From: brown@vidiot.UUCP (Vidiot)
Newsgroups: rec.video,rec.video.production
Subject: Re: Computing Surround Sound
Keywords: An odd question, but...
Date: 17 Nov 92 20:07:42 GMT
Article-I.D.: vidiot.4444
Reply-To: brown@vidiot.UUCP (Vidiot)
Organization: Vidiot's Hangout

In article <1992Nov16.052121.4390@foretune.co.jp> trebor@foretune.co.jp (Robert J Woodhead) writes:
<brown@vidiot.UUCP (Vidiot) writes:
<>In other words, take the audio you want for the rear channel, put it into
<>the left channel, take the same audio, 180 degree phase shift it and put that
<>result into the right channel.
<
<Ok, so what you are saying is: if I have 3 digital datastreams for L,
<R, and Rear, the left output stream is L+Rear (ie: add the values of
<the 16 bit samplings) and the right channel is R-Rear.

Yep.

<Ok, that seems easy enough.  Now for the next question:  Is it possible
<to trick the ear into thinking that the sound is coming from, say,
<the left rear or the right rear?

Even though there are two speakers for the rear, it is a mono signal, so you
can't get the audio to only come out of the left rear, or right rear, speaker.
Because of that, you can't get a pure left rear or right rear effect.  But,
what you can try and do is try and image the from the rear to one of the three
front channels by changing the amount of the rear effect in the left or right
channel, or both.

As an example...if you want the audio effect to go from the rear to the left
front, you would apply equal amounts of rear to the left and right channels
(taking into account the correct phasing requirement).  Then you would slowly
reduce the level of the rear going into the right channel, until you gave it
no rear in the right channel at all.  The speed of the reduction is determined
by how fast you want the audio to go from the rear to the left front.

This kind of effect is normally used to go from the front to the rear for from
the rear to the front.  ST:TNG does it alot for the "sound" of the Enterprise
going at the viewer or going away from the viewer (starting from behind them).
I've not heard them (or anyone for that matter) do a rear to left/right effect.

Get a Dolby Pro-Logic unit to check out what you are doing.

<I've never had a surround sound decoder (gonna get one soon) but I
<assume you basically have 3 speakers, one per channel.  If so, then
<the speakers are basically a triangle around the listener, right?

Actually, fully blown systems will have five speakers: left, center, right,
left rear and right rear.

I use phantom mode, since I don't have a center speaker (nor a place to put
it).

<That being the case, is it possible to position a sound within the
<triangle by mixing it proportionately into the three pre-encoding
<data streams (ie: a sound that you want to be heard right inside
<the viewers head would be equal on all channels, but this means
<that the right channel would be 0!), or is it the case that a sound
<can either be on the rear channel or somewhere on the line between
<the L and R speaker, or perhaps is there a "dead zone" inside the
<triangle you are not allowed to position a sound in.

Good question.  It would appear that using the numbers that a center triangle
value would be zero.  But, maybe they cheat the flyover stuff by phase shifting
the audio that is placed into the rear, from what is placed in the front.
Maybe that value is 90 degrees.  Just a thought.

<My eventual goal is to write some mixing software that will let me
<position and move sounds and mix them all together, by the way.

Good luck.  We know that it can be done.
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