Proponents of an alternate modulation scheme -- DMT -- claim that there are often substantial interoperability problems between CAP implementations from different suppliers.
In principle, CAP is similar to the analog QAM modulation scheme used in analog modems. Two carriers 90 degrees out of phase are combined at the transmitter and separated at the receiver thereby doubling the information capacity of a given amount of bandwidth. In general, QAM uses analog techniques to handle the quadrature signals whereas CAP uses digital techniques.
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