He says the intervals are arbitrary, but they are not, or at least not as arbitrary as that. An octave is 2x the pitch, a fifth is exactly 3/2x the pitch, a major third is exactly 5/4x the pitch etc.
This is fine for instruments which can play an arbitrary pitch like a violin or trombone. But for fretted instruments like guitar, or ones which use a different oscillator per note like piano, xylophone or harmonica, one has to make a decision. Either you tune it to play perfect intervals and can therefore play in only one key, or you tune it to a compromise which can play in any key and sound OK, but no key "perfectly". Equal temperament is one such compromise and is described in the article.
For instruments which play often play long chords like strings, the difference between a perfect interval (as an orchestra would play) and an equal temperament imperfect interval (as a general purpose synthesiser will play) is stunning. Some synthesisers like those from Access have modes to automatically correct the tuning of the third, fifth and maybe seventh harmonics to make them perfect.
This is fine for instruments which can play an arbitrary pitch like a violin or trombone. But for fretted instruments like guitar, or ones which use a different oscillator per note like piano, xylophone or harmonica, one has to make a decision. Either you tune it to play perfect intervals and can therefore play in only one key, or you tune it to a compromise which can play in any key and sound OK, but no key "perfectly". Equal temperament is one such compromise and is described in the article.
For instruments which play often play long chords like strings, the difference between a perfect interval (as an orchestra would play) and an equal temperament imperfect interval (as a general purpose synthesiser will play) is stunning. Some synthesisers like those from Access have modes to automatically correct the tuning of the third, fifth and maybe seventh harmonics to make them perfect.