multi channel effect from 2 channel tv

[JGklipsch]

Some tv shows say they are broadcast in surround sound. I was reading a post that said you can get multi channel effects from a two channel tv receiver if the broadcast has the multi channel signal. Can someone explain how this can be done?

[wife-said-no]

2 ways. The first is to use y sonnectors to make 2 cables into 4. The way that I think you are really talking about is in the electronics itself.

This way the processor controls the way that speakers are played in order to "project" a "phantom" signal. My Sony reciever does this in many ways. The first is to make surround channels when there are not any surround speakers.

I think that it all has to do with the reverberation time and a few other factors. I am not really all that knowledgeable in the exact breakdown. I can tell you that the virtual cinema on my Sony adds fantom side surrounds to my reart and it sounds really good.

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Come on honey why can't I spend some more money?

[dougdrake]

The way I do it is by running the signal through the TV tuner in my stereo hi-fi VCR, and sending the audio from the VCR to my a/v receiver's Dolby Pro Logic decoder. That gives me surround sound.

Doug

[JGklipsch]

I don't have a hi-fi vcr but I think that is how it would have to be done. To get multi channel surround sound (as opposed to virtual surround from the receiver)I assume you need to have a multi channel broadcast and a decoder that can receive the signal. Any way of using the decoder of the a/v receiver for tv reception?

[ERIK]

If a TV program says its broadcast in surround sound that means it is broadcast in Dolby Pro Logic. A Prologic decoder takes a two channel signal and decodes the other channels from it. A Pro-logic signal can be decoded into 4 seperate channels left, right, center and surrounds. Unlike Dolby Digital 5.1 the surround speakers in Pro-Logic are a mono signal.

My limited understanding of how Prolic works is this:

If the decoder sees a signal just in the left or the right the soung goes to the corresponding speaker.

If the decoder sees identical signals in the left and right the sound goes to the center channel.

If the decoder sees identiacl but ut of phase signals then the sound is sent to both surrounds.

Once again this is an over simplified version of how it works.

If your TV has audio outputs you should be able to send this signal to your reciever. Or give you VCR's audio outputs a try.

[dougdrake]

Good comments, Erik. To clarify, your TV or VCR audio outputs (whichever you are using to feed the audio to your receiver) must be stereo for it to decode the Dolby Pro from the signal.

[WMcD]

I believe most of the following is correct. I'm trying to dispel some misconceptions. I might be wrong in some minor points. Take it with a grain of salt.

Point 1. Yes, you can feed the t.v. output to an HT receiver, and it should work well.

Point 2. I think the question is "how can this be a "multi-channel" system when we really only have a two channel audio transmission system? This is true in f.m. stereo, vhs tapes, cd's, or even stereo vinyl. There are two channels. Period.

The answer is that we have a two channel storage and transmission system in many cases. Please consider that storage or transmission are essentially the same from the user end. (DTS and digital dolby are true multi channel. Maybe 5 or 6 discreet channels, like 5 or 6 phone lines. We are not discussing those here.)

The hardware under discussion is pretty simple. There are two channels of audio feed to the user. Yup, the little RCA connector for "left" which is white, and an "right" (the little RCA connector which is red). No joke; these are two channels of audio. If you hooked up a pair of head phones to them, you'd be hearing all the real signal. No hidden mystery channels. What you hear is what you get.

Nonetheless, we can fool around with the signals. We can add and subtract to make a L+R channel (for the center) and a L-R for the (surround channel). This is called a matrix in some publications. A poor choice of terms which just obscures what is going on.

Lets ignore "steeing logic" and just look at power and math. This is simply analog systems.

A. When two identical a.c. voltages (two sine waves in phase) ADD, we get twice the voltage or four times the power. This is 6 dB gain over one signal alone. P=+V*+V/R.

B. When we take two identical sine waves and reverse the polarity of one, they cancel entirely. This is SUBTRACTION There is no voltage and no power. P=+V*-V/R =0.

C. What happens when we add two a.c. signal which are random in phase? We get twice the power, or a power gain of 3 dB. This is kinda like having one raincloud putting out raindrops in a random way. If you add another raincloud, you get twice the rain. 3 dB power. However, the drops don't fall on top of each other to add up.

This is a bit of a strained analogy. We never have cancellation of raindrops like we do with reverse polarity a.c. signals. On the other hand, some of random a.c. in the two channels are fractionally out of phase and might add a bit, or subtract a bit.)

Lets look at the decoding end (at home) which just has a "processor" which has an input of a L channel and a R channel. This is total analog. The outputs can be stated in terms of input as L, R, L+R, and L-R. You might say, "Yeah, now we have four outputs, but it is not 4 channels, I just see L's and R's." Ya can't fool me. And you're correct. However, the power equations A,B,C above, tell us some things which might not be obvious.

Lets take the L+R Output and feed that processor output to the center main channel. Lets assume the a.c. signal is identical in the L and the R transmission/ storage channels. (Only two of them, natch. My main point.) The power being sent to the center channel is 6 dB higher. Ha. We have to turn down the center amplifier by 6 dB if we expect equal power to the three front channels.

