RETURN

Multiplexing

 
The Voice Frequency Carrier Terminal (VFCT)

The VFCT multiplexes up to 16 low speed (e.g. 75 baud) teleprinter channels into a SINGLE voice channel. Each channel uses a separate frequency to transmit and receive, and these frequencies are “stacked” into a SINGLE VF line.  They are commonly referred to as “Tone Packs“.

Channel       Frequency
-------       ---------
   1            425 Hz
   2            595 Hz
   3            765 Hz
   4            935 Hz
   5           1105 Hz
   6           1275 Hz
   7           1445 Hz
   8           1615 Hz
   9           1785 Hz
  10           1995 Hz
  11           2125 Hz
  12           2295 Hz
  13           2465 Hz
  14           2635 Hz
  15           2805 Hz
  16           2975 Hz

• Frequency Shift Keying (FSK) is employed. The CENTER frequencies for each of the channels is listed above.
• When a MARK (logic 1) is transmitted, the frequency is LOWERED 42.5 Hz (e.g. 382.5 Hz for Channel 1).
• When a SPACE (logic 0) is transmitted, the frequency is RAISED 42.5 Hz (e.g. 467.5 Hz for Channel 1).
• This 42.5 Frequency Shift Keying is employed on all 16 channels.
• There is a 85 Hz “guard” band between all channels.