A form of signal modulation in which the amplitude of the digital pulse carrier is modulated according to the amplitude level of the original signal. PAM samples an incoming analog signal, for example, measures its amplitude, and outputs a digital pulse of a representative amplitude. The outgoing pulse closely matches the amplitude of the incoming signal, but when digitized in an A-to-D converter, the digital output is the nearest of a number of standard amplitude values. PAM was used in this manner in early channel banks to interface analog PBXs and central offices (COs) to DS-1 digital circuits. The PAM-encoded signal subsequently was further encoded using pulse code modulation (PCM) before the signal was placed on the circuit. PAM is considered obsolete in this application, having been replaced by direct PCM-encoding of the analog signal, with the sampling performed by the same chips as contained in the A-to-D converter. However, variations on PAM remain widely used. Quadrature amplitude modulation (QAM) is such a variation. See also amplitude, channel bank, DS-1, modulation, PCM, and QAM.