[78-L] Stereo broadcasting

[DAVID BURNHAM]

I'm curious as to how they pulled that off, considering that the country
wasn't blanketed with NBC O&O radio stations carrying that other
channel. I'm assuming that this was done the same way FM stereo is done,
ie sum-and-difference matrixing. 

As with M-S stereo recording, in FM stereo one channel carries the sum
of both left and right - essentially the mono signal - while the other
channel carries the difference between the two, or what is either on the
left or right but not common to both.  

The sum could be broadcast over the television network, thus not
depriving the national audience of anything important, while the
difference could have gone out over radio to provide that extra spacious
three-dimensional stereophonic space-age sound.

I hope Mike Biel is reading this, because if I have something screwed up
somewhere I would very much like to be corrected. 

MS

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Your description of FM stereo isn't entirely accurate, unless it's just the way you're describing it.

The Sum, (or "M"), signal modulates the main carrier of the station the same way as the audio modulates the carrier of a mono transmission.  The difference, (or "S"), signal modulates a subcarrier which itself also modulates the main carrier on a stereo FM, (or television), transmission.  Within a stereo FM or Television receiver, these two signals are detected and run through a matrix which adds and subtracts the two audio signals to produce stereo left and right channels.  Without the action of this matrix, stereo cannot be derived from a sum and difference signal.  

However an acoustic matrix is possible, (I have done this), by feeding the sum signal to a centre speaker and the difference signal to two speakers placed on either side of this centre speaker with the speaker on the left in phase with the centre speaker and the speaker on the right out of phase with the centre speaker. Your phantom left and right channels will appear to be midway between the side speakers and the centre speaker on each side.

Interestingly, (to me anyway), any stereo LP also contains the Sum and difference channels in the groove - the sum being lateral modulation and the difference being vertical modulation.  An LP could be cut by feeding the sum to a lateral cutter and the difference to a vertical cutter and after the fact the LP would be indistinguishable from an LP cut in the normal fashion - left on one side of the groove and right on the other, 45 degrees either way from the perpendicular of the surface.  Alan Blumlein, (an engineer's name from the 30s which should be familiar to anyone on this list), recognized this during his experiments in stereo recording but advised against cutting stereo records with sum and difference signals.  To my knowledge, nobody has ever cut a stereo record using sum and difference signals.

db