Service manual

The figure also shows the Second Audio Program (SAP) frequency, which is a constant
carrier of an audio tone of 5fH (78,670 Hz), FM modulated by the SAP program content.
SAP is always monaural.
The material on the SAP audio may, in some cases, not be related to the video content.
Alternatively, it may contain a verbal description of the action in the video for those who
can not view the picture.
Some stations use it to transmit background music or book reading for the blind.
Figure: Block diagram stereo sound system
In figure above, a simplified block diagram of a stereo sound system in a TV set shows
the 'L+R' audio signal combined with the detected 'L-R' audio signal to develop stereo
'L' and 'R' audio.
The 'pilot' signal is doubled in frequency (to 2fH) and re-inserted into the detection of the
'L-R'. This to complete the detection.
The pilot signal also tells the control circuit when a pilot frequency is present. The 'L-R'
and 'L+R' audio are supplied to the stereo matrix. There, by addition and subtraction the
single 'R' and 'L' audio signals are output. The SAP audio is detected and made
available. In addition, the indication of a SAP signal is given to the control circuit.