Correcting Audio Delay in Cinemas

[Benjamin Hörbe]

Hi, As a sound editor for film productions, I've recently become increasingly aware of a common issue in cinemas – the misalignment of audio delay in cinema processors, particularly in my hometown. It's surprising how often this goes unnoticed, considering its significant impact on the audience's experience.
The standard 80ms delay, which seems to be widely accepted as a one-size-fits-all setting, doesn't always suit every room size. Sound travels at roughly 343 meters per second, so after 80ms, it has traveled about 27 meters. In many cinemas, which are significantly shorter than 27 meters, this delay causes the sound to noticeably lag behind the visuals.
In audio postproduction, there are several tools to measure the AV sync error. One such tool is SynQR. This iOS app makes the delay adjustment process straightforward. You simply play the test DCP that comes with the app in the room you want to calibrate. The app measures the audio video offset in real-time. Afterwards, you can set the audio offset based on SynQR's readings.
This is a very simple yet effective way to enhance the audience's experience.



SynQR's website: https://synqr.app

[Harold Hallikainen]

I think SMPTE calls for audio and video to be in sync at the "reference position." This can cause audio to be before video for audience members in front of the reference position. I've often thought that we should sync audio at the front row of seats. We are much more tolerant of audio after video than the other way around.

[Benjamin Hörbe]

I agree. Calibrating for the front row is a good approach in smaller venues.

[Dave Macaulay]

I do sync from the middle seating row. Using a sync test clip, it's not easy to tell small sync errors. I just record a video on my phone in slow motion mode, playback shows very small disparities. Doing this again from the front and back rows does show some offset, but watching a feature, it isn't perceptible in lip sync (to me, anyway).

[Steve Guttag]

Lip sync will vary based on equipment used (S1, S2, S4 projectors, various servers...etc.). I tend to also use a clip with an audible "clack" with a flash and judge from the reference position. I will always shift 1 frame each way to ensure that I have indeed found dead sync.

Using the reference position scales to large and small rooms alike. I agree with Dave in that small differences are imperceptible. The image is only updating 24 times a second so that is the granularity that you are trying to sync to. That comes to about 41.67ms. That is a pretty course amount, in terms of time delay. Using freedom units for a moment, because it is easier for sound speed...sound travels at about 1130ft/sec...and rounding to 1000ft/sec, that puts each "frame" at about 42-feet. Most theatres are 40-80 feet in length so you really have a sweet spot for your combination of server and projector latency.

One thing is for sure...it is one thing to have sound lagging behind the picture...your brain is well suited for that as all sounds, in real life, come after the eyes sees it. What is not natural and your brain will reject is sound preceding the visual. It is better to have the sound delayed one frame too much than one frame not enough.

There was/is an app for iOS...I think it was called "Catchin' Sync" where one can calibrate their phone/pad (take a book or something that will make a sound when dropped)...you record the drop and match the sound blip to the visual...that calibrates your phone...then you can use a test DCP to have a flash with a sound and then, again, set the sync to be dead on.

[Mike Babb]

I have checked sync for hundreds of screens at film festivals, screenings, premieres, and common multiplexes. Its more often incorrect than correct and usually off more than 40 ms.
I tend to aim for 1/3 from the front and not 2/3 back which I believe is wrong. I use a DCP and Sync Check box which is correct to 1ms.

The correct position of course is where the filmmaker is standing

As Benjamin mentioned, I just found one set at 80 and left it at zero.
80 was more appropriate for series one projectors but even then, not a global truth.

[Steve Guttag]

Don't forget, you can have two different things putting delay in too. You have the server, which can take delay away...but you also have the sound processor which can (and many will) add some delay on their own. There is also, normally, a lower-limit to the sound processor's contribution.

[Marco Giustini]

A "sync checker" should be a must for an installer but having installed hundreds of cinemas myself, that was not a tool that was ever provided by my employers unfortunately so I had to do my best and evaluate as Steve says. 80ms is the "fader 5.5" of delay. It is "good enough" but clearly cannot be correct for everything.

Bear in mind that the delay is taking into account the whole chain. If you set 0ms, it won't mean that your audio is perfectly sync'd at the speaker. Another good reason for actually measuring the delay somehow.

[Leo Enticknap]

Also, some rooms are more sync-triendly than others. A horseshoe-shaped auditorium with, say, a 40ft throw and where no-one is sitting more than that from the screen is near impossible to sound significantly out to anyone, even if all the delays are left on factory defaults. A shoebox-shaped room with a 120ft throw, on the other hand, will likely result in even an untrained eye/ear noticing that something is off from the back few rows, especially if Mike's and Steve's principle is followed (no-one should be able to perceive the audio as being ahead of the pix).

But of course it's a tradeoff: those shallow, arc-shaped rooms might be better for sync, but seating on the extreme sides will have very poor sightlines.

[Pete Naples]

I use one of these:
https://sync-one2.harkwood.co.uk/

I've a few DCPs, check it in two or three rows, taking into account the points raised above.

We all lip-read, wether we know it or not. So the brain can tolerate picture-before-sound to a larger degree than picture-behind-sound.

Some venues are a nightmare, I have one place with a fairly long throw, it's used for festival premieres and as such subject to more scrutiny than your average cinema. I tend to set sync a couple of rows in front of the projectors, because that's where studio people, film-makers etc always tend to go.

[Benjamin Hörbe]

The Sync One 2 is a very good and reliable device.
This one has a similar approach but lives in your iPhone: https://synqr.app

[Harold Hallikainen]

The LSS-200 can also measure audio sync. The sync measurement is part of a typically daily test show that checks screen luminance, chromaticity, and each loudspeaker ( including rough frequency response to detect failed drivers or amplifiers). Results are logged internally and can be posted to a web server for further analysis.

[Steve Guttag]

Unfortunately, the LSS200 went EOL last year as another part of the QSC/Cinema cinema thing.

[Harold Hallikainen]

That's too bad. My one and only patent! I still have one unit. I guess I can check our TV with it...

[Mike Babb]

Just adding in the box Pete mentioned above works well but I find my Sync Check 3 does better in lighted rooms whereas the Sync One2 seems to prefer darkness. I bought mine when it happened to be the last one on the shelf at Markertek, and have never seen one again for sale. The Sync One2 seems to have filled the gap. I have checked sync side by side and they measure indentical.

I have however checked side by side with phone apps and found a little variance. In any case, all these methods are better than no check at all which I suspect is the norm in cinema.

One note, I first saw the sync check box many years ago when working with MPS engineers in London, bought mine the next week, and have never had a filmmaker question sync since.