[BC] FM Stereo invention

[Bob Tarsio]

I did a little checking in an old file that I have about FM stereo stuff.
The systems that I mentioned in an earlier post an AM, and SSB system, were
actually one in the same. It was an AM sub carrier with a partially
suppressed lower sideband. The sub carrier frequency was approximately 23
KHz. This was actually a neat approach in an attempt I guess to preserve the
standard 41 KHz sub carrier frequency that we in widespread use at the time.
This vestigial side band approach was similar to NTSC television in that
enough of the low frequency information of the lower side band was passed
along to minimize carrier overshoots of the sub carrier. I am not sure what
effect the constant 23 KHz sub carrier modulation would have had on receiver
performance but it is interesting to think about. 

Bob Tarsio
President
 

www.Broadcast-Devices.com
 

-----Original Message-----
From: broadcast-bounces at radiolists.net
[mailto:broadcast-bounces at radiolists.net] On Behalf Of Bob Tarsio
Sent: Monday, February 19, 2007 14:36
To: 'Broadcasters' Mailing List'
Subject: RE: [BC] FM Stereo invention

Willie:

If you think about it, the balanced modulator method does the same thing as
the switching method under left or right only conditions that you cite. The
distortion generated in the switching method gets taken care of in the low
pass filter that is usually applied to the composite output. 

Some more FM stereo trivia! It's interesting to note the other contributors
to the FM stereo effort. Philco, EMI, and Crosby Laboratories also submitted
systems. There were I believe seven systems proposed in all. Crosby offered
FM/FM, One proponent offered an SSB sub carrier method, and there was
another that proposed an AM sub carrier. What a different world this would
have been if some of these other methods had been chosen. The road not taken
I guess!  

Bob Tarsio
President
 
www.Broadcast-Devices.com
 

-----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 14:23
To: broadcast at radiolists.net
Subject: Re: [BC] FM Stereo invention

In a message dated 02/19/2007 01:58:03 PM Eastern Standard Time, 
Robertm at broadcast.net writes:

> Look at it in these terms. In a switching system where the left and 
>  right channels are sampled at 38 kHz, if the signal is pure mono the 
>  left and right channels are identical and there is no switching 
>  component in a perfect world of zero rise time switches. If there is a 
>  difference between left and right, then that difference appears as alias 
>  of 38 kHz which is the sum and difference of the baseband frequences and 
>  38 kHz. In other words a 38 kHz DSB signal. That is what makes the 
>  switching and matrix system mathematically equivalent.

Right, and I have observed exactly this on my oscilloscope. However, the 
deficiency of the switching menthod becomes immediately apparent when there
is a 
large difference between the channels, such as one channel being silent. The

audio is chopped on/off at 38Khz, at nearly a 50% duty cycle. (Perfect
switches 
would make it 50%) That's gotta add distortion along the way. Someone else 
mentioned the harmonics, which can intermod with subcarriers (if present) or
do 
other nasty IMD tricks.

While the switching method may be *effective*, I don't think it's the best 
way. Someone else pointed out the decoder flaws, and again, those are good 
points... and may, in reality, negate the effects of the "cheaper" switching
method.

Willie...
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