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In telecommunication, the term baseband has the following meanings:

1. The original band of frequencies produced by a transducer, such as a microphone, telegraph key, or other signal-initiating device, prior to initial modulation.

Note 1: In transmission systems, the baseband signal is usually used to modulate a carrier.

Note 2: Demodulation re-creates the baseband signal.

Note 3: Baseband describes the signal state prior to modulation, prior to multiplexing, following demultiplexing, and following demodulation.

Note 4: Baseband frequencies are usually characterized by being much lower in frequency than the frequencies that result when the baseband signal is used to modulate a carrier or subcarrier.

2. In facsimile, the frequency of a signal equal in bandwidth to that between zero frequency and maximum keying frequency.

Source: from Federal Standard 1037C and from MIL-STD-188