Nordic Breeds WA4VZQ wrote:
I am sure Scott Dorsey knows more about this than I do, but CBS produced
two devices called the Audimax and Volumax that shifted the phase of the
audio as a function of frequency. Another term for these devices is
phase rotator. Kahn Communications also was in the market with its
SymmetraPeak. To the ear, the sound was unchanged, but to the
transmitter, the peaks became symmetrical. There is an excellent
discussion of these devices on James Tonne's (W4ENE) website
http://www.tonnesoftware.com/appnote...s/allpass.html and on Gary
Blau's (W3AM) website http://www.w3am.com/8poleapf.html. {A biased
opinion here — Jim's site contains some _excellent_ free software.}

The original Audimax/Volumax combination had no phase rotator. I worked
at an AM station that used them, and the chief engineer had installed a
phase reverse switch on the announcer mike and auditioned each announcer
to tell them which position to use. (Apparently they had used figure-8
mikes a year or so before I got there, and the announcers just used the
front of back of the mikes).

A lot of stations using the Audimax/Volumax would also have a phase
rotator in the chain, though. CRL made a popular one, and so did Garron.
Some folks made some boards tht dropped inside the Volumax for it too,
but I never used any of those. I went to the Optimod as soon as I could,
and it has a great phase rotator.

The phase rotator is a hell of a great gadget, it gives you a lot of loudness
without any perceived distortion. Mind you, for communications applications
it's no more effective just than aggressive clipping, but there are folks
who don't want aggressive clipping.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."