Listening to the tune

I’ve just been skimming the September 2003 issue of Klips, a newsletter published by Jim Tate Stereo in South Australia. Quite interesting, but I simply must object to one part that jumped off the page. It is part of a guide to listening to equipment with a view to choosing the best piece to buy:

Since the perceived pitch of a note consists of the sum of its fundamental plus its harmonics, a distortion that adds extra harmonics will shift the pitch of that note up slightly. Likewise, a distortion that results in the rolling-off of higher frequencies (thus reducing the amplitude of some harmonics) can lower the perceived pitch.

At the risk of offending, this is entirely wrong. Yes, most distortion adds harmonics. Indeed it adds harmonics not only to the fundamental (thus boosting some of the natural harmonics of the instrument so effected), but adds harmonics to the natural harmonics already on the recording. But harmonic distortion does not change the perceived pitch of the sound.

The sound of an instrument is determined by a number of different characteristics: its level, its pitch, its attack and decay and its timbre. For the most part (very deep bass sometimes excluded), the pitch is determined by the fundamental frequency of the sound. Distortion does not alter this. The character of the sound is determined primarily by attack, decay and timbre. Ignore the first two and consider a sustained note. A violin plays a note. A flute plays the same note. The fundamental frequency is the same for both. But the timbre is different, and that is why they sound different. Timbre is the determined by the proportion of harmonic components (and, in some instruments, non-harmonic components) in the note. Harmonic distortion changes the balance of these harmonic components. If harmonic distortion changed the perceived pitch, it would be impossible to play a flute and violin in tune with each other.

(On a more technical level, the ear is constructed somewhat like a Fourier analyser. It breaks down an incoming signal into several distinct frequency bands and processes them separately. That is why when you listen to a complex tone, like that produced by a violin, you can almost perceive the harmonic components on their own. But the fundamental frequency remains at its frequency, perceived by the relevant section of the cochlea and its associated nerve, regardless of the harmonic components accompanying the signal which are perceived in narrower parts of the cochlea and delivered to the brain by parallel nerves.)

Listening to the tune is a very useful technique for evaluating equipment, but for a different reason than that stated here. On low quality equipment, the tune can get lost in the muck. While harmonic distortion is a problem (at significant levels, particularly with odd-order harmonics, it can sound very harsh and make a violin sound less like it ought to thanks to the timbral changes), one of the most important factors distinguishing good from poor gear is spurious noise. This can come from ringing in speaker drivers, or simply lack of control so that they continue to produce sound after they are supposed to have stopped. If you listen not to the main tune in a piece of music, which is normally delivered at a fairly high level, but to one of the sub-themes playing along at the same time, you ought to be able to follow it. Unless, that is, the system is producing muck that obscures it.

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