What happens when there is just an L signal voltage into the processor as received from the storage/transmission system. The accoustic power comes out full strength from the left speaker, and it is 6 dB down in the center channel speaker. So, our acoustic output is mostly from the left channel speaker, something from the center, and none in the right. There is "leakage" but it is not too bad.

Same thing happens when there is just a signal from the right channel of the transmission/storge system. Again leakage, but it is not too bad.

PWK calls this a phantom center channel. The movie companies used the set up for years. Bell Labs tested the concept it in the '30s. It is "phantom" because we don't actually have a center channel in the transmision or storage channel. But, it can be created or re-created.

Okay, what can we do with the L-R processor output? We imagine an input to the processor which is identical a.c. signals, but one is reversed at the source of the recording or transmission in polarity. Let's call it a positive in the left and the same signal but reversed in polarity in the right. The processor is going to subtract the reversed right channel input. But if the right input is already reversed, now it is reversed again, and it is back in phase.

Therefore, if the people making the transmission or recording have planned properly, the L-R voltage encoded input signal appears strongly at the processor's L-R output. You can think of this as being L input minus a negative R input, which comes out of the processor as a strong representation of L-R.

This L-R input to the encoding is the "surround" output of the decoder. Of course it doesn't appear at the L+R center speaker at all because the two voltages cancel. And note the L+R, which is supposed to be in front, doesn't show up in the surround. It is cancelled.

Now you've spotted a problem. The L part and the R part of any signal is going to show up in the surround L-R. How do we make the brain think that the surround speakers are not putting out "real" L or "real" R from the front.

The answer is that we can use some electronics to assist our ear/brain in interpreting the output of surround speakers as not being the true source of direct sound. Essentially, the output to the surround amplifier as delivered to the surround speaker is delayed by at least 10 milliseconds. This means it "sounds" to be at least 10 feet farther away. So, quite correctly, our ear/brain calculates, the L-R output must be distant. However, the undelayed R or L from the front is the true source. This is a balancing problem of intensity and delay. Whales and SSBN's might have better internal computers. Still, if there are two sources of sound and one arrives late and is a bit faint, it is probably an echo and/or from far away.

The next question should be what is going on at the encoding end. The recording engineer for Monday Night Football, or Leno, or Star Wars, wants to take advantage of the processor and our set up at home. One thing the engineer can do for "surround" is put a microphone in the stands. Just a mono signal from the mike. Then the engineer puts the microphone output into the left channel which gets recorded or transmitted. The engineer also flips a switch for the feed to the right channel so that it is reversed in polarity going into the right storage/transmission channel.

Oh, for the commentators, they go into both the L and the the R transmission channels with no polarity change. The band goes into the left channel, solely. The cheerleeders go into the right, solely.

To reiterate, we only have two transmission/storages channels. However, at home, the additive and subtractive processor does its work. The commentator's sound add and are mostly heard in the processor's output (L+R) center, they're not expressed in the surround at all. Band is at the left. To the extent there is some leakage into the surround, their music is delayed and our brain tells us, the band is left, the surround is just an echo. The same for the cheerleaders.

Surround from the microphone in the stands? Because it is L-R in the encoding, it is most stong in the surround decode. And since the decoding system has delayed it, our ear/brain tells us it is from far away, even if the microphone is in the middle of the crowd.

Next question is: Why do they call it "5.1 Dolby sound." We've only discussed, above, a 4 channel system of encoding and decoding with all these pluses and minuses. How do all these numbers fit together?

The answer is that the outputs, note OUTPUTS, are under considered. There is the front left (L) and right ® and center (L+R). That is 3. There are typically two surround speakers even though they are being sent an extracted L-R signal. That makes 5 total. The 0.1 is when we add a subwoofer to simply extract the bass tones and send those to a speaker better able to re-create them.

Whoops, someone in the back of the room just asked whether HT systems can be used to play back recordings where there was no effort to mix this Left and Right and Center and Surruound when putting down a two track (channel) recording. Will our HT decoders do good things?

The answer is that even if it was not intended during the recording, there is probably some "information" in any two track (stereo) recording which will be decoded by an HT system in our homes.

If the two recording microphones were spaced in front of stage, the center channel decode is going to place the center performs pretty well. This was PWK's conclusion decades ago. His little network of resistors feeding a center channel amp, feeding a "Model H" as described in a Dope From Hope, does the L+R, as in an HT system.

The next question is whether there is "surround sound" or L-R in old recordings. Yes, there is, potentially. It is not L-R (planned), but it is random. This is why I described the 3 dB gain of random phase, versus 6 dB of in phase, above.

Hall sound is going going to be random. So to the extent any hall sound gets into the front center, it will not be getting expressed strongly. The in phase signals (center) will be expressed stongly. Therefore we have a situation where the recording engineer created a recording where L+R is stong because of microphone placement, and reverberant sound is always less.

What is the lesson, even if the above is confusing?

First, storage and transmission channels are often two channel systems at this date. It is a misconception that there are hidden channels for transmission when they say "surrond" or "Dolby."

Second, the addivive and subractive processes can allow us to extract information from a two channel system, nonetheless.

Regards,

Gil

Sorry if I've created confusion.

[boa12]

yea u plug a stereo component into an a/v receiver that has at least dolby prologic decoding & ahhlaaa. sorry but it's that simple