[BC] FM Stereo invention

[Bob Tarsio]

The mathematics of stereo wave modulation can be applied to both methods and
the result is the same. I don't think that with today's components there is
a real difference in performance between the two methods. The balanced
modulator method provides excellent results without the need for the alias
filtering at the output that switching modulators require. Phase linear
filters were difficult to synthesize back in the early 60s which is what
made the balanced modulator method novel. Today phase linear filters to do
the job are available in integrated circuit form for a couple of bucks. I
honestly think that most of the deficiencies in stereo mod/demod occur in
the receiver. Cheap demodulator circuits often provide adequate results but
fail to provide what was available at the transmitter. On the transmit side,
matrix matching is important for the balanced modulator method but again
with precision components available today not really very hard to achieve
good results. The parallel to today is MPEG encoding. MPEG encoders are
expensive and provide excellent compression. Many decoders are cheap and
provide adequate performance but don't always achieve levels of performance
that a more expensive decoder would. The same logic can be applied to stereo
generation. Stereo generators are relatively expensive compared to the
demodulator circuits found in most receivers. The deficiency in performance
is likely to show up where less money has been spent. 

What spoils stereo separation at the transmitter side is inattention to
detail in stereo generator setup, path anomalies from the stereo generator
output to the exciter and termination at the exciter. Of course the exciter
and transmitter can have an effect but most of the time it is misadjusted
stereo generators and poor path performance that ruin the numbers. 

Bob Orban's original 8000 and 8100 Optimod stereo generators used the
balanced modulator technique elegantly with no real penalty in stereo
separation. Broadcast Electronics used a novel method of digitally
synthesizing the 38 KHz signal read from a sine ROM. The method for
generating the composite waveform was essentially balanced modulation as the
sine ROM values were multiplied by the incoming reference which was the L=-R
audio. Again, that original BE stereo generator provided excellent numbers.
It was also a novel hybrid approach to stereo wave generation. 

Bob Tarsio
President
 
www.Broadcast-Devices.com
 



-----Original Message-----
From: broadcast-bounces at radiolists.net
[mailto:broadcast-bounces at radiolists.net] On Behalf Of Dennis Cope
Sent: Monday, February 19, 2007 12:54
To: Broadcasters' Mailing List
Subject: RE: [BC] FM Stereo invention

Willie..

In the lab I got much better stereo separation with the TDM method over the
Matrix.  Especially in the receivers demux.  IMD and THD was also better.

Dennis
WESR, WCTG, NOAA



-----Original Message-----
From: broadcast-bounces at radiolists.net
[mailto:broadcast-bounces at radiolists.net]On Behalf Of WFIFeng at aol.com
Sent: Monday, February 19, 2007 12:30
To: broadcast at radiolists.net
Subject: [BC] FM Stereo invention


In a message dated 02/19/2007 12:15:10 PM Eastern Standard Time,
mark3xy at gmail.com writes:

> As most of you know, the FCC-approved stereo FM system was jointly
>  developed by GE and Zenith.  The difference was that Dr. Adler of
>  Zenith viewed it as a time-division-multiplex scheme (L and R
>  alternately sampled at a 38 kHz rate) while Tony Csicsatka at GE used
>  a matrix and balanced modulator to generate the L-R subcarrier.
>
>  He was undoubtedly a brilliant engineer.

The interesting thing is that both of these methods generate virtually the
same result! Some of those FM stereo generators on-a-chip use analog
switches at
38Khz to chop the L and R signals alternately. The end result is a
"standard"
stereo signal. Pretty slick "shortcut".

Willie...

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