Film-Tech Forum ARCHIVE: Hypothetical mono Pro Logic "noise reduction"

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Posted by Kieran Hall (Member # 10268) on 04-09-2019, 06:52 PM:

I was having a bit of a think the other day about how best to deal with a mono archive print we're told may have sprocket damage from the source printed into it (was thinking it'd be worth trying just routing "Lt" to centre with no NR to reduce hum, but we don't have the print yet so who knows) and this got me wondering.

Assuming you're dealing with a bog-standard dual VA mono track, my understanding is that Pro Logic decoding it, with only the centre channel actually routed to a speaker and whilst applying no NR, would steer the content identical to both channels to the centre, instead of naively summing them as I assume format 01 does. Ignoring the fact that the processor (certainly our CP500 afaik) won't apply Academy EQ to this Franken-format, this would seem in principle to provide some (very) rudimentary form of noise reduction, as damage, dirt etc. would be scattered unevenly across left and right.

Is my thinking sensible here and, if so, was this ever "supported" by any of the manufacturers? I ran this by one of my colleagues and it got him going "hmm... why isn't that really a thing?", so we'd be keen to hear!

Posted by Steve Guttag (Member # 268) on 04-09-2019, 09:05 PM:

The CP500 allows for user formats so you might be able do do your own format with/without academy filter and with/without NR.

How to best deal with a damaged print differs by print and just how it is damaged.

Once upon a time, we received a sprocket ridden print where it drifted between the tracks and what we did was put a switch box on the cell feed so we could choose which track was the good one and then raise the gain to compensate for the fact we only had half the track modulating. Then throughout the show, as the damage drifted, we'd switch which cell was active. It worked but was a very manual means of dealing with it.

Depending on the modulation level of the damage, NR may either help or accentuate the problem as Dolby NR applies no NR above 50% modulation so you might actually attenuate subtle signal leaving just the buzz of the damage.

Ideally, you'd want a single-ended NR system like DNR of National Semiconductor https://www.amstzone.org/LM1894-NS.pdf

Even then, it may or may not help with this sort of noise. Perhaps making a notch filter around 96Hz (4x24 or the frequency of a sprocket ridden print) would help a bit. An RTA could be an aid in getting the frequency right and taking the noise out.

Posted by Martin Brooks (Member # 1269) on 04-09-2019, 10:47 PM:

ProLogic would send identical right and left content to the center channel. But if there are any out of phase components, they'll go to the surrounds. Any any non-identical in-phase components will go to Left and Right front, but I suppose you could shut those off.

So I assume what you're thinking is that the noise is not the same in the left and right channels?

Posted by Marcel Birgelen (Member # 6801) on 04-10-2019, 07:18 AM:

quote: Steve Guttag
Even then, it may or may not help with this sort of noise. Perhaps making a notch filter around 96Hz (4x24 or the frequency of a sprocket ridden print) would help a bit.

I don't think a simple notch filter will do it, since it will not create a 96Hz tone, but rather more like a resonance pattern at 96Hz. Still, something that could be filtered, but would require some more sophisticated tools.

Posted by Steve Guttag (Member # 268) on 04-10-2019, 07:18 AM:

See my fear on that is that the noise would be so pervasive it would also not see the actual signal's contribution and not route to center either so dialog could waver in level.

I'd still favor a hard-center route and block the affected channel (disconnect). The overall S/N level gets worse (only have the track) but it would keep the nasty buzz out.

Posted by Steve Kraus (Member # 476) on 04-10-2019, 02:43 PM:

Obviously, if the damage is confined to one channel, eliminating it is the way to go. But just out of curiousity, what does "Clean Up" on a Dolby Model 364 offer?

Posted by Mitchell Dvoskin (Member # 751) on 04-10-2019, 02:54 PM:

> with a bog-standard dual VA mono track

I think the easiest way to do this is to disconnect the wire from the photocell to the right channel input of your Dolby processor and to move the wire running from the left channel amp to the center channel speaker, then turn off the amps for the center/right/surrounds.

If you can not turn off the center/right/surround amps seperately, remember that many amps need some sort of speaker load or the output transistors will blow. In that case, just swap the amps left/center output, and disconnect the amps input to the center/right/surrounds.

Posted by Sam D. Chavez (Member # 1841) on 04-11-2019, 12:42 PM:

quote: Steve Kraus
Obviously, if the damage is confined to one channel, eliminating it is the way to go. But just out of curiousity, what does "Clean Up" on a Dolby Model 364 offer?

It was basically 1/2 noise reduction as in 5dB of NR engaged which helped with the worst of the noise. Don't recall exactly how this was engaged. There once was a Cat. 43 I think, that was used in recording studios. It used the Cat. No. 22 card in a fixture with four sliders pots that adjusted the specific freq. band for up to 10dB of effect each band. In the 364, each of 4 bands was preset at the factory.

Posted by Bill Brandenstein (Member # 7758) on 04-11-2019, 08:30 PM:

Forgive me for treading in here since I don't actually run 35mm. But I do certain kinds of audio on a professional basis. Occasionally I've run into 96Hz noises on 16 or 8mm tracks due to bad dubs. And 24Hz noises on 16mm optical tracks.

In a word: don't try to filter this. Whether points dug into the emulsion from teeth, or misalignment of the track so holes were picked up in the dub, it doesn't matter: these are impulse noises (or at best, sawtooth), not sine waves. In other words, the noises are rich in harmonics for octaves. You can't filter out enough frequencies to get rid of this. There are digital restoration tools that cope with such problems, but not for real-time playback.

Unhooking half of the audio and remapping Lt or Rt to center seems most likely to be successful, IMHO.

Posted by Louis Bornwasser (Member # 3063) on 04-25-2019, 05:03 PM:

Did it all the time in the CP-50 days. Just unplugged one of the cat. 22 cards as required.