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Mid/Side method for recording, mixing and mastering, and playback.

DVDdoug

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've got to figure out where I'm going to setup my second system that can actually play back a multi-track recording.

If you do your processing in advance, you can make a multi-channel file and it doesn't require any "processing" for playback (other than the usual surround decoding). I made a 5.1 channel DVD from a concert with a mono track. Lots of different tricks (and lots of time). I mixed-in applause from different parts into the rear channels and I panned all of the dialog between songs to the center channel. I used complementary EQ to create the 3 front channels (during the music) and used a similar method, as well as reverb and delay to get the rear channels. This was mostly "just for fun" and I can select the original mono or the surround from the DVD menu.

All AVRs have various "soundfield" or "upmixing" settings. That's the easiest way to do it. I use a "Theater" or "hall" setting to get some delayed reverb in the rear speakers. How can I use an AV Receiver for music? There also usually "matrix" settings with are based on L-R and L+R. The Dolby Pro Logic modes are also based on M/S tricks, but they use steering (automatic panning) to move the sound and it doesn't usually work well with music.


...In the 1970s when quad was popular I built a L-R amplifier and routed it to the rear speakers. (So it was only 3-channels with both rear speakers getting the same signal and the left & right front unchanged.)
 
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Tim Link

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If you do your processing in advance, you can make a multi-channel file and it doesn't require any "processing" for playback (other than the usual surround decoding). I made a 5.1 channel DVD from a concert with a mono track. Lots of different tricks (and lots of time). I mixed-in applause from different parts into the rear channels and I panned all of the dialog between songs to the center channel. I used complementary EQ to create the 3 front channels (during the music) and used a similar method, as well as reverb and delay to get the rear channels. This was mostly "just for fun" and I can select the original mono or the surround from the DVD menu.

All AVRs have various "soundfield" or "upmixing" settings. That's the easiest way to do it. I use a "Theater" or "hall" setting to get some delayed reverb in the rear speakers. How can I use an AV Receiver for music? There also usually "matrix" settings with are based on L-R and L+R. The Dolby Pro Logic modes are also based on M/S tricks, but they use steering (automatic panning) to move the sound and it doesn't usually work well with music.


...In the 1970s when quad was popular I built a L-R amplifier and routed it to the rear speakers. (So it was only 3-channels with both rear speakers getting the same signal and the left & right front unchanged.)
@BenB has his own method of up-mixing, and that's what I'm interested in hearing. To hear his pre-upmixed recordings I need to set up my 5.1 system somewhere in the house. My main rig is already taking up the main listening space. The rest of the house is cluttered at the moment with years of accumulation from all of us family members. I've got to clear out some space before I can get my 5.1 system setup decently.
I've done some processing in advance myself, but at this point I'm interested in on-the-fly methods that can up-mix 2 channel in various ways. Of what I've heard, I like my method the best so far. I've not been fully happy with the standard AVR sound field and up-mix settings, preferring basic Dolby Pro Logic in movie mode over just about anything else. I tell it I only have left, center, and right speakers. Movie mode actually uses the center channel. For me, a prime goal is to get rid of the phantom center. It surprises me how many of these up-mix settings tend not to use the center, or suppress it. Apparently a lot of people actually like the phantom center sound for music. For the love of comb filtering, I guess. Seriously though, I think it has to do with the fact that often the center channel is not the same kind of speaker as the front side channels. It's usually a sideways speaker intended to fit neatly under a TV. And, the center is acoustically positioned in the room quite differently than the side channels, so it doesn't always integrate into the soundstage well, tending to call attention to itself. The surround channels are kind of interesting but in the end I always turn it off.
 
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BenB

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The software I created in matlab (actually octave since matlab is expensive) doesn't run in realtime, and I haven't given any thoughts to a realtime implementation, until now. I could probably tweak it in a way that wouldn't hurt performance and would reduce the computations to be implemented for realtime processing in a language like C or C++. I have no plans to do that, though. I do think it's important to explore your options so you know what the idealized version of your approach would be.
 

BenB

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yea, you can't do this with arithmetic. this also isn't the only/first software doing it, but I tested a few and this one does it the best.

I'm curious how you tested the software. From my reading, it sounds like Dolby was concerned mostly with placing sounds that were happening in isolation. For example, it would do a good job with a conversation between two people, one in the center and one on the right. Each person takes their turn talking, and is reproduced in the center channel and the right speaker. I'm trying to address different sounds in different channels at the same time. (My apologies to the people at dolby if I've misunderstood their approach.)
I'm considering taking 4 mono tracks and mixing them into stereo, and then separating them. In this way I will know what truth is, and can tell how close I got to it. Did you do something like that already?
 

dasdoing

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I'm curious how you tested the software

I didn't really test how pure the extracted center is, but the other software would produce artifacts in the center. It was in the context of improving a 70ies mix in a DAW.

From my reading, it sounds like Dolby was concerned mostly with placing sounds that were happening in isolation.

the intention initially was to lock dialogue to the middle of the screen. C=L+R does that. the problem really is that stereo separation gets compromised. So a real phantom center does actually improve the sides.

