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  #1  
Old 11-01-2004, 10:24 AM
CLee CLee is offline
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Default LCR verses LoRo stereo question

So I've been doing films for a while in good old LoRo Stereo, checking them on VHS with a pro logic reciever, everything seems to work out OK, and they translate well to theatres. But I'm wondering, technically what is the difference between a LoRo mix and a Dolby encoded LCR mix. Done some reading on the Dolby site and it seems there is some gain control going on to increase seperation, but it seems they would be pretty similar.

BTW, I'm talking analog Dolby stereo not Dolby Digital.
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  #2  
Old 11-01-2004, 10:38 AM
JKD99 JKD99 is offline
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Default Re: LCR verses LoRo stereo question

Hi
The main disadvantage to Lo/Ro when compared to true Lt/Rt is that there's no true center channel. So, the dialog will never seem anchored to the screen, and FX and Foley (not backgrounds) will seem unnaturally wide.
If you're just screening on video and TV, then it's not so noticeable, but in a theater it's much more apparent.
In addition, although this is not as big an issue, there's no surrounds (duh).
If you've been making video dubs, and screening on video, get yourself Dolby Surround Tools and do an Lt/Rt and compare. I think you'll like what you hear!

Hopethishelps, good luck!
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  #3  
Old 11-01-2004, 05:16 PM
Richard Fairbanks Richard Fairbanks is offline
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Default Re: LCR verses LoRo stereo question

JKD99 is referring to the difference between hearing a discrete stereo mix (2 speakers) vs an LCR mix. He's right, the center channel really helps anchor the "center" in a large room.

Techinically, You can nearly duplicate a Pro Logic encoder's Lt/Rt output by Mixing to Left, Center, and Right channels. To manufacture an output, send your Left and Right to a stereo output buss with unity gain. The Center channel is fed to both channels of the output buss equally, and reduced in level by 3 db. This means that a reference level tone sent to either left or right will come out on it's respective output buss channel at full reference level. A reference level tone sent to the center mix channel will emerge on both channels of your output buss, and each output will measure -3db. You may already know all of this. A Pro Logic encoder does exactly this, but with one change. After deriving two signals from three channels, the two are then sent through an "all pass" type of phase shift network. The audio sounds pretty much the same going in as coming out (to my ear, anyway). So, for an LCR mix, deriving two channels as I just described and sending them into a Pro Logic decoder will give you the same results as feeding your left, center, and right mix channels into a Pro Logic encoder, and sending THAT to the decoder. I have done this and not been able to hear a difference.

You will have much more trouble getting the surround channel(s) to behave without using an encoder. Remember that all-pass network I mentioned? There is another one (similar but different phase delay) used on surround information, before the surround information is added to the Lt/Rt signal. (Pro Logic is mono surround. The mono surround signal is all-passed and then dropped by 3db before being fed equally to the output buss, just like the center channel. Unlike the center channel, the surround signal is phase-reversed to the left side of the Lt/Rt output. Because the surround signal is out of phase between the Lt and Rt signals, the surround channel cancels itself to nothing when Lt/Rt is combined to mono.) The two different phase delays, along with surround information being out of phase, is used by the Pro Logic decoder to figure out what is supposed to go into the surround speakers during playback.

Whew. Anyhow, you can mix LCR without an encoder, using only a Pro Logic decoder, with great results. The gain control you talk about is part of the decoder. Because the center speaker is derived from in-phase signals present in the Lt/Rt mix, which is probably most of your mix, the center speaker tries to play loudly all the time. The decoder tries to counter this tendency, by altering the balance of all the speakers in realtime, to favor the dominant one. It does this with varying degrees of success. Many unexpected things happen. This is why you really must always check your work through the decoder. It is best to perform your mix listening through the decoder at all times, if an Lt/Rt is your final delivery.
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  #4  
Old 11-02-2004, 11:46 AM
CLee CLee is offline
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Default Re: LCR verses LoRo stereo question

