Commonly used pure (sine wave) test tones are 404 Hz, 1004 Hz, and 2804 Hz. The frequencies that are used in testing usually fall within the voice frequency band. Most measurements of absolute power in transmission are made in milliwatts or in units that are directly related to milliwatts. Since the power in telephone circuits is small, the milliwatt is used as the basic power measurement unit, just as the foot is used as the basic measurement of length. When you use the appropriate units of power related to specific references, you can measure absolute power, relative power, and power gains and losses. Each form of measurement has a specifically defined reference point. Based upon the transmission parameter that is measured, you can use different forms of decibel measurement. In order to describe relative power in terms of decibels, you must define the reference point from which you measure. For these reasons, a convenient mathematical expression of relative power, the decibel (dB), is commonly used. In addition, transmission is concerned with an extremely wide range of absolute power values. In transmission, the common interest is in power ratios rather than in absolute power. Since the power encountered in transmission systems is relatively small (compared to the power of a light bulb), power is usually expressed in milliwatts, abbreviated mW. Hertz equals one (0.00000000125) cycle or one oscillation per second and measures the waves or frequencies of electric changes each second.Īs is common in most electrical systems, power is measured in units of watts, abbreviated W. The unit used to measure frequency is the hertz, abbreviated as Hz or seen with the f symbol. Many performance standards are stated in terms of power at a particular frequency. The quantities commonly used to describe various aspects of transmission performance are frequency and power. Analog Voice MeasurementĪny waveform can be characterized in terms of frequencies and power. Some signaling transmissions are in-band and some are out-of-band. 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. Hence, any signal carried on the telephone circuit that is within the range of 300 to 3300 hertz is called an in-band signal. The voice pass band is restricted to 300 through 3300 hertz. However, the transmission of speech does not require the entire VF channel. Therefore, the bandwidth of the VF channel is 4000 hertz. (Sometimes this band is called a message channel.) Bandwidth is the difference between the upper limit and lower limit of the pass band. Zero to 4000 hertz is the pass band of a telephone system voice channel-a VF channel. The ranges of frequencies that are passed are said to be in the pass band. 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. However, most of the energy necessary for intelligible speech is contained in a band of frequencies between 2. 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. The vertical axis is relative energy and the horizontal axis is frequency. The figure in the Analog Voice Measurement section shows the typical distribution of energy in voice signals. Analog signals are often depicted as smooth sine waves, but voice and other signals are more complex than that, since they contain many frequencies. Human speech, and everything else you hear, is in analog form, and early telephone systems were analog as well. Analog Voice CharacteristicsĪnalog is defined as a signal that has a continuously and smoothly varying amplitude or frequency. Refer to the Cisco Technical Tips Conventions for more information on document conventions. This document is not restricted to specific software and hardware versions. There are no specific requirements for this document. What is the reference point for the measurements? In transmission, the first step to answer this question is to decide upon these questions: But how can you define good and bad quality in an objective way? Anyone who uses the telephone experiences both good and bad connections, and can probably describe the quality of a particular connection in a subjective way. The quality of a transmission system is defined by the difference between spoken voice at one end and reproduced voice at the other end. This document discusses how analog voice signals are measured, the units used, and the points of reference used when you measure.
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