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LaserDisc dual mono tracks
#1
It's been observed by myself and others that dual mono tracks on LD always have a minor volume difference between the L and R channels. I believe the reason for this is to prevent everything being sent to the centre speaker by a Dolby Pro Logic receiver.

Dual mono tracks on modern formats don't seem to do this and have identical L and R channels. So my question is; are modern receivers able to distinguish between plain 2.0 stereo tracks and matrixed DPL 2.0 tracks in a way that LD-era amps could not?

The reason I ask is that I've previously been 'correcting' all of my LD mono tracks by deleting one channel and then duplicating the other prior to muxing. But I'm worried that by doing so I'm dooming everything to the centre speaker on a multi-channel receiver. I only have a 2-channel set up currently so I'm unable to test any of this for myself.
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#2
I can't speak for older receivers as I didn't get into home theater until way until the DVD era, but on all modern equipment doesn't distinguish between dual mono, stereo, and Dolby stereo. That's why on the BD forums you'll see people ask if it's a matrixed track or straight stereo so they can turn on/off any upmixers (tbh that's a dumb way of doing things and on many receivers turning on 'pure direct' or whatever will disable your crossover but that's another topic.)
Playing a dual mono track doesn't make sound play purely from the center channel when using DSU/neural-x. The opposite in fact as it will mainly play from your front speakers while phasing sound into the center, of course if it's true dual mono it'll be pretty much the same audio going into all of your front sound stage.
As for why LDs have the volume difference between channels, I have no idea. Someone can correct me, but my assumption would be it's a mastering mistake or an attempt at faux stereo.
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#3
I read that for some reason bass goes out of the subwoofer if reduced to 1 channel, but is restored to the sub if you add the 2nd one back. I myself doesn't have a sub, though I have a 5.1 set up, as I prefer to redirect the bass to the 5 speakers.

However I do prefer any track released mono as 2.0 on LD to be preserved as is, as I think that mono tracks on prints may have been stereo on most prints to give a more massive sound (more spread) using as noted any level difference, or EQ difference.

If the track is analog mono 1 channel on the LD, then fine, we don't have a choice. But if it is 2.0, it's cooler to have both channels and let everyone decide if they want to get rid of one or not. That is, unless there's some damage rot on one channel, in which case I understand.
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#4
So if run a dual mono track with identical channels through a multi-channel receiver set to auto-detect then it will treat it as DPL and send everything to the centre speaker?
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#5
Dolby stereo/pro-logic works on signal phase to steer the channels, slight variations in volume are tolerated by the decoder so long as the signals are in (or out of) phase. Back in the day mixers would have to put a slight delay on certain music cues/effects if they wanted a wide left-right soundstage otherwise the decoder would send it all to the centre (which is the opposite of a wide soundstage). Discrete obviously doesn't have these restrictions and allows for phantom imaging between speakers.
Dolby have always recommended that mono be played back through the centre (ie using pro-logic) as most home cinema centre speakers are optimised for dialogue (and Dolby are obsessed with dialogue being locked to the centre channel). My A/V receiver is set for DPLII for laserdisc PCM soundtracks so mono always goes to the centre, if there's any 'leakage' I've not noticed.
Historically the left audio channel was designated as the mono channel for backwards compatibility, whether that was the reason for the slight increase in level I couldn't say. I do agree that mono soundtracks should be preserved as was ie dual mono PCM, if people want to sling a channel out at their end its up to them.
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#6
Listen to a mono 2.0 on headphones using VLC, then switch it using the commands to 1.0 mono folded down. You will notice the sound is more spread in 2.0, but goes dead center and kinda "nosy" when folded. I believe one of the reasons the mono tracks are/were dual was to make the sound more massive and the mono mix less noticeable.

Remember, for example, with the Capitol Beatles records, there were complaints when the CD releases had some tracks folded down, as it changed the sound, due to the original records having different processing applied to each channel. In these case, they were either more bass in one, more treble in the other, or slight reverb in one etc.

I think the same preservation precaution should be done to mono LD and VHS tracks, as you might unwittingly mess up the EQ and final sound.
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#7
Once stereo became the standard then dual mono was necessary for backwards compatibility. In the analogue world there are probably enough minute variations in the signal path/processing to give the separate L/R channels slightly different qualities which may explain the 'spread' (but would still fold to mono for dolby pro-logic), whereas with digital it would be clinically identical
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#8
Yes, we should not overlook the possibility that director's who had mono mixes of their film made (and sound mixers) use tricks to make the absence of stereo unnoticeable to audiences. As the sources used on LD are either optical or magnetic, I think they should be kept as they are. There's also the possibility that foreign dubs of mono films aren't fully mono, but have stereo dialogue placement in some scenes, with the M&E track still being mono. If you fold that, or delete one track, you lose part of the mix.
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#9
Doubling the track merely doubles the impact and when things went stereo most optical tracks and later video releases followed suit. The 2.0 mono merely keeps the form inside the systems geared for stereo. Since most main L and R speakers are superior to center channels it was kept this way for most things. 1.0 mono became more commonly used as a space saving device. Personally I prefer 2.0 and giving the listener options. 1.0 can be put back to 2.0 by forcing it on your receiver but this usually is a direct mode that takes out any of your settings.

I agree the slight fluctuation is likely an analog thing vs digital precision. I notice it in almost all of the captures I do.

The bigger problem is the giant volume boost on almost all vhs releases after the early 90’s.
Damn Fool Idealistic Crusader
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#10
What about using some kind of "tri-mono" inside of a 4.0 container? This way you're forcing the processor to use the entire front soundstage without bleeding into the rears.
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