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Re: Not the mistake I was referring to.

Posted by thetubeguy1954 (A) on March 9, 2007 at 13:52:53

In Reply to: Not the mistake I was referring to. posted by Pat D on March 9, 2007 at 13:04:42:

Pat D-Cake,

I stated that specifications are derived from measurements. Although I'm fairly sure I knew that to be a correct statement BEFORE making that comment I did a little research. Here's info on THD as taken from the link below, which FYI is about pro-audio components.
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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%
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As you can clearly see it's the measurements that provided the specifications, which is precisely what I said. Now whether or not you're willing to believe what the manufacturer says is up to you or anyone else who reads them. I find it quite amusing that objectivists bow at the altar of measurements, yet don't trust that what's provided by the manufacturer is correct. That caution of course is not without some merit for as the article quite clearly states without knowing the conditions under which the measurements were taken one cannot know the validity of the specification!
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Pat D-Cake you state: I have not corrected your grammar. Not this time, but you have in the past. I was simply using it as an example of objectivist misdirection as opposed to addressing the actual topic.

Thetubeguy1954

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