stereo to surround revisited

[smack5099]

The mix in question had to go out as is. I've been out of town for a week and just got back, and got to experimenting.

As it turns out the subwoofer wasn't the problem (or I should say, wasn't the *only* problem). After I rolled off the low-frequencies in a test, I played the stereo track on the surround system only to find that the left, right and center channels were also going nuts (the excessive presence of the woofer must have masked the issue before). It seems the tracks are bleeding into each other in a sense. For example, a piano piece plays with very slow, pulsing quarter notes over the sound of a shower (or some other diagetic sound). Every time the quarter note hits, the shower sound boosts significantly, then goes back to its normal level during the quiet parts of the piece. The only sections that are completely unaffected are those with only one track present. It's as if anytime two tracks play together, the volume of one is boosted by the other. (I haven't had the time to test and see what the method to the madness is, i.e. which tracks are boosted by which other tracks and to what extent, as it is very time-consuming, though it seems from limited testing that tracks that are visually higher may be boosted by the tracks below it).

1) This mix doesn't behave this way on any stereo or mono system I've tried. Unfortunately, I don't have access to any other surround system than this one to test.

2) This surround system plays commercial DVDs fine. I watched Apocalypse Now on it and it sounds brilliant.

3) I've bounced this in a few different ways. Final Cut to Quicktime to DVD. Final Cut through Compressor to AIFF and/or AC3. Pro Tools to AIFF.

4) I can't imagine this has anything to do with it, but just in case, I'll mention that the video is in true 24fps, and I've gone through various methods of getting it from 24 to 29.97 for the DVD. Letting Compressor convert it, leaving it alone and letting DVD Studio Pro change it during the burn, exporting to Quicktime (w/embedded audio), conforming to 23.98 and running through Compressor. I've used both embedded audio and audio imported directly from Pro Tools to AIFF and into DVD Studio without any connection to the video (I let it go out of sync in the latter case for the sake of testing). The problem never goes away.

Any idea what this phenomenon might be?

[Mark Reis]

This is the flaw in matrix encoding. You're experiencing steering or pulling. The frequencies in the shower and piano are such that the piano pulls the shower from its location into the channels being used by the piano without any change in volume. If you pull a sound from the left into the center, it seems to be significantly louder because it is now playing at the same volume through twice as many speakers.

I've had the same problem with a fountain and a chirping bird. I ended up changing both sounds.

The steering occurs on Decode, not Encode, and is why you must monitor your mix through an encoder/decoder setup in order to detect and correct these problems before they happen.

The short version is that the sound with the most energy (my term- a function of frequency and volume) will pull other sounds with similar frequencies to its location.

You have to remix to correct this and that might include panning/position, EQ, compression, or new sounds.

There are also noteable differences between an average home dolby decoder and the dolby 564. Apparently not everything is built to the same spec as the professional equipment.

[cabuford]

This is a stereo audio mix, correct? plain old stereo, no dobly pro logic, no dolby digital 5.1?

If you're playing a stereo mix on a surround sound system, you should only hear it in Left and Right and sub. If you are hearing center and surrounds, the system is probably set on either Dolby pro logic and trying to decode it that way, or its got some psuedo surround setting (hall, club, etc found on many home systems). Home theatre systems have all kinds of settings that, personally, I think confuse the average use and do nothing for the sound. Find the system manual and turn off any EQ, "room" and surround settings. Most displays now have little pictures of how many speakers are being used in the setting. Obviously you want 2, not 5 for playback of this mix.

Hope this helps, its kinda hard to explain.

[Mark Reis]

A stereo mix is a pro logic mix as far as the decode process is concerned. It looks a the audio, checks phase and levels, and then directs sound to the speakers it thinks it belongs in.

It seems to me like he wants to monitor the audio in surround and have it sound like it would when monitored in stereo. This is not easy to accomplish even when working in surround from beginning to end.

This week, the Daily Show started playing VO in my surrounds. It happens sometimes.

[cabuford]

A stereo mix is a pro logic mix as far as the decode process is concerned. It looks a the audio, checks phase and levels, and then directs sound to the speakers it thinks it belongs in.

exactly. I guess what I was trying to say is he needs to turn off the pro logic decoder, as he has not mixed for pro logic. therefore the decoder is trying to decode something not encoded and putting things in speakers they do not belong in. At least thats what I'm thinking is happening.

[mnoxious]

First off, I applaud you for being so tenacious in testing your mix. I usually just test stereo mixes in stereo but now this post has opened my eyes to see how important it is to test a stereo mix also on a surround system.

