Since the rate of change at the bit centers should be zero, the output of the circuit should be zero. Random noise has random rates of change at the sampling instants which cause the circuit to output a random voltage. This noise voltage is rectified, filtered, and sent to a comparator. If the noise voltage is below a preset threshold, the comparator turns on the carrier detect signal at the digital interface and also turns on a data gate which allows the received data to go out to the interface. An unmodulated signal will also trigger this circuit since there would be no noise at the sampling instants to inhibit the comparator.
Performance There are several measures of a modem'sperformance.
One of the most important is its bit error rate. Any well designed modem should not produce any errors if the signal has no noise or distortion in it. The true measure of a modem's quality is its performance under weak signal conditions when the signal to noise ratio is low. These modems were tested to determine their error rate performance at various signal levels and frequency offsets. The results are summarized in the table below.