be considered by the transmission engineer. In this section we will talk about these aspects regarding frequency response across a well-defined voice channel. We will be required to use dBs, dB-derived units, and numeric units. The basic voice channel is that inclusive band of frequencies where loss with regard to frequency drops 10 dB relative to a reference frequency. There are two slightly different definitions of the voice channel, North American and CCITT: North America: 200 Hz to 3300 Hz (reference frequency, 1000 Hz). CCITT: 300 Hz to 3400 Hz (reference frequency, 800 Hz). We sometimes call this the nominal 4-kHz voice channel; some others call it a 3-kHz channel. (Note: There is a 3-kHz channel, to further confuse the issue; it is used on HF radio and some old undersea cable systems.) To introduce the subject of a "flat" voice channel and a "weighted" voice channel we first must discuss some voice channel transmission impairments. These are noise and amplitude distortion. We all know what noise is. It annoys the listener. At times it can be SO disruptive that intelligent information cannot be exchanged or the telephone circuit drops out and we get a dial tone. So we want to talk about how much noise will annoy the average listener. Amplitude distortion is the same as frequency response. We define amplitude distortion the variation of level (amplitude) with frequency across a frequency passband or band of interest. We often quantify amplitude distortion as a variation of level when compared to the level (amplitude) at the reference frequency. The two common voice channel reference frequencies are noted in the preceding list. To further describe amplitude distortion, let us consider a hypothetical example. At a test board (a place where we can electrically access a voice channel) in New York we have an audio signal generator available, which we will use to insert audio tones at different frequencies. At a similar test board in Chicago we will measure the level of these frequencies in dBm. The audio tones in