From: john@zygot.ati.com (John Higdon)
Subject: Re: Am stereo
Originator: wdp@gagme
Organization: Green Hills and Cows
Date: Thu, 8 Oct 1992 22:44:35 GMT

On Oct 6 at  0:57, Bill Pfeiffer writes:

> What benefit would KEEN gain by dropping the 'existing' stereo?
> Mr Higdon mentions the existance of competeing FM in the market programming
> country.  I personally see no benefit in eliminating the existing stereo,
> even though keeping it might have only marginal benefit.  I'm sure John will
> set me, and us, straight in short order :-)]

It boils down to a cost+effort vs benefit ratio. Here are some stereo
AM facts of life:

The "standard AM" portion of the chain must be super circumspect.
Harmonic distortion must be in the low single-digit department as well
as the IM (particularly what is known as "IQM") distortion. The former
is relatively easy; the latter is much more difficult. Everything from
power supply anomalies to tank and antenna tuning affect IM. Your
typical AM station has a two-digit IM distortion figure. This is
intolerable in stereo since it shows up as "phony stereo".

The C-Quam system (among others) was designed to operate optimally with
a synchronous detector. And, happily, most stereo receivers are so
equipped. However, virtually the entire mass of standard radios
use the common envelope detector (and usually half-wave at that) which
chokes on the intentional carrier phase shifting that carries the Q
channel (L-R). The amount of distortion on a standard radio is
therefore roughly proportional to the amount of stereo separation in
the program material. (An extreme case is the loss of one channel; mono
radios go to mush.)

Some stereo radios (notably the Ford-Philco units) cannot tolerate
any carrier pinch-off. This means that a safety margin must be created
so that the carrier NEVER actually reaches 100% negative modulation. To
compensate for this conservatism, the signal is usually squashed
heavily. But then, many mono radios object to Q channel phase shift AND
a sustained "greater than 90%" modulation. So we back it down even more
to around 85%. All of this is necessary due to receiver problems, but
remember: radio is a closed system. It requires and transmitter AND a
receiver.

And finally, the phase, level, and eq relationships between the I
channel and the Q channel are very critical and unstable. As tubes age
(not all of us are blessed with solid-state transmitters), so does the
signal they amplify. Also, changes in the antenna system due to aging,
moisture, and temperature radically affect the stereo system. None of
these things are noticed with a simple mono transmission. A few percent
difference in the IM is undetectable in mono; the same change in stereo
can mean the difference between pleasant listening and audio torture.

In essence, stereo AM is a lot of work. Those within the listening area
of KEEN know that I am not a slacker and do what it takes to make it
work. But to what end? No one notices or cares. There are so few
receivers out there that the whole exercise seems pointless. It has
been fun and enlightening, but now it is time to cater to the bulk of
the radios which employ half-wave envelope detectors. It would be fun
to just modulate at 100% again and be done with it!

-- 
 John Higdon  |   P. O. Box 7648   |   +1 408 264 4115     |       FAX:
 john@ati.com | San Jose, CA 95150 | 10288 0 700 FOR-A-MOO | +1 408 264 4407

Newsgroups: rec.radio.broadcasting