Philips PM 5508 Operating Manual page 14

13
The sound subcarrier is generated in a 5.5 MHz (6 MHz in the B-version)
oscillator. Via a switch, this subcarrier can be frequency-modulated with
1 kHz by means of a modulator, operating according to the diode switching
principle. This subcarrier oscillator itself can be switched off if desired.
In an adder stage the sound subcarrier is added to the composite video signal.
From there it is applied to an RF modulator and, via an RF attenuator, to
output socket "RF". The instrument is provided with an stabilised power supply
with short-circuit protection circuit.