Re: [ARSCLIST] Sampling Theory (was Fred Layn's post on the Studer list re: Quantegy)

["Steven C. Barr" <stevenc@xxxxxxxxxxxxxx>]

----- Original Message -----
From: "Mike Richter" <mrichter@xxxxxxx>
> Suppose the subject "cannot hear" anything above 15 KHz. That means that
> higher pure tones (sine waves) are not audible on standard tests or by her
> own experience. Set up two oscillators, one fixed at 8 KHz sine wave, the
> other adjustable for frequency and level but generating square waves. The
> challenge: adjust frequency and amplitude until the two match. Nothing
> about the test except frequency response of the amplifier and reproducer
> (speaker[s] or headphones) must extend beyond 25 KHz.
>
> Simply put, no adjustment will cause the two to match. The best fit will
> still leave the 8 KHz square wave sounding 'sharper'. That despite the
fact
> that elementary Fourier analysis shows the first difference between the
two
> signals is at 24 KHz, which the subject "cannot hear".
>
> Once the subject has mastered that issue, repeat with sine and triangle
> waves. The difference is more subtle, but still in dozens of tests the a
> amtch cannot be achieved. Most subtle, so occasionally it fails, is square
> vs. triangle. In that, the primary difference is phase of the harmonics
> relative to the fundamental.
>
> Conclusion: the ear is not linear. It is sensitive to the effects of
> amplitude and phase of frequencies well beyond that at which it can sense
a
> pure tone.
>
> We did this experiment back when AES was working on the standards for
> CD-DA. A friend who was a member (and who has since passed away) joined me
> for the planning and testing, but it was to no avail. Of course, the
> objective was to get the sample rate above 44.1 Ksps so that phase could
be
> retained where it mattered most. Even in my home audio system, which is
> good but far from the sort to satisfy 'golden ears', the difference
between
> CD and direct-to-disc recording on the best material (e.g., from
Sheffield)
> is audible and is in line with the tests above. Piano sound, where up to
> the 13th harmonic is important, is seriously affected and where one can
> easily distinguish among fine instruments from the LP, the lines are
> blurred by the effects of sampling on CD.
Actually, I can provide an example from "real life!" We harmonica players
usually use overdriven amplifiers to try and recreate the original sound
of blues as played in Chicago in the early 1950's. An overdriven tube
amplifier provides distorted sine waves...but still rounded waveforms.
It sounds distorted, but pleasantly so!

A solid-state amplifier, because of a greater high-frequency response and
transient response, will cut off abruptly when the signal passes a certain
level...producing a series of essentally "flat-topped" waves similar to
square waves. As a result, an overdriven solid-state amp produces a
nasty, edgy, harsh sound.

This is why firms like Fender still use vacuum tubes...

...stevenc
http://users.interlinks.net/stevenc/