How is a Distortion measurement made?
Distortion is an audio quality measurement that compares the audio signal level from the mobile at a specified (desired) frequency to the level of signals present at other frequencies. The audio signals from the mobile are typically measured after digital demodulation of the traffic channel.
The test set makes two measurements to determine distortion. First, the test set measures the total audio signal level. Next, a precise notch filter is then used to remove the signal at the specified frequency. The remaining signal level indicates the level of unwanted signals (distortion). Distortion is expressed as a percentage of the total audio signal.
Distortion can be measured in the range of 100 Hz to 10 kHz.
The distortion measurement can be used to perform the audio harmonic distortion test. For example in an analog system like AMPS, two audio signals are used: a 1004 Hz tone and the 6000 Hz Supervisory Audio Tone (SAT). The 1004 Hz tone is notched out to make the measurement.
