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Good summary...that was my understanding back in the day. There were 
"logic decoders" and "full logic decoders" and many more marketing words.




On 2017-09-12 8:40 AM, Karl E. Fitzke wrote:
> Thanks again, Eero!
> 
> 
> So if I understand you well, the apparent wider separation is achieved by slightly accentuating any relative difference among the various channel levels at any given time.  The "logic" then appears to be in the time constant ballistics (trigger, attack, and release times) and perhaps also a bound on the amount of sustained gain .  Interesting trick.
> 
> 
> -Karl
> 
> ________________________________
> From: Association for Recorded Sound Discussion List <[log in to unmask]> on behalf of Eero Aro <[log in to unmask]>
> Sent: Monday, September 11, 2017 4:38:28 PM
> To: [log in to unmask]
> Subject: Re: [ARSCLIST] SQ (CBS) Quad Decode
> 
>> I want to learn more about what is meant by "logic" features
> A basic quad matrix decoder provides only about 3dB channel separation
> between certain channel pairs;
> Left Front - Left Rear
> Right Front - Right Rear
> Left Front - Right Rear
> Right Front - Left Rear
> 
> The channel separation is reasonably good only between the two front
> channels and between the two rear channels. It is practically the same as
> the sound carrier, LP, tape...
> 
> To enhance the channel separation subjectively, so called "logic decoders"
> were developed. They use gain riding at all decoder outputs.
> 
> For example, if there is a higher level after the matrix decoding in the
> Front Right channel, that channel is boosted slightly and (in some
> decoders) all of the other three channels are damped down a bit. There are
> certain time constants for the gain changes.
> 
> All four outputs have a similar gain riding system. The system is
> tracking all
> the time the matrix output levels and does the gain riding dynamically
> as the music changes. As a result, the listener subjectively hears a better
> channel separation between all of the four channels than what a matrix
> decoder as such can provide.
> 
> I'm not sure if you'd be happy to do the same with two channel stereo?
> In order to make two channel stereo sound "wider", or to make an impression
> that the sound is coming from two loudspeakers, you would ride the gain
> of the
> L and R mixer faders along with the music.
> 
> Eero
> 
-- 
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada                             647 479 2800
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.