to the local serving exchange. This circuit is analog. Based upon the CCITT definition, the voice channel occupies the band from 300 to 3400 Hz. We call this the passband. Attenuation distortion can be avoided if all frequencies within the passband are sub- jected to the same loss (or gain). Whatever the transmission medium, however, some frequencies are attenuated more than others. Filters are employed in most active circuits (and in some passive circuits) and are major causes of attenuation distortion. Figure 3.3 is a response curve of a typical bandpass filter with voice channel application. As stated in our definition, amplitude distortion across the voice channel is measured against a reference frequency. CCITT recommends 800 Hz as the reference; in North America the reference is 1000 Hz. 2 Let us look at some ways that attenuation distortion may be stated. For example, one European requirement may state that between 600 and 2800 Hz the level will vary no more than - 1 to +2 dB, where the plus sign means more loss and the minus sign means less loss. Thus if an 800-Hz signal at - 10 dBm is placed at the input of the channel, we would expect -10 dBm at the output (if there were no overall loss or gain), but at other frequencies we can expect a variation at the output of -1 to +2 dB. For instance, we might measure the level at the output at 2500 Hz at -11.9 dBm and at 1100 Hz at -9 dBm. When filters or filter-like devices3 are placed in tandem, attenuation distortion tends to sum. Two identical filters degrade attenuation distortion twice as much as just one filter. 3.3.2 Phase Distortion We can look at a voice channel as a bandpass filter. A signal takes a finite time to pass through the telecommunication network. This time is a function of the velocity of Figure 3.3 Typical attenuation distortion across a voice channel bandpass filter. Crosshatched areas are response specifications, whereas the wavy line is the measured response. 2Test frequencies of 800 and 1000 Hz are not recommended if the analog voice cha