Paul McGowan / PS AUDIO
We often take standards for granted. Yet, inevitably, there’s a long history behind their creation.
Take the idea of measuring distortion. Today, it seems rather obvious that before we listen to a new design, we’d want to measure its ability to pass a single unimpeded. To do that we place it on our Audio Precision analyzer and run a full sweep. We can see if it has flat frequency response and how much noise and distortion is present.
But how did we get here?
Best I can tell, in 1933, a new group was formed called the American Standards Association (ASA) with the goal of generating standards for measuring sound quality.
In those early days, the biggest problem of sound reproduction was achieving a flat frequency response. So this new committee focused its efforts on that problem by developing the “Octave Band Method.” This method measured the frequency response of an audio system by breaking the frequency range into eight bands. Broken apart like this, it was then possible to measure the amplitude (loudness) of each separate band and then graph the results by hand.
The first frequency “sweep” had been created.
Move forward in time by ten years, and we find the engineers now concerned with looking deeper into sound quality than just frequency response. Now, they wanted to look at distortion. The earliest efforts to establish a standard for maximum THD levels were led by the Radio Manufacturers Association (RMA) in the 1940s. The RMA worked with the Institute of Radio Engineers (IRE) to develop a series of practices for measuring and specifying the distortion characteristics of audio equipment. They would name their measurement findings with a three-letter acronym: THD. (Total Harmonic Distortion, i.e., how many “extra” unwanted frequencies of sound were added to the original pure signal).
The pioneers involved in this work include names you’re likely familiar with: Henry Kloss, James B. Lansing, and Walter G. Schottky. These engineers were instrumental in establishing the RMA/IRE standards for THD, and much of their work was conducted through listening tests with trained and untrained listeners and, gasp, comparing the performance of different types of audio equipment.
Move forward another ten years to the 1950s, when the International Electrotechnical Commission (IEC) developed the first standard for measuring THD. This standard was called IEC 268, and it specified a method for measuring the THD of audio amplifiers.
The IEC 268 standard used a pure sine wave as the input signal to the amplifier and then measured the level of added harmonics (frequencies higher than the original) in the output signal. The level of the harmonics was expressed as a percentage of the original signal level.
Over the years, the IEC 268 standard has been revised and updated to include a method for measuring THD+N, which takes into account both the harmonic distortion and the noise in the audio signal.
There’s plenty more history to all of this, but I found it interesting to look back a bit and see how all that we take for granted came to be.