serted in New York are all inserted at a level of -16 dBm, one at a time. In Chicago we measure these levels in dBm. We find the level at 1000 Hz to be +7 dBm, our reference frequency. We measure the 500-Hz tone at +3 dBm; 1200-Hz tone at +8 dBm; 2000-Hz tone +5 dBm, and the 2800-Hz tone at 0 dBm. Any variation of level from the 1000-Hz reference value we may call amplitude distortion. At 2800 Hz there was 7 dB variation. Of course, we can expect some of the worst-case excursion at band edges, which is usually brought about by filters or other devices that act like filters. The human ear is a filter, as is the telephone receiver (earpiece). The two are in tandem, as we would expect. For the telephone listener, noise is an annoyance. Interestingly we find that noise annoys a listener more near the reference frequencies of a voice channel than at other frequencies. When using the North American 500-type telephone set with average listeners, a simple 0-dBm tone at 1000 Hz causes a certain level of annoyance. This value applies when looking toward the subscriber from the local serving exchange. Looking into the network from the local serving exchange the value drops to 3 dB. C.7 dB APPLIED TO THE VOICE CHANNEL To cause the same level of annoyance, a 300-Hz tone would have to be at a level of about +17 dBm; a 400-Hz tone at about +11.5 dBm; a 600-Hz tone at about +4 dBm, and a 3000-Hz tone also at about +4 dBm for equal annoyance levels for a population of average listeners. The question arose of why should the transmission engineer be penalized in design of a system for noise of equal level across the voice channel? We therefore have "shaped" the voice channel as a function of frequency and "annoyance." This shaping is called a weighting curve. For the voice channel we will be dealing with two types of weighting: (1) C-message, used in North America, and (2) psophometric weighting as recommended by CCITT. Figure C.1 shows these weighting curves. Weighting networks have been developed to simulate the corresponding res