I guess what I'm trying to figure out is... Given no surround channel info, what does the Dolby encoder do to an LCR mix to create an LtRt that would be different than a straight stereo mix. In the mix the center information should be down 3db when things are paned to center to keep them equal power, and everything else seems to be on the decoder side. I'll have to reread the Dolby stuff to figure out what the "all pass" encoder stage does. Also it's my understanding that ProLogic and theatrical Dolby Stereo decoders are similar.
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  #5  
Old 11-03-2004, 12:15 AM
CCash CCash is offline
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Default Re: LCR verses LoRo stereo question

This may seem too elementary, but here are two examples I noticed the first time I used a Dolby encoder: Music didn't encode the way I expected. Generally, there's a lot more correlated info in the left and right than I would have thought, making the mix really center heavy. It makes sense, but I didn't anicipate it would be so dramatic. I suppose if you use Richard's encoding method (great post, by the way) and decode through a Dolby box, you'd get very similiar results... but if you're mixing through the encoder/decoder you can manipulate the outcome easier than when doing a fold-down (using panners, divergance and center %). I *would* like to find more time to concretely prove it to myself.

One other thing, on the negative side, is the artifacting. If you solo a left or right speaker (or stand in fornt of one), it's weird to hear things like esses jump to the outside speakers. I can't say I find it distracting when listening to all the speakers together, but I can see how it would be if you ran into something odd or extreme.
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  #6  
Old 11-03-2004, 06:39 AM
Richard Fairbanks Richard Fairbanks is offline
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Default Re: LCR verses LoRo stereo question

Quote:
Given no surround channel info, what does the Dolby encoder do to an LCR mix to create an LtRt that would be different than a straight stereo mix. In the mix the center information should be down 3db when things are paned to center to keep them equal power, and everything else seems to be on the decoder side. I'll have to reread the Dolby stuff to figure out what the "all pass" encoder stage does
I tried to answer these questions, I'm sorry I was not clear. The all pass filter applies a frequency-dependent time delay such that for any given frequency the amount of phase shift is the same. This is not like a standard delay circuit, which delays all frequencies by the same amount of time. Because it can potentially "smear" transients, you might think it would be very audible but I find it to be an extremely sublte effect, not worth considering. Electrically, though, it has a very important role for the decoder hardware. This is because a different amount of "all pass" phase shift is applied to all surround signals. The value of this technique is that you are able to send the same signal simultaneously to front and surround speakers. Even though you are sending an identical signal to both, the use of differnt all pass filters on front and surround channels make them look electrically different from each other, so that the decoder detects the difference and routes them to their proper speakers. It was a clever choice by Dolby.

To answer your specific question about your LCR mix, it is the presence of the all pass filters that the encoder has that your simple stereo mix does not. For LCR, and no surround, the all pass filters do nothing useful so there is effectively NO DIFFERENCE with or without the encoder, as long as you have -3db center pans. PT stereo panners are not, but I believe the surround panners are. Somebody correct me if I am wrong.

While this may seem like good news to you, it does not mean you should be mixing without hearing the mix being decoded! The decoder will put information into the surrounds whether you want it there or not. The decoder tends to make stereo sound MUCH less wide, subtle and not so subtle panning will most likely be lost, and there are the inevitable steering problems, where dialog will get pulled away from the center into surrounds by all manner of synth pads and reverb tails. You cannot always predict what will happen. While the actual balance of elements within your mix will probably not sound much different, their positioning may become distracting. Get Dolby hardware, or at least a plugin package (I hear Circle Surround is okay too, but I've never tried it). As long as you are getting a decoder you might as well get an encoder and do it right, and no longer fear the surround speakers. It will be a good investment.
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  #7  
Old 11-03-2004, 06:50 AM
Richard Fairbanks Richard Fairbanks is offline
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Default Re: LCR verses LoRo stereo question

Quote:
This may seem too elementary, but here are two examples I noticed the first time I used a Dolby encoder: Music didn't encode the way I expected
I think you mean entire encode/decode chain, right? Yes, stereo is usually dramatically reduced in width. This is why there has historically been such a struggle to keep music and effects out of the center speaker. Some techniques cause horrible mono compatibility problems, though, so are nearly useless for broadcast.

