I've commented before that this Maryland* company made a device that does what SDA does but at line level. It was used in a tape loop like an EQ. It also later showed up in some Radio Shack EQ's. I always felt it may have been at least a partial inspiration for SDA. Here are links to the patents. The designer Paul F Bruney also has 6 speaker patents. Here are links to them. I'm posting this because I think its interesting and germane.
* the first patent in 78 was done when he lived in New Orleans
SDA like circuit:
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO %2Fsearch-adv.htm&r=8&p=1&f=G&l=50&d=PTXT&S1=bruney-paul$.INNM.&OS=in/bruney-paul$&RS=IN/bruney-paul$
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO %2Fsearch-adv.htm&r=7&p=1&f=G&l=50&d=PTXT&S1=bruney-paul$.INNM.&OS=in/bruney-paul$&RS=IN/bruney-paul$
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO %2Fsearch-adv.htm&r=6&p=1&f=G&l=50&d=PTXT&S1=bruney-paul$.INNM.&OS=in/bruney-paul$&RS=IN/bruney-paul$
Link to all patents including speaker patents:
http://patft.uspto.gov/netacgi/nph-P...A%0D%0A&d=PTXT

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