To advertise here contact: admin@tbi.net

Return to the Multiplexing Overview page.

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.

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.