Objectively comparing pro audio signal processing products is often impossible.
Missing on too many data sheets are the conditions used to obtain the published data.
Audio specifications come with conditions. Tests are not performed in a vacuum with random parameters. They are conducted using rigorous procedures and the conditions must be stated along with the test results.
To understand the conditions, you must first understand the tests. This article introduces the classic audio tests used to characterize audio performance. It describes each test and the conditions necessary to conduct the test.
Apologies are made for the many abbreviations, terms and jargon necessary to tell the story. Please make liberal use of the Rane Pro Audio Reference to help decipher things.
Also, note that when the term impedance is used, it is assumed a constant pure resistance, unless otherwise stated.
The accompanying table (back page) summarizes common audio specifications and their required conditions. Each test is described next in the order of appearance in the table.
By its name you know it is a measure of unwanted signals. Distortion is the name given to anything that alters a pure input signal in any way other than changing its magnitude. The most common forms of distortion are unwanted components or artifacts added to the original signal, including random and hum-related noise. A spectral analysis of the output shows these unwanted components.
If a piece of gear is perfect the spectrum of the output shows only the original signal—nothing else—no added components, no added noise—nothing but the original signal. The following tests are designed to measure different forms of audio distortion.
THD. Total Harmonic Distortion
What is tested? A form of nonlinearity that causes unwanted signals to be added to the input signal that are harmonically related to it. The spectrum of the output shows added frequency components at 2x the original signal, 3x, 4x, 5x, and so on, but no components at, say, 2.6x the original, or any fractional multiplier, only whole number multipliers.
How is it measured? This technique excites the unit with a single high purity sine wave and then examines the output for evidence of any frequencies other than the one applied. Performing a spectral analysis on this signal (using a spectrum, or FFT analyzer) shows that in addition to the original input sine wave, there are components at harmonic intervals of the input frequency.
Total harmonic distortion (THD) is then defined as the ratio of the rms voltage of the harmonics to that of the fundamental component. This is accomplished by using a spectrum analyzer to obtain the level of each harmonic and performing an rms summation.
The level is then divided by the fundamental level, and cited as the total harmonic distortion (expressed in percent). Measuring individual harmonics with precision is difficult, tedious, and not commonly done; consequently, THD+N (see below) is the more common test.
Caveat Emptor: THD+N is always going to be a larger number than just plain THD. For this reason, unscrupulous (or clever, depending on your viewpoint) manufacturers choose to spec just THD, instead of the more meaningful and easily compared THD+N.
Required Conditions. Since individual harmonic amplitudes are measured, the manufacturer must state the test signal frequency, its level, and the gain conditions set on the tested unit, as well as the number of harmonics measured. Hopefully, it’s obvious to the reader that the THD of a 10 kHz signal at a +20 dBu level using maximum gain, is apt to differ from the THD of a 1 kHz signal at a -10 dBV level and unity gain. And more different yet, if one manufacturer measures two harmonics while another measures five.
Full disclosure specs will test harmonic distortion over the entire 20 Hz to 20 kHz audio range (this is done easily by sweeping and plotting the results), at the pro audio level of +4 dBu. For all signal processing equipment, except mic preamps, the preferred gain setting is unity. For mic pre amps, the standard practice is to use maximum gain. Too often THD is spec’d only at 1 kHz, or worst, with no mention of frequency at all, and nothing about level or gain settings, let alone harmonic count.
Correct: THD (5th-order) less than 0.01%, +4 dBu, 20-20 kHz, unity gain
Wrong: THD less than 0.01%