David Pickett wrote:
> I dont expect them to ever sound as good as an Ambisonic recording,
> but I bought some SQ-encoded LPs today. I get pleasant results
> playing them out of phase with the same on two rear channels at -6 dB.
>
> My reason for writing is to ask whether anyone here knows what an SQ
> decoder actually did. Despite all the BS Ben Bauer spouted when he
> presented it to the AES in London (or was it the BKSTS?), I seem to
> recall that it wasnt too sophisticated and perhaps, knowing this, one
> can synthesize something better than the above in a DAW.
MAG's distaste for SQ is well documented,
and in December 1977 there was even a fight
between him and Benjamin Bauer on the
pages of New Scientist. (This was refereed
by Barry Fox (writing as Adrian Hope).)
However, in July and August 1977 (refs at end),
Gerzon published the design of an Ambisonic
decoder that included an SQ mode (along with
nine other modes). In Part 1, Gerzon wrote:
"SQ decoders cannot be designed to give
full ambisonic results; there is even a
mathematical theorem to this effect. The
decoder for SQ provided is, however, less
phasey in quality than the SQ designs on
the market, and was designed specifically
for incorporation into this design. It is not
in accordance with CBS Laboratories' SQ
specification, but in the author's opinion, it
is better than decoders that are."
In Part 2, the equations for decoding SQ are
given as:
W'' = 0.73*Sum
X'' = -0.73*j*Sum
Y'' = 0.73*Diff - 0.73*j*Diff
where Sum = Left + Right
and Diff = Left - Right
As far as I can tell, W'' is the W' signal after
the shelf filters (and the SQ mode did not use
shelf filters). Also the W' signal appears to be
the W signal after removal of the Sqrt(2)
weighting. Anyone interested in implementing
this decoder will need to read the refs, which
will be somewhere in the Ambisonic
Motherlode.
Regards,
Martin
Michael Gerzon, "Multi-System Ambisonic
Decoder":
Part 1: "Basic Design Philosophy",
Wireless World, vol. 83 no. 1499,
pp. 43-47 (1977 July).
Part 2: "Main Decoder Circuits",
Wireless World, vol. 83 no. 1500,
pp. 69-73 (1977 Aug.).
later parts never written & published.
--
Martin J Leese
E-mail: martin.leese stanfordalumni.org
Web: http://members.tripod.com/martin_leese/
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