Anyone interested in hi-fi will have heard of distortion. Many amplifiers, for instance, are marketed with claims of a low distortion factor or low levels of something called Total Harmonic Distortion (THD). In the following article, we’ll examine the causes of distortion, define the basic types and the extent to which distortion effects music playback.
Linear versus non-linear distortion
There are two basic types of distortion. Non-linear distortion describes the ways in which an outgoing audio signal differs from the ingoing signal. The frequencies present in the outgoing sound that are not given in the incoming audio signal are, quite simply, elements of distortion that entered along the way. Note that all amplifiers produce non-linear distortion as no electronic device is capable of reproducing a pure signal without adding harmonics. Diodes, transistors and amplifiers will all contribute to slight distortions.
With linear distortion, on the other hand, the signal is not fundamentally altered. This means that no additional frequencies are added. Instead, the sound produced differs from the pure audio signal due to changes in wave amplitude or phase. This can lead to the cancelling of certain frequencies or increased volume levels for others. It is not, however, the type of distortion that “colours” the sound.
Harmonic versus non-harmonic distortion
In order to understand harmonic distortion, we first need to cover the concept of harmonics. The pitch of a tone is determined by the what is known as the “fundamental tone” or simply “fundamental“. This is the loudest and lowest frequency in a periodic waveform which can be defined as a series of waves that form as multiples of a fundamental tone. For instance, if the fundamental is 100 Hz, the next wave that forms in a periodic waveform will be 200 Hz. This is called the first overtone in relation to the original fundamental. It’s also called the second harmonic — which can be a little confusing. Basically, the number for the harmonic is simply one up from the overtone. The second overtone/third harmonic for our 100 Hz fundamental will be 300 Hz, etc.
When a certain note is played on the piano, say middle C, the middle C is the fundamental but that is not all we hear. We also hear all the overtones that vibrate along with the middle C. Their sound will be softer, yet will contribute to the piano’s distinctive sound. Clarinets, for instance, sound like clarinets because the sound they produce contains dd harmonics with larger amplitudes ocompared to the even harmonics produced. The odd overtones are therefore more easily perceived by the human ear and give the clarinet it reedy, slightly plaintive sound.
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A recording will contain fundamental tones and their surrounding harmonics from the recorded voices and instruments it contains. The trick is not to add to these harmonics, yet this can easily happen as the audio signal is processed. A measurement called Total Harmonic Distortion indicates the difference between the harmonics present in the outgoing signal produced by an amp compared to the pure incoming signal. Tube amps are famous for their warm sound, an effect they produce by creating many even-order harmonics. This means more 2nd, 4th and 6th harmonics as opposed to 3rd, 5th and 7th which are more present in solid state amps.
Non-harmonic distortion like clipping is the result of additional frequencies being added to the signal which are not multiples of the fundamental tone. Since its relationship is not proportionate, it’s not harmonious in a mathematical or aesthetic sense. The non-harmonic distortion of an over-driven solid state amp is very much a part of the sound of heavy metal as well as some electronic and experimental genres. Clipping chops off the tops of the sine waves in the audio signal resulting in a very harsh sound. This effect is more difficult to pull from tube amps with their harmonious dominant second harmonics. Non-harmonic distortion can be used for interesting effects in certain types of music in which case it will actually be a part of the pure audio signal, but it is definitely not something that should be added to the signal by the audio gear used to process it.
Key points regarding distortion at a glance
- Distortion can be divided into linear and non-linear
- Non-linear distortion can be further divided into harmonic and non-harmonic
- Linear playback is a desirable sound that represents the pure audio signal without added distortion
- The fundamental tone refers to the lowest frequency in a periodic waveform which determines the pitch of a sound
- Overtones are frequencies that are created along with the fundamental tone and are multiples of the fundamental tone
- Harmonics refers to the relation of the overtone to the fundamental tone
- Absolute linear playback is an impossibility in the real world as electronic equipment will always add harmonics
- Harmonic distortion is considered undesirable although some people prize the “warm” tone tube amps produce through added even-order harmonic
All pictures: Property of Teufel Audio
