Service manual

Page 5
ADJUSTING TWIST
The microphone generates two tones when any key on the keypad is depressed. The following
table illustrates the row and column tones generated:
Column Tones (Hz)
1209 1336 1477
Row
697
1 2 3
Tones
770
4 5 6
(Hz)
852
7 8 9
941
* 0 #
The actual frequencies generated are:
Keypad
Location
Reference
Tone (Hz)
Actual
Tone Output (Hz)
Percent
Deviation
Row Tone 1 697 699.1 +0.30
Row Tone 2 770 766.2 -0.49
Row Tone 3 852 847.4 -0.54
Row Tone 4 941 948.0 +0.74
Column Tone 1 1209 1331.7 +0.57
Column Tone 2 1336 1331.7 -0.32
Column Tone 3 1477 1471.9 -0.35
DTMF twist is the ratio between the levels of the column tones and the row tones generated by
any single keypad button. Because the higher frequency (column) tones are more readily
attenuated than the lower frequency (row) tones, the high frequency tones of any DTMF pair are
sent at a slightly higher level. The ratio of column/row tones is factory set to telephone industry
standards and no adjustment is needed for most applications. However, if particular systems
require levels other than the standard ratio, they can be adjusted. To increase the column/row
ratio decrease the value of C4. To decrease the ratio, increase the value of C4.
By generating the tones individually as a single tone, any adjustment of twist is made easier.
This can be accomplished by temporarily the circuit board plating from U3 and ground. The burst
mode jumper JPR4 must also be temporarily cut for single tone operation. A single row or
column tone can now be generated by simultaneously pressing two keys in respective horizontal
row or vertical column. Reinstall the jumpers as they were before when done.