Film-Tech Forum ARCHIVE: Difference between LT/RT and Left Right stereo?

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Posted by Richard May (Member # 2627) on 05-18-2006, 10:39 AM:

We run video a lot. I have the signal going through the non sync circuit of a CP65. Non sync is set for full decoding so we get L/C/R/S and sub. Some videos come in marked Channel one:left....Channel two: right. Some come in Marked Ch1: Left Total....Ch2: Right Total. They both decode the same way. Why?

Posted by Steve Scott (Member # 630) on 05-18-2006, 11:14 AM:

Pro Logic Decoding in the CP65 split any stereo signal into the configuration you're experiencing. As far as the terms go, I know that LT/RT applies to the preamp's reading of the Dolby Stereo tracks, as they are combined into the SVA track on film.

I haven't spent as much time with video, but it would seem that those LT/RT video sources may be referring to a pro logic recording.

Posted by Pete Naples (Member # 761) on 05-18-2006, 11:35 AM:

Lt/Rt means Left Total and Right Total.

In film terms this means that it's a total of the left or right track, which of course is normally pro-logic encoded, plus the NR encoding.

So if these videos are following the terminology correctly, they are NR encoded.

However in my experience I haven't had to provide outboard (of the VTR) NR decoding for a LONG time. Most pro VTRs, (betaSP Umatic, VHS/SVHS, 'C' etc) have on board NR.

So in short it's probably nothing more than mis-applied terminology.

Posted by Gordon McLeod (Member # 33) on 05-18-2006, 01:29 PM:

all Lt and Rt mean is that it is a equivelent of a print master and NR would only be required if marked as such and often they will only get labeled l and r
and in that case tone would be required to set the nr threshold

Posted by Bill Gabel (Member # 1105) on 05-18-2006, 01:43 PM:

As Pete said it's mis-applied terminology.
I handle alot of screenings and some post-houses write the wrong terminology on the labels of the tapes.
As both Scott and Pete said.

Left Total-Right Total, not left track-right track. Indicates the presence of matrix encoding of four channels on a 2 track stereo master.

In most cases Left channel-Right channel is just a stereo track that has not been matrix encoded. By sending the tracks into the matrix you might be sending wrong information into the speakers that the film maker did not intend. It might sound right but it may be wrong.

Posted by Richard May (Member # 2627) on 05-18-2006, 02:01 PM:

I would have to assume then that all the tapes we have run here are LT/RT. Even the ones that were just marked Left/Right, ran perfect through the matrix decoding. When I say perfect, I mean no dialogue in the surrounds. Only in center channel. The balance between all the speakers seemed correct. There were maybe one or two tapes that didnt sound right. They had surrounds so loud that it drowned out the stage speakers. BTW, with colors bars and tone on the head of the tapes, you can set input level using the Cat222 card. Even though the noise reduction circuit is not active, you can set level with double greens to even out the left and right inputs to give you correct decoding.

Posted by Manny Knowles (Member # 1171) on 05-18-2006, 04:45 PM:

Some programs are indeed only mixed for Left/Right only. To be absolutely clear (and correct) they should be labeled "Lo/Ro."

I'm not used to seeing a capital letter "t" in Lt/Rt.

As mentioned, Lt/Rt designation does not automatically imply the NR is employed.

Posted by Steve Guttag (Member # 268) on 05-18-2006, 04:52 PM:

I've run quite a bit of 2-channel stereo tapes (Lo/Ro) in theatres. I think without exeception, they sounded vastly better using a "Pro-Logic" matrix decode on them. Is it technically correct, no. But, it is highly unlikely that the person that mixed the tape thought about Left and Right speakers being 40-feet apart. There is simply no hope of a phantom center channel at that point.

By using the Pro-Logic decode, the dialog normally finds its way to the middle. If you use a reference decoder like Dolby's DP563, you can force the decode to just L,C and R and omit the surrounds. You can also kill the surround signal in several other ways, depending on the electronics you have.

I have had the makers of the film indicate they only wanted Left and Right but after hearing it on the "big screen" they invarably as to put the "Pro Logic" back on.

I wish the Lo/Ro designation would catch on since it take the guess work out of the intended format.

Posted by Frank Angel (Member # 248) on 05-21-2006, 04:27 PM:

I too have run lots of "Film" Festival video and for the most part, videomakers rarely have any concept of matrixed 4->2->4 playback. I always decode to get a center channel and as Steve notes, it always sounds better than straight Lo, Ro stereo. Some even think that's they way their Lo, Ro sounds without benefit of the decoding. Without the decoding, the dialogue wanders conspicuouly; decoding firms it up and locks it in the center. Surprise, surprise when I tell them no, it wouldn't sound that way with just L & R playback; you are getting a center channel for free, thanks to Ray Dolby.

Posted by Dan Zastrow (Member # 3796) on 05-21-2006, 04:39 PM:

Unfortunately, in the “Film” festival world, the directors seem to have very little understanding of the technical specification of their own “film”…sound, aspect ratio or even format. When you have a filmmaker putting a DigiBeta tape in your hands when you were very specific that you only have a BetaSP deck and he can’t understand why that is a problem!?!

Posted by Manny Knowles (Member # 1171) on 05-22-2006, 06:37 PM:

i blame the industry that created more than one beta format. they should've renamed each and every one.

Posted by Phil Hill (Member # 371) on 05-22-2006, 06:44 PM:

That's silly! Everyone knows it dumbass SONY's fault for the BETA confusions...oh and shitty film products!

Posted by Manny Knowles (Member # 1171) on 05-22-2006, 06:56 PM:

phil, you're madder than a wet hen!

Posted by Phil Hill (Member # 371) on 05-22-2006, 07:02 PM:

HA!