This is a picture of the RCA BTS-1A Broadcast FM Stereo Generator. The BTS-1A was
the first production FM Stereo Generator to become available after FM stereo was
authorized by the FCC in 1961. The BTS-1A was a simple, elegant design, using only
two tubes. The Gates model M-6146, a popular Stereo Generator of the time, required
a total of eleven tubes to do the same job.
Here is the block diagram of the BTS-1A. For those of a technical bent, who wish to understand the operation of the BTS-1A, please note that the block labeled "Time Delay Network", is actually a 15 kHz lowpass filter, designed so its time delay matches that of the "23 -53 kc Bandpass Filter" at the corresponding subcarrier sideband frequencies. Click here to see the full BTS-1A schematic, excepting the details of the filters.