Anyone else ever notice weird processing during Pro-logic? I was watching PBS (one of the few channels I can get in surround) and all of a suddend the dialog started coming out of the surrounds, then flipped back to the center. Back and forth it kept going distinctly. When the dialog was coming out of the surrounds, the center sounded like it was turned completely off.
I've never noticed this with my previous processor.
Maybe it is PBS. I only get one or 2 other surround channels. I'll have to see if they ever do it.

I've never run into that with either of the Pro Logic II processors I've used (the 990 and the 950). It is most likely due to a Pro Logic encoding slip-up on the particular program you were watching.

Probably is PBS. I have seen something like this before. TV stations often will convert the audio feed to an M-S matrix as it falls back to mono better, and it gives them some control over making the vocals more prominent.

If someone at the station accidentally plugged in the channels in reverse everything that is supposed to be center ends up in the L/R feeds and the L/R stuff ends up in center.

It is pretty crazy, you hear all the background noise and such, and vocals are extremely distant and muffled. Depending on what other encoding they did this could lead to the center info also ending up in the surrounds.

I get a number of HD channels with surround sound via cable and I've never had the issue you described. On a couple of occasions I have had the surround sound change to what sounds like stereo for a minute or two the go back to surround. I attributed it to the station and not my processor. Let us know if your other TV channels with surround switch around.

I get that weird surround sound glitch too.. I just thought it was a free be of Dish Network. LOL i hate it.. but I can't seem to solve it either.

Shawn's got an excellent point about the potential for the signal to get flipped at the station - as the old saying goes: garbage in, garbage out.

jrlouie, when you said "Pro Logic" I thought of a two-channel broadcast with Pro Logic encoding. Are you talking about an HD channel with Dolby Digital? If so, it's not as likely to be Pro Logic's fault (unless it's a Dolby 2.0 signal, in which case we're back to the same spot we were when I originally posted) and it becomes even more likely to be a fault with the source material. It reminds me of an example I have run into - a local affiliate taking the 5.1 sound of an HD broadcast and "downmixing" it to stereo in the middle of a broadcast by stripping out every other channel, leaving just left and right. Since dialog is mostly in the center channel, the results are highly annoying and the only solution (short of calling the station or driving to the station and pounding on the control room door) is switching to the SD channel.

I've occasionally noticed something like that and always felt it was due to something on the source end and really had nothing to do with the 990. Actually when I think about it one of the PBS stations I get does that.

A simplistic and basic description of Pro-Logic processing of a two-channel signal goes something like this:

If there is something in the left channel input that is not in the right channel, send to the left output. If there is something in the right channel input that is not in the left channel, send to the right output. If there is something in the left and right channel inputs that is the same and is in phase, send that to the center speaker. If there is something in the left and right channel inputs that is the same and is out of phase, send that to the surround speakers. If the same signal exists in left and right, but the amplitudes are different and/or if the signals are not in phase but something other than 180º out of phase, then the signal may be sent in varying amounts to different outputs.

The processing is somewhat more sophisticated than that, but the basic ‘encoding-decoding’ for all surround sound, other than end-to-end totally discrete handling of the signals, is still based on these principles.

So what happens when, somewhere in the multi-link chain of audio signal handling, including in and out of several digital audio buffers some of which are in satellite or cable digital receiver/decoders, one channel is momentarily delayed or its phase is temporarily flopped until the problem is recognized, sometimes automatically by the piece of equipment where the problem originated, and corrected? The PL processing sees the dialog portion of the signal as being present in both L and R channels, but not in phase. If you were a PL processor, where would you send this out-of-phase dialog signal?

This is a 2-channel mix encoded with pro logic. Unfortunately, I cannot get HD. But the cool thing is my apartment complex provides free satellite TV. Basically they get a few channels over-the-air and those are in surround if the show is broadcast that way, and my PBS shows have almost always been in surround. That's about it. Everything else, almost all the data comes out of the center channel, so I assume those aren't in surround.
I would've attributed it to the program, but I just hadn't ever noticed it with my previous processor. But that doesn't rule out that it wasn't the program.
This evening I'll try to watch a little PBS again, and also maybe one of the other channels I get in surround.

That's what I was thinking of initially. It's pretty typical for pure stereo signals to "collapse" to the center when Pro Logic processing is applied (although Pro Logic II isn't as bad an offender in this regard as the original Pro Logic was). BB4TB's explanation of matrix decoding is an excellent one and offers a likely explanation for what you were encountering.

It is very likely that it is just coincidence. Many stations are reducing staff right now, and when they send an intern in to swap equipment and a mistake occurs it can go for a long time without being noticed.

I can't even begin to describe the issues the 'local' WB has around here. They have a ground loop at the studio that is passing the hum through to the cable feed. Only that channel has the hum. :rolleyes: Been going on for weeks now. They also have issues with whatever machine records the feeds for their syndicated shows. Every few seconds there is a glitch where the audio speeds up and down for a fraction of a second. Weird stuff. No other channels do this so I know it is the WB.

So it may have been something that was in both channels, just out of phase, therefore routed to the surrounds, and in this case then a problem with source material.

I would think so. That is the problem with "finding problems" when watching broadcast TV. There are so many things that go wrong at the individual stations that I hate to trouble shoot via braodcast TV. My ABC affiliate has the worst picture and sound in the world. All other stations can come across great but if you tried to adjust to that station it would drive you nuts.

By the very nature of M-S processing, the S (or side channel) has a lot of out of phase information in it. If the TV station is doing M-S processing, and someone plugged the M into the S feed it could very well send your center channel into the surrounds.