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- SCA.1
- If this station were a plain old-fashioned monaural FM station, all it
- would have to do is feed in the audio signal -- voice, music or funny
- sound effects -- which you want the listeners out there in Radio-Land
- to hear.
-
- The signal would modulate the carrier, be amplified, fed to the antenna
- and radiated. It would come swooping down into your ordinary, Monaural
- FM receiver. The receiver would say, hmm... The frequency is F + K*M
- I therefore have to subtract F and divide by K to give my loyal owner
- the instantaneous value of the signal waveform, which is M. Voila! Out
- of the FM demodulator comes M in livid high fidelity.
-
- (I don't think the following information is needed for present purposes,
- but I include it so somebody won't say I am oversimplifying things!!
- Now, even in simple monaural FM there is one trick that we haven't
- mentioned. This is called pre-emphasis and de-emphasis. Very early in
- the game, it was noticed that in FM systems if you fed in no modulation
- at all and listened to the receiver's reconstructed value of M (after it
- did the arithmetic noted above), there was a NOISE output from the
- receiver, even at fairly strong signal levels. The noise was
- particularly noticeable because its amplitude increased with frequency.
- Thus a quite noticeable high-frequency hiss was present on even fairly
- good signals. Somebody then had the bright idea that they should
- effectively "turn down the treble control" at the receiver. A fixed
- frequency compensating network called a "de-emphasis" network was
- designed and standardized to do that. But then the music had its highs
- "de-emphasized", so an "inverse" network called a pre-emphasis network
- was added at the transmitter.
- CONTINUED IN FILE SCA.2