live gear for and event we ran with John Waters doing commentary over a film last year, and I had to patch in at a point that made no sense to me, caused a horrible buzz, and gave me no control over relative volume level. I'm hoping to have a cleaner option in the future.

[Sascha Roll]

Justin,

the DTS XD10P features a "NonSync" Stereo Input for alternative sources (via RCA jacks). You could feed a MIC/Mixer Signal in there.
As you don't have speakers behind a perforated Screen (obviously ?) causing Feedback when people are Standing in Front of it... It should Work for you.

As Others have mentioned, some sort of more modern, flexible DSP solution Like QSYS would make it a lot easier for you... Also giving you more Options to "manipulate" the Signal to Not overdrive your FM Transmitter.

Still, you will need to keep the 35mm capable Professor for preamp/Noise Reduction/Decoding.

[Ryan Gallagher]

I think one nuance of his cited show was it was billed as “live commentary”, IE talking during the film.

in that case I would assume patching a live mix into two channels or aux-in on the little mixer they already use to get their 2.0 mixdown would have been the most direct path with current gear, but perhaps it lacks free channels to do so?

[Ryan Gallagher]

Glad they got their noise/levels issue sorted. Yeah there is a routine calibration against Dolby Tone test film that is needed on a regular basis as the optical reader diodes age to keep the two projectors at the correct volume.