User's Manual

RPT 633
Page 3 - 8 TM1184 Issue 1
Switching to Speech
24. To switch into the speech mode, either as part of a speech paging call or for open
channel speech, the encoder generates two 1ms pulses 4ms apart as shown in
Figures 3 and 4.
Figure 3: Speech Call
Figure 4: Open Channel Speech
25. The pulses are inverted at PL1-4 and the leading edge of the first pulse results in
IC1-6 (OA) going high which switches PL1-2 low for 4.5ms (see Figure 3). The
trailing edge of the first pulse results in IC1-7 and IC3-1 presenting two lows to IC3-
12 and IC3-11 respectively, so that IC2-1 goes high. This has no immediate effect
on the circuit since the resetting of IC1a after 4.5ms removes one of the two low
states which caused J to go high.
26. The negative-going edge at IC1-6 clocks IC1b so that IC1-10 goes high for 2ms.
The trailing edge of the second pulse sets IC2-1 high again, but this time IC1b
resets after 2ms, clocking IC2-12 and resulting in IC2-3 going high. This ensures
that IC3-4 remains low during the speech mode.
27. 4.5ms after the commencement of the second pulse, IC1-7 resets high so that the
transmitter is now keyed in the speech mode.
28. Timings of IC1a and IC1b, controlled by RV2 and RV3 respectively, are absolutely
crucial to the reliable switching of the circuit into speech and must be accurately
set.