Digital milliwatt
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In digital telephony, the digital milliwatt is a test signal consisting of eight 8-bit words corresponding to one pulse-code modulated cycle of a sinusoidal signal approximately 1 kHz in frequency and one milliwatt, rms, in power.
The digital milliwatt is stored in ROM. A continuous signal of arbitrary length, i.e. an indefinite number of cycles, may be realized by continually reading out and concatenating the stored information into a data stream to be converted into analog form.
The digital milliwatt is used in place of separate test equipment. It has the advantage of being tied in frequency and amplitude to the relatively stable digital clock signal and power (voltage) supply, respectively, that are used by the digital channel bank.
Source: from Federal Standard 1037C and from the Code of Federal Regulations, Telecommunications Parts 0-199 (edited for Wikipedia).