quadrature audio

[The Dark force of dance]

Y-ellow Martin.

At 02:12 PM 04/14/99 +0200, Martin Czech wrote:
>By the way:
>
>what could be usefull applications for a wideband audio quadrature
>circuit?  Ie. a circuit that offers some delayed versions of the input
>and where the outputs have (allmost) 90 DEG phase difference?
>
>So far I only know about one application : Single Sideband Modulation
>a la Weaver.

That's exactly what you need for Dolby Prologic encoding as it happens. The
surround channel is encoded by creating 2 90degree versions of it self
which are 180 degrees apart. This is then added back to the left and right
signal. The decoder interprets this as a difference signal initially and
some discrimination sorts out the surround signal. Together with a
discriminator that isolates the mono component for the centre channel,
these 4 signals are then used to drive voltage followers and onto VCAs. The
VCAs are used to steer the rear channels so that they appear to be stereo.
They are not!

In fact there's nothing terribly new about this stuff. Which is why Dolby
Labs could neither patent it nor keep it a secret. The only thing they can
do is prevent you from using the name. "Dolby Prologic".

And in reality, because Dolby prologic surround is such a fudge in the
first place, virtually any scheme to add more speakers works quite well.
The number of speakers possible is only limited by the number of ways you
can think of tacking more on. But it all starts with a near constant 90
degree phase shift. Dolby Prologic must be one of the few systems out there
where the encoding process is far simpler than the decoding.

Hope this helps.

be absolutely Icebox.

 _ __        _                              
| "_ \      | |         batzman at all-electric.com / aek at all-electric.com
| |_)/  __ _| |_ ____           ALL ELECTRIC KITCHEN               
|  _ \ / _` | __|___ |  The Elementary Urban sanity CD
| |_) | (_| | |_  / /   out now on Transmission Communications
|_,__/ \__,_|\__|/ /    
                / ,__   http://www.all-electric.com
Goodfortune    |_____|