THD 1 – 1/10/2-14
Distortion
Distortion is characterised in the Master Handbook of
Acoustics (Everest & Shaw), as ‘any change in the waveform or harmonic
content of an original signal, as it passes through a device. The result of
nonlinearity within a device’.
This distortion not only includes having a variable
input/output relationship with the signal, but may also modify the basic
content of the signal.
One example of the behavioural characteristics of this nonlinearity
may be the change of harmonic content within the signal with respect to the
frequency domain. There are two
predominant measures of this which I will discuss briefly in the coming posts;
Total Harmonic Distortion and Intermodulation Distortion. These are two methods
which have been used since the 1940s, to characterise the effect a device has
on a signal. There is a big catch to both of these methods, which I will also
discuss.
Total Harmonic Distortion
Total harmonic distortion (THD) is the measure of a system
response and viewed in the frequency domain, when the system is given an input
of a spectrally pure sine wave at a given frequency (often 1kHz).
The total energy of the output (in the frequency domain) is
measured, and compared with the level of the excitation tone. This relationship
is characterised as a percentage, and known as the percentage of total harmonic
distortion.
The benefits of this method of nonlinear behaviour
characterisation, is that you can clearly see (graphically and numerically) the
effect a device has on a signal. The down side is that this method by
definition only works with a spectrally pure signal, and in modern measurement
systems the noise floor of the system must also be taken into account. Pure
sine tones do not accurately represent the complex and highly variable signals
presented as music, and therefor do not relate to what realistically occurs when
a system distorts musical signals. Furthermore, links to the perception of
distortion of pure tones may not relate to the perception of the distortion of
complex tones.
For more information: see Temme 1992 – Audio Distortion
Measurements – a Brual and Kjaer text on methods of measuring distortion.
Shmilovits 2005 characterises THD in his paper ‘On the
definition of Total Harmonic distortion and its effect on measurement
interpretation’ as either a comparison to a signals RMS, or its harmonic
content and advocates the latter. As such, I will be using this method in the
analysis on the effect of the nonlinear devices.
In the following blog post, I will give an example of a
total harmonic distortion measurement, and compare what is measured with a
complex signal and a spectrally pure signal.
In the coming week I plan to try and find a method of using
THD measurement to characterise the distortion effect of a device on complex
music.
I will also look for supporting literature.
I have found a number of papers on the perception of
distortion, but none specifically looking at the links to loudness perception.
Generally most work is done on quality.
No comments:
Post a Comment