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From: dplatt@coherent.com (Dave Platt)
Newsgroups: rec.audio,sci.electronics
Subject: Re: Extracting Mono from Stereo
Keywords: mono stereo signal processing
Message-ID: <12922@coherent.com>
Date: 28 Oct 88 18:34:44 GMT
Article-I.D.: coherent.12922
References: <341@ivucsb.UUCP> <343@ivucsb.UUCP> <3836@ncrcae.Columbia.NCR.COM> <785@mplvax.nosc.MIL> <4115@teklds.TEK.COM> <13393@cisunx.UU
Reply-To: dplatt@coherent.com (Dave Platt)
Distribution: rec
Organization: Coherent Thought Inc., Palo Alto CA
Lines: 49

In article <13393@cisunx.UUCP> rm@unix.cis.pittsburgh.edu (Mingo) writes:
> On the back of one of Brian Eno's "Ambient" albums, he described his
> "ambient" speaker setup, which was achieved by adding a third speaker
> in back of the listener, which was connected to one terminal of the
> right speaker and one terminal of the left speaker.  Wouldn't the third
> channel have only the part of the signal that was common to right and
> left?

Actually, it will have the portion of the signal that's different...
the ambient speaker is usually connected between the "hot" terminals of
the two front-channel speakers.  The ambient speaker will produce L-R,
or R-L, depending on whether you've hooked the "hot" side of the
ambient speaker to the left or right main speaker.

This system was popularized some years ago by Dave Hafler, I
believe...  he was the originator of the Dynaco component line, and
referred to this ambient-extraction system as "Dynaquad".  A full
Dynaquad system consisted of the two front speakers (connected
normally), and one or two smaller rear-channel speakers connected
between the "hot" terminals of the front-channel terminals of the
amplifier.  A single rear-channel speaker can be connected with either
polarity.  If two rear-channel speakers are used, they are connected in
series with their "-" terminals wired together;  each one's "+"
terminal is connected to the corresponding "+" terminal of the front
channel.  So, the left-rear ambient speaker produces L-R, and the
right-rear produces R-L.

One additional tweak to this setup is to have a small "center-channel"
speaker, whose "-" terminal is connected to the amplifier "-", and
whose "+" terminal is connected to _both_ L and R "+" terminals via a
pair of resistors.  This speaker thus receives an L+R mix signal at a
low volume level, and helps "stabilize" the soundstage and gives better
positioning clues for voices and instruments located at the center of
the soundstage.

Dynaquad isn't a particularly sophisticated system;  it doesn't do the
sophisticated sound-stage processing that today's digital processors
are capable of.  However, it works surprisingly well on well-recorded
live material.  It's inexpensive, too... all you need is two small
speakers for the rear channel, some wire, and an L-pad to control the
ambient-speaker volume level.

Note for those who are thinking of trying DynaQuad: make sure that your
amplifier/receiver uses a "common ground" system, in which the "+"
outputs are driven with the same polarity and the "-" outputs are
held at the same (ground) potential.  Bridged amps, amps that swing both
"+" and "-" outputs, and similar setups may not work with this
system... check your owner's manual to see if there are any warnings
about doing this sort of thing.