Axiom Home Page Forums General Discussion Home Theater 7.1 question

When you have a receiver that is 7.1 when you play 5.1 or 6.1 material through it what information gets sent to the extra set of rears is it seperated info or just more of the same that is going to the other set of rears.Thanks for any input.
Jake

Jake

If you are just playing the source without any add'l receiver "processing", when you play a 5.1 source, you will not get any sound out of the rear speakers. When listening to a 6.1 source (DD EX, DTS-ES matrix/discrete), you will hear the same material in both rear speakers.

That said, most receivers will allow different modes to be applied. I don't bother with these, just play it as it was intended to be heard.

Alan

Dolby Pro-Logic IIx ("DPLIIx") will allow you to take both 2.0 and 5.1 material and "extend" it into the 6th and 7th speaker, using some pretty good stearing technology (phase related information in the surround and main channels).

While you will never get "more" sound from this matrixing technology, "where" the sound comes from will change. That is, the technology is not extracting any sonic information not otherwise exisiting, and ostensibly playing audibly, through other speakers in your set-up. What it does, though, is analyzes the sonic information in the surround channels (4th and 5th), compares them, and then figures out which sound should be coming from in between the two speakers. Where a sound is equal in strength in both rears, the 6th and 7th channel will play that sound in "mono" and, if the stearing technology is working properly, the sound will not be heard out of the 4th and 5th channels (some echo or reverb might still be heard there - but it will sound more or less "discrete" in the 6th and 7th channel"). When the signal is the same in both 4th and 5th speakers, but its level is different, depending on the difference in level, a varying amount of sound will be piped through to the 6th and 7th speaker.

Thus, for example, lets say the sound of a helicopter is heard equally in the rears ("A fly-over occuring directly over the listener's head) - the processor will figure that that the sound should only come from the rears, and will play the sound back in equal strength in both the 6th and 7th rears. If the fly over is a "panning" effect (i.e. from rear left to rear right), the engineers will, in all likelihood, slowly increase the sound occruing in the left rear and gradually increase the level of the signal in the right surround. The DPLIIx processor will take this sonic cue and place the sound in the 6th channel, then the 7th channel and then the rear right channel, for what will sound like a seemless pan.

Got the theory?

Getting it to work properly is a different matter. There will likely always be some bleed back into the side surrounds (4th and 5th), and the 6th and 7th channels are rarely totally discrete. But, it does allow for a great amount of seemless panning, much like your front channel allows for a seemless pan up front.

Before the ubiquitous advent of the center channel, stereo "phantom" centers created this pan in much the same way - by gradually increasing the sound in one channel, while slowly reduing the sound in the other channel. Other sonic cues, such as echo, reverb, etc. help enforce the effect. Our ears interpret this change in volume, phase, etc. as sonic "movement", but it is nothing more than an illusion. The center channel takes a lot of the guess work out of the pan, just as the rear surrounds do.

So, in response to the above poster, who prefers to listen to the source "as it was intended," I would argue that the sound was "intended" to be discretely located "panning" or more fully enveloping in the rear and that mostly, this is done with phase and volume tricks, relying on the listener's ear to figure out the cues and process them similarly. The addition of the rear speakers, I believe, helps us to better hear the soundtrack in the manner in which it was intended, just as the center channel does on stereo 2 channel movies.

Thanks Capn that is what i was looking for.
Jake

no problem. Glad to help. My 7.1 set-up cefrtainly isn't the best (rears are too close to me, as the couch has to be butted up against that wall), but in a larger room, the effect can be very convincing and really help to expand the surrounds "around" you.

I hope maybe some others jump in with their impressions

Jake, just following up a bit on Adrien's excellent explanation of surround processing, it should be kept in mind that in the concert hall the majority of sound reaches us as reflected ambience from the sides and back, rather than as direct sound from the front. Extracting this ambience which is present to varying degrees in all recordings, but is confined artificially to the number of channels available in the format, and then sending it to the surrounds where it belongs, increases the reality of home listening. We're not confined to the number of channels as were the producers of the recording; the Stereo Police won't bust us for using too many speakers.

"Extracting this ambience which is present to varying degrees in all recordings, but is confined artificially to the number of channels available in the format, and then sending it to the surrounds where it belongs, increases the reality of home listening."
John,
Wouldn't this only work though if the recording had been made using some sort of surround mixing in mind? Ala DVD-A/SACD? If it is just a 2 channel recording, the processor has no idea what signals are intended for the rear's right? So it is just going to send the entire signal back there, and in an actual listening environment, not all of those signals would be reflected, right?
I'm really asking, not trying to correct...

And Adrien, since that was such a great explanation of what a 7.1 system is doing, I have a question to you.
I just bought this 2500, which has 7.1 capabilities, but I have no wall at the back of my listening area (it opens into the kitchen behind the livingroom). I have been considering setting up a pair of wires hooked up to the rear speaker terminals that I would hide under my shelves that the system is on. Then, whenever we wanted to watch something in 7.1 I would just take my M2Is (that I have on top of the shelves for the front effects) and set them on the counter that separates the livingroom and the kitchen, stands and all, and hook up the wires from the rear surround outputs.
Kind of a long-winded intro to this question;
Would there be any sonic issue with them not being on a wall? Or, to ask it a different way, would there be any issue with there not being a hard surface behind the speaker set-up for sounds to reflect off of?
Since I got this reciever I have thought that this would be an easy way for me to have my cake (my front effects DSPs w/ the M2Is up front for the odd CD that I like the DSPs for) and eat it too (plopping them on the counter for 7.1 soundtracks). BTW, the rear surrounds would be about 2 feet above our heads, and about three feet behind us.

>>Would there be any sonic issue with them not being on a wall? Or, to ask it a different way, would there be any issue with there not being a hard surface behind the speaker set-up for sounds to reflect off of?

For reasons I don't... um... remember... bookshelf speakers often sound better when they are pulled way out into the room away from the back wall. You lose some sound reinforcement from the back wall so may need to set the levels up a couple of dB but you'll need to do level setting anyways.

I think this would work quite well.

Tom, actually that's exactly what the better surround processors(e.g. DPLII, Logic 7)can do with 2-channel material, i.e. they "know" what should stay in the front speakers and what should be extracted and steered to the surrounds(not with 100% perfection, of course). This is because when a sound reflected from a surface reaches the recording mike(or a listener)a split second after the direct sound it's out of phase with the direct sound. The surround processor can measure these phase differences. Slightly oversimplified, direct in-phase material on the left channel stays in the left front speaker, out-of-phase material in the left channel is extracted and sent to the left surround speaker; the right channel is processed similarly. Another process is that direct sound equally strong in both channels is extracted and sent to the center speaker, forming a real center(adjustable with the DPLII center width control)rather than the "phantom" center that we experience with those sounds when listening with two front speakers.

The amount of ambience present in 2-channel material varies depending on how closeup the recording mikes were and whether "ambience mikes" farther back were also used. A very closeup recording with less ambience present will benefit less from surround processing, but to some extent using the dimension control on DPLII to increase the sound directed to the surround speakers can help.

John,
Very interesting.
I am going to have to start experimenting with these modes with music.
I've always been a big fan of Yamaha's DSP technology, hence my use of M2i's for a set of front "presence" speakers. But I never knew that some of these programs could do what you are talking about for surround technology for 2 channel music.
Interesting.
Thanks!