Quote:
One other thing, on the negative side, is the artifacting. If you solo a left or right speaker (or stand in fornt of one), it's weird to hear things like esses jump to the outside speakers. I can't say I find it distracting when listening to all the speakers together, but I can see how it would be if you ran into something odd or extreme.
This is one of the reasons Dolby Surround and Dolby Pro Logic limit the frequency response of the surround channel to 7Khz, and time delay them. Since the surround speakers are often closer to the audience, things like esses can be distracting. By limiting high frequency response and delaying the surround speakers, you hopefully hear the main speakers first and the distracting effects are less audible.
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  #8  
Old 11-03-2004, 07:58 AM
CLee CLee is offline
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Default Re: LCR verses LoRo stereo question

Richard,
Thanks much for your responses, you really do reinforce what I already suspected. I would like to do 4:2:4 monitoring on all my film projects, unfortunately the budgets don't justify the investment in room treatment and speaker purchases (I'm mixing in a pretty narrow room and when we considered surround speakers we ended up with a really small sweet spot).

On the last 3 features I did I suggested going to a dub stage to do a proper LtRt and Dolby digital mix, but no one was willing to spend the extra cash, and there really is no good film surround mixing room in Chicago. One of these films will pay for a dub stange, and then I hope to visit one of you fine DUC folks. So, I agree with what everyone is saying regarding using encoders, but untill Chicago film post budgets improve I guess I'll be sticking with checking out my mixes through ProLogic.
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Old 11-03-2004, 03:12 PM
Richard Fairbanks Richard Fairbanks is offline
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Default Re: LCR verses LoRo stereo question

The budgets don't justify an investment in room treatments and speaker purchases?? Holy cow! Do you at least have chairs? If you are doing quality work in spite of this then you indeed have talent and my respect. You ARE doing QUALITY work, aren't you? Really, it is worthwhile to invest in improvements you can carry with you to your next space, and I would think monitor items would qualify. At least you are checking your mixes through your receiver's Pro Logic. You may want to consider setting it up at work to mix through, with temp surround speakers, sending your stereo mix into it the way you are already doing and having the benefit of "instant feedback".

BTW, I lived and worked in Chicago for over a decade, although it has been a while, and the phrase "holy cow" actually brings memories with it.

I worked at a place called Universal Recording on the corner of State and Walton above a liquor store. One of the finest sounding large rooms I have ever worked in! Too bad it isn't a studio anymore. (It also had one of the worst sounding voice studios I've ever heard, too.)
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Old 11-03-2004, 04:07 PM
CLee CLee is offline
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Default Re: LCR verses LoRo stereo question

Universal! it's gone, never worked there unfortunately. The 1st feature I finished we did sync tests at Zenith, that's gone too. Scott Smith was going to open a dub room, that never happened.
Really, it's been my thinking that I would rather mix in a good stereo room than a bad surround if I'm doing the mixing here. All my directors have agreed so far. I've been in rooms that rent speakers to mix and are stereo the rest of the time and I wouldn't work in any of them. The cost of 3 new M&Ks, moving a wall and changing all the surface treatments are not justified. Not if I'm the dialog editor, foley artist, foley recordist, SFX editor and mixer.

I was having a meeting about doing a film and like all meetings told the director and producer that I can take the project to premix at which point I can finish it in stereo or we can go elsewhere and do a proper surround mix. At thet point the producer said something like "I was at my friends house and he had a computer with surround, why don't you. Just get some speakers and set them up!" I figured explaining reflections, calibration and sweet spots wasn't worth it.

I think i do good work. And yes I'm sitting in a chair as I type this.
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