/mcpAnalog Voice Characteristics Analog is defined as a signal that has a continuously and smoothly varying amplitude or frequency. Human speech, and everything else you hear, is in analog form, and early telephone systems were analog as well. Analog signals are often depicted as smooth sine waves, but voice and other signals are more complex than that, since they contain many frequencies. The figure in the Analog Voice Measurement section shows the typical distribution of energy in voice signals. The vertical axis is relative energy and the horizontal axis is frequency. The figure in the Analog Voice Measurement section shows that the voice frequencies that contribute to speech can extend from below 100 hertz to above 6000. However, most of the energy necessary for intelligible speech is contained in a band of frequencies between 200 and 4000. In order to eliminate unwanted signals (noise) that can disturb conversations or cause errors in control signals, the circuits that carry the telephone signals are designed to pass only certain frequencies. The ranges of frequencies that are passed are said to be in the pass band. Zero to 4000 hertz is the pass band of a telephone system voice channel−a VF channel. (Sometimes this band is called a message channel.) Bandwidth is the difference between the upper limit and lower limit of the pass band. Therefore, the bandwidth of the VF channel is 4000 hertz. However, the transmission of speech does not require the entire VF channel. The voice pass band is restricted to 300 through 3300 hertz. Hence, any signal carried on the telephone circuit that is within the range of 300 to 3300 hertz is called an in−band signal. Any signal that is not within the 300 to 3300 hertz bands, but is within the VF channel, is called an out−of−band signal. All speech signals are in−band signals. Some signaling transmissions are in−band and some are out−of−band. Analog Voice Measurement Any waveform can be characterized in terms of frequencies and power. The quantities commonly used to describe various aspects of transmission performance are frequency and power. Many performance standards are stated in terms of power at a particular frequency. The unit used to measure frequency is the hertz, abbreviated as Hz or seen with the f symbol. Hertz equals one (0.00000000125) cycle or one oscillation per second and measures the waves or frequencies of electric changes each second..