I wrote: > > >> (if anyone is interested in more details on that process, let me know >and I'll post it). >
Mark replied: >Yeah, post it, I for one am interested, or e-mail me if you don't feel like >posting to the whole list. > You asked for it, you've got it. And I promise I'll get to the part about the Silent Black UN Helicopters(tm) mentioned in the subject, and also luxury cars. Seriously. Disclaimer: This is all from memory, therefore I might get some small details wrong here or there. IIRC, some of this might have come from the Dolby website originally. Before I can explain how phase is used to fold four channels of audio into two audio tracks, there are some basics that I need to cover. Some of this is *greatly* oversimplified, and a lot of it some of you know. 1) What is sound? Basically, sound is our perception of changes in air pressure at our ear drums. That change in air pressure causes our ear drums to move back and forth triggering a chain reaction resulting resulting in us "hearing" what has caused the change. 2) What is a wave-form? Most of us have seen a picture of a sine wave with a line running through the middle of it, touching the beginning of the wave and the end of the wave. This is a graph of air pressure (the vertical axis) over time (the horizontal axis). The center line is the room with no sound. Any distance above the center line represents an increase in air pressure (compression), and distance below the center line represents a decrease in air pressure (rarifaction). If you increase the vertical size of a wave-form, it is perceived as an increase in loudness. If you increase the length of the wave-form, you are decreasing the number of times that wave-form can repeat itself, giving it a lower frequency (measured in Hertz or cycles per second), resulting in our perception of a lower pitch. 3) What is phase, in reference to audio? Phase refers to the relationship between two waves. If you have two sine waves of the same amplitude and frequency, and they start at the same time, they are said to be completely in phase with each other, or zero degrees out of phase. If one wave starts half way through the cycle of the other wave (the graph starts moving up for one wave while it's moving down for the other), they are anti-phase, or 180 degrees out of phase. There are other possible phase relationships, but the only ones we are interested in for this discussion are zero degrees and 180 degrees. 4) What is phase cancellation? If you have one wave that is trying to compress the air a certain amount, and an identical wave trying to rarefact the air by an identical amount, the result is no change in air pressure, or no sound. In other words, waves that are 180 degrees out of phase with each other cancel each other out. This is how some luxury cars manage to be so quiet; they have small microphones around the outside of the car to pickup the outside sound, then the phase invert that sound (make it 180 degrees out of phase) and adjust the EQ to make it sound like it would once it came through the car body, then play that anti-phase or phase-inverted sound through the car sound system and that cancels out the sound from the outside. Silent Black UN Helicopters(tm) would work on the same principle, only the sound of their blades and motor would be phase-inverted and played through speakers on the outside of the helicopter. OK, now that that's taken care of, on to the good stuff. :-) The phase relationship of certain sounds is used in analog surround sound systems (like Dolby Pro-Logic) to fold 4 audio tracks into 2. Front center speaker: The dialog traditionally comes from a speaker in the front of the theater, usually behind or directly above or below the movie screen. If you have any sound you want to go to the front center speaker, you pan it exactly center when mixing. This means that the dialog will be exactly in phase in both the left track and the right track. Any such info is also removed from the left and right track, so it goes *only* to the front center speaker. Surround speaker(s): Any sound that should go to the surround channel is panned all the way to the left (hard left) during the mixing process, and then a copy of that sound is made, phase-inverted, and sent to another channel on the mixer and panned hard right. During playback, the system detects these sounds as being 180 degrees out of phase with each other, and sends them to the surround channel. Even though there are usually 2 speakers in the surround channel in home systems (more in movie theaters), these speakers carry the same sound. Usually they are wired anti-phase from each other (or at least the tweaters are wired anti-phase). In other words, if you had a yellow cable going to connector 1 of surround speaker 1, and a blue cable going to connector 2 of the same speaker, then on speaker 2 you would have the blue cable going to connector 1 and the yellow cable going to connector 2. (This is not how it is actually handled with home systems; usually the wires are connected the same but the tweater element is wired anti-phase inside the speaker.) Since the surround speakers are anti-phase, they cause phase cancellation in the air. If these speakers were sitting next to each other, the sounds would sound like they were coming from outside left of the left speaker and outside right of the right speaker. Sounds sent to the surround channel are also removed from the main left and right signals. (Also, to be certified for Dolby Pro-Logic, a home system must also apply a specific EQ and delay to the surround channel -- or is that for THX certification? I can't remember. :-) Front left and front right speakers: After the sounds going to the front speaker and the surround channel are removed from the main left and right channels, the remaining sounds from left and right tracks go to the front left and front right speakers, respectively. So what happens if you're listening to something encoded for analog surround sound but only have a front left and a front right speaker? Anything that is panned center will sound like it's coming from directly between the speakers. Anything that is anti-phase between the two speakers will cancel in the air, and sound like it's coming from to the left of the left speaker and to the right of the right speaker. Broadcast televisions shows that are in surround sound are in analog surround, and function this way. It is possible to listen to surround sound on four or five separate speakers without having a decoder. WARNING: This will only work if you have a bridged amp (most amps aren't). If you try this with a non-bridged amp, *you will blow up your amp*. If you have a bridged amp, and the speakers plug in by having two separate leads on each speaker wire, then you wire up the front left and front right speakers as per normal. Then for the front center speaker, the one that has everything that is in phase between the two channels, wire one of the leads into the top connector for the left speaker and the other lead into the bottom connector for the right speaker (or vice versa). For the surround speaker(s) hook one lead into the top connector of the left speaker and the other lead into the top connector of the right speaker. All of surround speakers can be hooked up this way, but if you want two that are wired anti-phase, hook one up to the top connectors, and the other to the bottom connectors. Once again, if your stereo amp is not bridged, *YOU WILL BLOW UP YOUR AMP* if you try this. Of course, there are some tv sets that wire their stereo speakers antiphase (or have a chip that phase-inverts one speaker or at least the treble from one speaker) to give everything a more spacious sound. Analog surround actually sounds *worse* on those televisions because the surround channel is being brought back into phase and now sounds like it's coming from the center... I hope that explains things clearly enough. If anyone would like a clarification on any of this, just let me know. Reggie Bautista _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp.
