binary 1 is represented by sending a signal burst of the same phase as that of the previous signal burst sent. A binary 0 is represented by a signal burst of a phase opposite to that of the previous signal transmitted. The signals are demodulated at the receiver by integrating and storing each signal burst of 1-bit period for comparison in phase with the next signal burst. 10.9.3 Critical Impairments to the Transmission of Data The effect of various telephone circuit parameters on the capability of a circuit to transmit data is a most important consideration. The following discussion is intended to familiarize the reader with problems most likely to be encountered in the transmission of data over analog or mixed analog-digital circuits. We make certain generalizations in some cases, which can be used to facilitate planning the implementation of data systems. 10.9.3.1 Phase Distortion. Phase distortion "constitutes the most limiting impairment to data transmission, particularly over telephone voice channels" (Ref. 8). When specify- ing phase distortion, the terms envelope delay distortion (EDD) and group delay are often used. The IEEE (Ref. 1) states that envelope delay is often defined the same as group delay-that is, the rate of change, with angular frequency, of the phase shift between two points in a network. Let's try to put this another way that will be easier to understand. We are dealing with the voice channel, which will be transported in an analog network or in a mixed analog/digital network. It is a band-limited system. Phase distortion arises from the fact that not all frequency components of the input signal will propagate to the receiving end in exactly the same elapsed time. This is true when the signal passes through filters or their equivalent. Particularly troublesome are loaded cable circuits and FDM carrier circuits. Even PCM channel banks have filters. Figure 10.10 shows a typical frequency-delay response curve in milliseconds of a voice channel due to FDM equipment only. For the voice channel