[BC] FM Stereo invention

[Robert Meuser]

Switching the audio at a 50% duty cycle does not unto itself produce 
distortion. PDM transmitters do this during the regular modulation 
cycle.  The trick is properly filtering harmonics. If done properly, 
that flaw is eliminated. So the limits really come down to the matching 
of resistors and balancing of modulators in the matrix method plus the 
effect of time delay in the two different paths. I think you would have 
a hard time getting more than 50 db separation reliably and repeatedly. 
Switching is limited primarily by the filter phase and amplitude 
response.  At the end of the day i think once a filter is designed, the 
switching approach would be better.  We are really having a historical 
discussion as the switching method was greatly improved by several 
developers and the matrix when done completely in the digital domain can 
be virtually perfect.

R



WFIFeng at aol.com wrote:

> 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...
>  
>