Attenuation Distortion

Attenuation Distortion is the change in amplitude of the transmitted signal over the Voice Band. It is the frequency response curve of the Voice Band.

Attenuation versus Frequency

To measure Attenuation Distortion, the phone line has a test frequency transmitted from 0 - 4 kHz into the line at a standard amplitude of 0 db. The loss of signal or attenuation is measured at the receiving end and compared to a standard reference frequency: 1004 Hz.

db is short for decibel which is a relative unit of measure (similar to a unit like a dozen). It is a log unit and a +3 db gain will indicate an amplitude of 2x the reference. It is a logarithmic ratio between input voltage and output voltage. It is calculated by the following formula:

db =10 x log (Vout/Vin)

The resulting information is graphed on an Attenuation vs. Frequency chart. Attenuation is a loss of signal amplitude - the receive signal is a smaller amplitude than the transmitted signal. It is indicated by a positive db. It is also possible to have a signal appear at the receiving end with a larger amplitude than when it started - this is indicated by negative db.

The attenuation is due to the many pieces of electronic equipment and transmission media that the signal has to pass through, some can amplify the signal (make it a larger amplitude) and some may attenuate the signal (make it smaller).

There are maximum and minimum acceptable limits for Attenuation Distortion for phone lines. The Basic channel conditioning is:

Frequency RangeLoss (db)
500 - 2500-2 to +8
300 - 3000-3 to +12

The above Loss is a range of acceptable values for the frequency range. In the Basic Channelling Conditioning, it is acceptable to have a loss in signal in the frequency range of 500-2500 Hz of "8 db loss to -2 db loss" referenced to the amplitude at 1 kHz. Note that on the graph on the previous page that this is shown as -8db and +2 db.

+3 db attenuation is equal to -3 db in signal amplitude and +8 db attenuation equates to -8 db in signal amplitude.

Envelope Delay Distortion

If the Propagation Delay changes with frequency than we would have the condition where the lower frequencies such as 300 Hz may arrive earlier or later than the higher frequencies such as 3000 Hz. For voice communication, this would probably not be noticable but for data communication using modems, this could affect the phase of the carrier or the modulation technique used to encode the data.

When the Propagation Delay varies across the frequency range, we call this Envelope Delay Distortion. We measure propagation delay in microseconds (us) and the reference is from the worst case to the best case.

18d. Non-Linear Distortion

Non-linear distortion is distortion that changes the waveshape of the signal. If the signal was transmitted as a sinewave and arrived as a squarewave, this would be an example of severe non-linear distortion. Amplitude modulated carriers would suffer drastically if the original wave shape was distorted.

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TelecomWorld 101

Copyright July 2013 Eugene Blanchard