Sony IT-M708 Operation Manual page 10

Spectrum structure of Line 1
Voice Base band
０． ３KHz
Control data: ASK modulation by On and OFF of 450 KHz carrier
Data transmission speed: 780bps
Extension call: AM modulation, dial side 281.25KHz, receiving side 321.43KHz
Compander IC is equipped for improving better calling quality.
note
ASK: Amplitude Shift Keying
Spectrum of Line-1
Voice carrier of
receiver side for
the extension call
Voice carrier of
caller side for the
extension call
3.4KHz
281.25KHz 321.43KHz 450.KHz
Control signal
carrier
10