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Out of Phase surrounds
       The Pro Logic Gold Standard.
     
     The MACINTOSH Matrix Decoder seen below top center is The Pro Logic "Gold Standard".  It
contains an SSM-2125A.  I think the design was by SSM,  Analog Devices did the fab.  

     There was a die shrink.  As I remember,  there was  a bit too much shrink and the headroom
died,  more than expected or desired.  

     
Before the headroom issue was know about, the entire stock of original 2125s  were
destroyed.  Because we had tested and approved the original chip,  we had a few rails of the
engineering samples.  But all original production versions were history  There was no fall back
option for AD.  With the exception of headroom,  the SSM chip was/is what we wanted Pro Logic
decoders to sound like.

     I made a 6 channel A/B switch that  allowed me to compare a DUT with "The MAC".          

     For one decoder IC that I was testing,  I listened to a 1 minute clip from Back to the Future,  
126 times one evening and over 250 more times the next day.  Pad of paper, "O", OK, "X"
problem.  The chip only failed 37% to 43% of the time, else OK.  When it burped, it blew chunks.  I
had to develop a new test and test hardware.  This allowed the problem to be duplicated in
Japan.  Something they had not been able to do.  Using my mew test setup it was found that an
internal IC trace was too small.  The chip needed to be re-masked.  

     Before I made the new test we had no way to prove our concern with hard data.  With
millions of dollars at stake.  That new data changed what was getting to be a difficult political
instance back into an unfortunate technical one.