The purpose of this standard is to describe practical methods for measuring the transmission characteristics of a telephone set by means of objective measurements using a test connection. Although the various methods generally produce similar results, some deviations can be expected depending on telephone characteristics. Measurement conditions should be noted when reporting results. The test results thus obtained may be used as a means of evaluating or specifying the transmission performance of a telephone set on a standardized basis. This standard is intended for measuring the transmission performance of conventional two-wire analog telephone sets and digital telephone sets. These telephone sets can be equipped with handsets or headsets using carbon or linear transmitters. This standard covers measurements over the frequency range most useful for speech, 100 Hz to 5000 Hz. This standard is not intended to be applicable to special devices such as noise exclusion transmitters, loudspeaking telephones, or noise exclusion receivers equipped with large ear pads. Drawings and test procedures, as well as other pertinent information, are included as an aid in setting up the tests. The approach taken in this standard to measure digital telephone sets is to convert the digital signal to or from the telephone set via an interface and a reference codec to an analog signal, and then to use standard analog measurement techniques. Although not covered in this standard, direct signal processing in the digital domain could be used. Where an input signal, electric or acoustic, is required for making the objective measurements, either of two specified signals may be used, i.e., sinusoidal or continuous spectrum. The two types of signal will give essentially identical results for telephone sets measured in the linear region of their operation, when the measurement is sufficiently noise free. For telephone sets incorporating nonlinear devices, however, the continuous spectrum signal is preferable. Nonlinear devices include carbon microphones, compressors, automatic gain control (AGC) circuits, etc. The basic accuracy and reproducibility of measurements made in accordance with this standard will depend primarily upon the accuracy of the test equipment used, the care with which the calibration and the measurements are conducted, and the inherent stability of the telephone set under test. Where a given set of measurements is repeated in the same laboratory and by the same operator, a relatively high degree of reproducibility should normally be obtained. However, when comparing measurements made by different laboratories, allowances should be made for the influencing factors mentioned above.