A couple of years ago I had to output a mix on a variety of formats, stereo, LTRT (encoded Dolby Pro Logic II) , and 5.1 all of which were done in Pro Tools LE and all for the same DVD. And in one spot in the normal stereo mix while listening with Pro Logic Decoding turned on, some tennis ball sounds started coming from the surround speakers. This type of thing happened on a movie I rented around the same time as well.

Mark Reis seems to be explaining what cabuford is talking about. Basically Dolby Pro Logic attempts to decode any stereo signal into surround. Now I'm not an expert on how it does this (and maybe someone who is can pipe in) but as I understand it the algorithm looks for audio that is the same in the left and right channels and puts them in the center channel, audio that is panned hard left and hard right get sent to the left and right channels respectively, and audio that is 90 degrees out of phase get sent to the surrounds. Usually, when encoding a 5.1 mix for a Dolby Pro Logic stereo mix the LCRLsRs tracks get encoded in such a way that the Pro Logic algorithm is able to decode it properly. However, when a stereo file is not encoded for Pro Logic and a receiver attempts to decode the signal, it sends what's doubled to the center, panned to Left and Right, and what's 90 out of phase to the surrounds. So occasionally you'll get some sounds out of place if you don't check for this like Mark Reis mentioned.

So maybe the pulsing quarter note summed with the other diagetic sounds causing the Pro Logic decode process to miss direct your audio. (which is what the other guys were saying)

But, I have to ask. During the process of testing this mix did you take time out to watch all of Apocalypse Now? If so was it the Redux version or the Complete Dossier version? After frustrating sound days I understand the need to unwind with such a great sounding movie. That Walter Murch, you got to hand it to him. Every sound man/woman needs to watch The Conversation, just to remind oneself of the dangers of our sound obsessions.

[smack5099]

I had stated in the earlier thread that there were no speaker settings beyond decibel levels. However, this turned out to be wrong, as the remote control has a button to switch between stereo and surround sound. When switched to stereo, the problem went away (there are still minor issues, but I think they are simple issues with the mix that I will test the next time I get a chance).

I guess the question is: how many surround system owners know to switch between surround and stereo based on the DVD they're watching? And will other stereo DVDs behave the same way when not set correctly? (I haven't had a chance to test this, as all I had on hand were surround mixes and old movies in mono.)

Anyway, thanks for the help, everyone.

[cabuford]

I think in the DVD encoding you can make a specific stream the default, but on alot of players you can have a default as well. I never seem to have a problem with DVD's though. They seem to default to the 5.1 mix, or switch to stereo if a PCM stereo mix is the best option.
The complication here is DVD surround (digital 5.1) is different than ProLogic (decoding a stereo signal). Throw in DTS and it's a wonder anyone can figure it out ;D I'm pretty sure if you leave the ProLogic decoder off, the DVD will play in true 5.1 if it has it and stereo if not.

Either way, this seems like something the DVD mastering house should be worried about. I'm assuming you only burned a DVD to reference with the video. I'd provide them with the highest bit stereo mix you can give em and technically your job is done. That's just my 2 cents tho.

[mnoxious]

i agree with cabuford. I think it sounds like you did everything necessary to ensure a good mix that translates to a variety of home theater set-ups. And yes he is also right in saying that whoever authors the DVD can assign whichever audio stream to be the default. But unfortunately the average consumer doesn't know how to set up their home theater so things like Pro Logic are turned on because "surround is better than stereo right?" or even worse the TV has some sort of processing built in that mimics surround and really messes up your mix (i've had that happen). Plus the average consumer has to configure the DVD player along with the receiver to properly decode a Dolby Digital or DTS file and most don't read the instructions.

I don't know how many homes I've walked into where they had a widescreen tv that was stretching a dvd because they haven't set up the DVD player and the tv to properly play 16:9 picture. If people can't get the picture right I don't know if we can expect anything remotely correct for the audio. But we continue tenaciously to give our best. And I've always thought that the best compliment an audio guy can receive is no compliment at all. If they don't notice anything wrong I guess it was pretty good.

[cabuford]

I don't know how many homes I've walked into where they had a widescreen tv that was stretching a dvd because they haven't set up the DVD player and the tv to properly play 16:9 picture.

oh man this is my biggest pet peeve! I see it more the other way, the 16:9 stream playing on a 4:3 TV, so Everyone is tall and skinny. It takes up more room on the screen though so it must be better, right!

My first digital surround receiver would show that it was getting 5.1 digital signal, display 6 little speaker icons on the display, but wouldn't be playing true surround unless this one button with a green light was on. I can't tell you how many movies we got about 30 minutes into before after thinking "this doesn't sound cool enough" that the green light was off.

So yah, you're job is probably done. The consumer will probably have all 5 speakers on the same bookshelf anyway