White facsimile transmission
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In telecommunication, the term white facsimile transmission has the following meanings:

1. In an amplitude-modulated facsimile system, transmission in which the maximum transmitted power corresponds to the minimum density, i.e., the white area, of the object.
2. In a frequency-modulated facsimile system, transmission in which the lowest transmitted frequency corresponds to the minimum density i.e., the white area, of the object.

Both of these terms are probably obsolete except in specialist usage, as most fax machines now use the digital ITU-T fax standards, which encode the image digitally over a QAM-modulated signal.

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

The original article can be found at: http://en.wikipedia.org/wiki/White_facsimile_transmission.
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