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The circuits shown in the drawings are to be used in the receiver part of a modem to detect the transmission modes of the received data, i.e., full speed, half speed, or service mode.
English (United States)
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Fast Determination Of Transmission Modes In A Modem
The circuits shown in the drawings are to be used in the receiver part of a
modem to detect the transmission modes of the received data, i.e., full speed,
half speed, or service mode.
During the start-up time. the receiver has to detect the
presence of the data signal to determine if it must operate in full
speed, half speed, or service mode. Depending on the transmission
mode, there are three classes of training sequences:
- Half speed: two tones at Nyquist frequencies f1 and f2,
with f1 = fc + 1/2T, and f2 = fc - 1/2T, fc being the
carrier frequency (1800 Hz) and 1/T being the symbol rate
Full speed: three tones at Nyquist and carrier frequencies.
Service tone: one tone at carrier frequency fc.
If there is enough energy on the line, the receiver is organized as shown on
fig. 1A. In phase and in quadrature components X and Y are fed through Hilbert
filter 1 and analog-to-digital (A/D) converter 2 into digital filter 3, for analyzing
separately the tones at Nyquist and carrier frequencies which are likely to be
received. Filter 3 comprises T/2 delay elements and summing and substracting
Once the delay lines in the filter 3 have been filled up, estimates of side band
energy Es, carrier energy Ec and also noise energy En can be obtained in the
output lines of filter 3.
These three estimates are used for the determination of the received training
signal by the decision circuit shown on Fig. 1B....