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4. Distortion

Precisely we should name it "non-linear distortion", to distinguish it from the linear distortion, but simply distortion is commonly used.

Distortions are some of the most nasty effects of poor sound quality. If they are audible, they are always an indication for

- grossly wrong operation of the equipment,

- totally misaligned equipment,

- defective equipment.

In general it can be said, that

The presence of audible distortions in the studio is always an indication for a dramatic error and requires immediate action.

In fact every piece of equipment in the studio is producing a bit of distortion. Under normal operation conditions this should remain at such a low level, that it will not be audible.

The limit, where distortions are getting audible is between 1% and 3%, depending on the sensitivity of the listener and the kind of music. Most sensitive for the detection of distortions are pure tones like either test signals or tones of piano. Orchestral, band music or speech are more insensitive to distortions.

The following list shows the distortions which can be expected from different type of audio equipment:

- microphones: 0.01 - 0.1%
- mixers, amplifiers 0.1% - 0.5%
- record players 1%
- tape machines 0.5% - 1%
- cassette recorders 1% - 5%
- transmitters 1%
- receivers 1%
- loud speakers 1 - 5%

The distortions produced by any of these equipment will go up dramatically, as soon as the maximum level for the equipment is exceeded. When clipping is reached, the distortions may become as high as 30%.

Therefore most studio equipment has a safety margin for excessive level,
the so-called head room.

 

Different equipment has different headrooms:

- mixers, amplifiers 15dB - 20dB
- tape machines 6dB
- cassette recorders 6dB
- transmitters 0dB

This table shows, that especially transmitters and tape recorders are sensitive to over-modulation. When sending signals to any of these devices (recording studio, continuity) highest care must be taken to keep the level within the required limits.

Peak programme meters should to be used to measure the signal levels sent to such an equipment.

4.2. Definition of Distortion.

The distortion of signals is always caused by some equipment. When distorting the signal, the equipment is putting some additional signal to it. As a matter of physics, this new signal will always have frequencies, which are plain multiples of the original signal. These are called harmonics.
We can therefore say, that distorting a signal is producing harmonics.

Depending on the kind and the cause of distortions, different harmonics will be produced. Basically we distinguish symmetrical and unsymmetrical distortions. In symmetrical distortions the positive and the negative half-waves of the signal will be distorted equally. In asymmetrical distortions, the positive and the negative half-wave of the signal will be distorted differently.

- Symmetrical distortion will produce only odd harmonics,
(e.g. only 3nd, 5th, 7th, etc.)

- Unsymmetrical distortion will produce odd and even harmonics,
(e.g. 2nd, 3nd, 4th, etc.)



Fig. 4.2.1.

Graphical representation of a differently distorted sine wave.
a.) Symmetrical distortion of the sine wave (clipping). The spectrum shows the strong amplitude of the fundamental frequency and odd harmonics.
b.) Unsymmetrical distortion of the sine wave. The spectrum shows the fundamental frequency and odd and even harmonics.



The definition of harmonics is therefore rather simple: we just have to determine how much of harmonics are produced by some equipment. If more than one harmonic is produced, all of them have to be added geometrically. The total amplitude of them is related to the amplitude of the original signal. The ratio is then given in percent or dB.

Formel
Formel

Uf1 is the voltage of the original signal with its fundamental frequency
Uf2 to Ufn are the voltages of the harmonics with multiples of the fundamental frequency.

The distortions are measured with special distortion measurement equipment in the following way:

A pure sine wave with no harmonics is applied to the input of the equipment under test. The frequency is normally 1000Hz and the level should be the normal operation level for the equipment (e.g. standard studio level). At the output the signal is measured with a filter, separating the fundamental frequency from the harmonics. The level of both is determined and put into relationship. This is done either by means of suitable scales or in modern equipment by microcomputers.



Fig. 4.2.2.

The test set-up to measure the distortion of an equipment.




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