The GedLee metric - a new way to measure distortion?
Distortion measurements do not always tell the whole truth about what equipment sounds like. Should we pay more attention to some distortion products than others?
Hi-Fi News a few months ago had an interesting article on the measurement of distortion and the correlation of such measurements with subjective experience.
Distortion occurs in all electronic and digital audio systems. The shape of the waveform of a signal is altered leading to the production of new frequencies from the frequencies present in the signal.
Distortion is measured by inputting a sine wave, which is the simplest signal consisting of only one frequency, into the equipment under test. If this is 1 kHz, then the harmonic distortion products generated will be 2 kHz, 3 kHz, 4 kHz... all the whole-number multiples of 1 kHz up to and beyond the limits of human hearing. The level of the distortion components is then expressed as a percentage of the whole signal. A figure of 0.1% is considered adequate, but even lower figures are desirable.
But there is a problem. It is possible for a piece of equipment to measure very well, yet still not sound entirely clean. The suspicion is that not all distortion products are equally offensive to the ear.
Listening tests conducted by a number of researchers tend to show that higher-order harmonics are more troublesome than lower order such as 2nd, 3rd etc. This implies that it would be helpful to weight the individual harmonics in the measurement according to their nuisance value.
This is not new science. As early as 1937, according to the Hi-Fi News article, the Radio Manufacturers' Association of America suggested that higher order harmonics should be progressively more heavily weighted. The BBC refined this in 1950 and again in 1961.
The problem was however that in these early days of electronics it was extremely difficult to measure the levels of the various harmonics accurately. Even today, this is not a standard facility on most test equipment.
However, Earl Geddes and Lidia Lee have suggested that not only is this essential, but that distortion products at low levels also be more heavily weighted, and have come up with the 'GedLee metric' which is basically a formula for calculating a figure from measurements that will correspond closely to the subjective experience of distortion.
This is a worthy enterprise. If there could be one figure that all manufacturers could quote and could be relied upon, it would be very much easier to make comparisons between different equipment.
However, the chances of such a common system being adopted are not good. For one thing, manufacturers whose current products do not fare well under the GedLee metric will not be inclined to use it. For another, the GedLee metric is not the 'god equation' of high fidelity. There could well be better ways of measuring the subjective effect of distortion.
Perhaps the ultimate solution would be to publish tables of harmonic spectra, then users could calculate their own figures based on whatever metric they think is best for their purposes.
By David Mellor, Course Director of Audio Masterclass, previously published in Record-Producer.com or in print, republished by Audio Masterclass September 1, 2008