Data modulator/demodulator

A data modulator/demodulator (MODEM) for a bidirectional data communication system, comprising:

• main channel modulating means for modulating input first main data to a first main channel signal;
• subchannel modulating means for modulating input first subdata to a first subchannel signal having a frequency spectrum different from a frequency spectrum of said first main channel signal;
• multiplexing means for frequency division multiplexing said first main channel signal and said first subchannel signal to thereby produce a first transmit signal;
• transmitting means for transmitting said first transmit signal over a first metallic cable;
• splitting means for receiving a second transmit signal identical in format with said first transmit signal over a second metallic cable having substantially the same attenuation characteristic as said first metallic cable and splitting said second transmit signal into a second main channel signal and a second subchannel signal;
• main channel demodulating means for regenerating second main data from said second main channel signal;
• subchannel demodulating means for regenerating second subdata from said second subchannel signal;
• said first main data selectively having one of a plurality of predetermined data rates;
• data rate information inputting means for generating a control signal matching said one data rate of said first main data;
• first level controlling means for controlling the output level of said first main channel signal to a predetermined level in response to said control signal;
• and
• second level controlling means for controlling the output level of said first subchannel signal to a predetermined level in response to said control signal.