I'm considering taking 4 mono tracks and mixing them into stereo, and then separating them. In this way I will know what truth is, and can tell how close I got to it.

well, a simple test would be to mix two different sine waves, one centered the other paned. but passing this simple test wouldn't guarantee that an algorithm can separate complex signals. a better test is to use a stereo correlation meter. you sent center to one channel and L or R to the other, correlation should be zero (meaning, they are 100% different from each other)
 

goat76

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I think the resulting quality of all attempts of trying to extract more channels out of a stereo mix will be highly dependent on how the audio mixes have been done in the first place. Some stereo mixes have a lot of out-of-phase content going on and will simply not be 100% mono-compatible, I think we will run the risk of making a mess if we try to extract a center channels out of a mix that has a lot of "stereo tricks"/phase differences going on.

And what changes will be heard if we play a particular stereo mix that doesn't have any distinct phantom channel content, like recordings made with just a pair of microphones in a stereo recording configuration, will a recording like that even work in a similar way as a multi-mono recording if we extract a center channel out of the mix?

Well, if the sound system in use can be configured from 3 to 2 channels with a push of a button depending on if the content sounds better with one or the other setting, there will not be a problem in deciding what works best on a song-to-song basis. :)
 
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Tim Link

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I think the resulting quality of all attempts of trying to extract more channels out of a stereo mix will be highly dependent on how the audio mixes have been done in the first place. Some stereo mixes have a lot of out-of-phase content going on and will simply not be 100% mono-compatible, I think we will run the risk of making a mess if we try to extract a center channels out of a mix that has a lot of "stereo tricks"/phase differences going on.

And what changes will be heard if we play a particular stereo mix that doesn't have any distinct phantom channel content, like recordings made with just a pair of microphones in a stereo recording configuration, will a recording like that even work in a similar way as a multi-mono recording if we extract a center channel out of the mix?

Well, if the sound system in use can be configured from 3 to 2 channels with a push of a button depending on if the content sounds better with one or the other setting, there will not be a problem in deciding what works best on a song-to-song basis. :)
I've done a lot of listening with my array, and as far as I can tell it makes nothing worse. In cases where there is not much phantom center it still sounds great. The one downside is that music without a strong phantom center actually sounds perfectly fine on regular 2 speaker setups, and works better for more positions in the room. I had a friend over yesterday listening to my array and he was sitting in the sweet spot so I sat off to the side. I hear a solid mono center from the side, but the stereo effects are diminished and spacial separation is all but lost.

With my setup the potentially lost signals due to phase issues are restored so long as the speakers are kept close together, like I do. Keeping the speakers close is the key. Any spaced speaker arrangement, whether 2 speaker stereo or more, will have coherency problems whenever sounds are not pinned to a particular speaker. Phantom images between speakers are bad news. Any phantom images should be to the outside of the speakers, never between them. It's actually not quite that simple. More accurately, any phantom images should be achieved by crosstalk reduction, so if someone is using something like BAACH or ambiophonics then it's ok to have a phantom beween speakers because the crosstalk reduction will eliminate the comb fitlering and crosstalk induced imaging confusion.
I consider my system to be a low tech alternative to BAACH, not an alternative to up-mixing alogrithms like Dolby, which are intended to be used with widely spaced speaker arrays. I've played with Dolby Pro Logic and it does a great job of isolating sounds panned center, left, and right and depositing them in the correct speaker. I was surprised that I didn't like Dolby as much as my own channel mixing matrix. I'm pretty sure the reason is that my arrangement achieves some crosstalk reduction while Dolby is a virtual approach. I would have thought that real multi - channel mixes would be the ultimate, but now I'm more sold on well made 2 channel recordings listened to with crosstalk reduction. I think the old mantra that we only have two ears, so ultimately 2 streams of sound are all we need is true, so long as only 1 stream reaches each ear, which means the streams don't cross into the wrong ears. With my system they do cross a little bit, but in much less harmful way. The crosstalk is reduced, and what does reach the wrong ears is out of phase across the head, which means it does not contain reverse location information to compete with the correct signal reaching each ear. Instead it just adds spaciousness.

Using more channels is similar to what Microsoft was experimenting with for VR goggles. Depth effects are difficult to really get right because the goggles will have to track your eye convergence and adjust the scene to make foreground and background items go in and out of focus appropriately. Because of this difficulty, Microsoft proposed having multiple transparent displays at various distances from the eye to deal with this. This is similar to more channels in multi-channel audio. It works better than not doing it. But what about objects at distances that are between screens? They're going to be blurry because they have to be partially projected on two displays at once. This is the same as the problem with sounds that are between speakers. Utlimately it's a flawed approach unless your going to make curated material that doesn't produce anything not distanced precisely at a particular screen depth, or with speakers at a particular speaker location.

In a nutshell, I think the ultimate solution is crosstalk reduction for audio, not more channels - although more channels is better for providing lower quality imaging effects over a larger area of the room. For VR headsets it's eye tracking to calculate convergence and then adjustment of the image and mechanical lens focusing accordingly. Not easy.

One last thought - it should be possible to combine something like BAACH with multi-channel. You could have 7 speakers arrayed across the front of the room, and something like BAACH could allow phantom images between any two speakers to be free of crosstalk anomalies. It would basically require 6 times the processing power of BAACH, and might sound slightly better.
 
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