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Chapter 10, Signaling Encoder (AF Generator 2)
DTMF Sequence Encoder
Twist
Twist is the ratio of amplitudes (in dB) between the high frequency and low
frequency tone in each DTMF pair. A positive value indicates a higher amplitude
for the high frequency tones. A negative value indicates a higher amplitude for the
low frequency tones.
The amplitude of the combined tones is set in the
AFGen2 To field.
Twist and Pre-emphasis Interaction
Twist and Pre-emphasis affect the relative levels of the high and low tones within
each symbol (tone pair). If pre-emphasis is off, twist sets the difference in
deviation (in dB) between the high and low tones. If twist is off, pre-emphasis
places a 6 dB per octave difference in deviation between the high and low tones. If
both twist and pre-emphasis are on, the two effects are summed.
For most conditions, set
Twist to 2.5 dB, Pre-Emp on, and 60% rated deviation
(3 kHz for a typical 5 kHz deviation rated receiver).
Examples of Twist and Pre-emphasis Interaction
Example 1:
3 kHz deviation, Twist 0 dB, Pre-emphasis off.
The level of each low tone and high tone individually generate 1.5 kHz deviation.
The tones are summed to produce 3 kHz deviation.
Example 2: 3 kHz deviation, Twist 2.5 dB, Pre-emphasis off.
The high tone has 2.5 dB (a factor of 1.334) more deviation than the low tone. The
two tones are summed to produce 3 kHz peak deviation. Therefore, the low tone’s
deviation is 1286 Hz and the high tone’s deviation is 1714 Hz.
Example 3: 3 kHz deviation, Twist 0 dB, Pre-emphasis on.
There is a 6 dB per octave difference between the high and low tones. For
example, if sending a 1, which has a low tone of 697 Hz and a high tone of
1209 Hz, the high tone has a deviation of 1209/697 = 1.735 times the low tone’s
deviation. The high tone’s deviation is then 20
× log(1209/967) = 4.78 dB higher
than the low tone. Since their sum must equal 3 kHz, the low tone’s deviation is
1097 Hz, and the high tone’s deviation is 1903 Hz.
Example 4: 3 kHz deviation, Twist 2.5 dB, Pre-emphasis on.
If sending a 1 (697 Hz low tone and 1209 Hz high tone), the high tone’s deviation
is 1.334 (see example 2)
× 1.735 (see example 3) approximately 2.314 times the
low tone’s deviation. Since the peak deviation of their sum is 3 kHz, the low
tone’s deviation is 905.5 Hz and the high tone’s deviation is 2094.5 Hz.
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