[sdiy] [synth-diy] lock-in amplifiers

Donald Tillman don at till.com
Sun Jan 26 19:44:21 CET 2014

On Jan 26, 2014, at 9:30 AM, cheater00 . <cheater00 at gmail.com> wrote:

> At what point do you get lost?

The big paragraph featuring f and f0.  "We only care about the f-f0 part."  Why?  What are we doing?


When stereo was introduced in FM broadcasting around 1960 they had to transmit separate L and R signals and at the same time be fully compatible with mono.  So the mono-compatible L+R was transmitted normally, and an L-R difference signal was AM modulated at 38 kHz and added in... *except* that would be difficult to demodulate, it would be noisy, and waste energy on the 38 kHz carrier.  

So the L-R signal was AM modulated but with the carrier removed, and a low level sync'd 19 kHz pilot tone was added.  To retrieve the L-R signal an FM radio has to detect the 19 kHz pilot signal, double that, and use synchronized detection to demodulate the L-R audio.  Then add/subtract that with the L+R to provide L and R signals.

  -- Don

Don Tillman
Palo Alto, California
don at till.com

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