[BC] FM Stereo invention

[Robert Meuser]

Bob:

It seems that with the technology available at the time that would have 
been a bad system. My reasoning is as follows:

First is the creation of the VSB signal. Besides limitations of the 
matrix available at the time there would be time delay caused by the VSB 
filtering.That delay would need compensation in the L+R path.There would 
also as is the case in VSB TV the requirement for pre-distortion to 
correct for detector limitations. This is another time delay in the L-R 
path. Then there is the loss of overall modulation capability due to the 
AM carrier, unless it is partially suppressed which I did not see as 
part of the system spec.

A prototype system may have had reasonable results in the day but I dont 
think that going forward it would have been nearly as resilient as the 
system we have. If you read the fine print and reports on the Crosby 
system, which many today still think would have been better, there are 
serious limitations that would not have been a future friendly as the 
system that was selected.  FM stereo is one case of systems selection 
where a wise choice was made.

R




Bob Tarsio wrote:

>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...
>_______________________________________________
>
>
>  
>