The Denon track has some leading and trailing silence. Inside this, the tone fades in/out over about 800 samples. This is making some of the spectrum plots above uglier than they should be. The remainder consists of a sequence repeating every 441 samples. The spectrum of this using an FFT size of 44100 and rectangular window looks like this:
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A quick attempt at generating this signal from scratch using rounding doesn't quite match on all samples. This is probably simply due to different accuracies used in the calculations, and I'd rather not spend any time trying to find the exact recipe.
The Alpine track is rather nasty. It fades in/out over about a second, so that has to be trimmed. This is where it gets weird. Counting zero crossings puts the frequency at 1024.8 Hz. Most periods have 43 samples, a few 44. Poking at the signal, I noticed it had a growing DC offset (so not truly DC, for the pedantic). For a better look at this, I calculated a moving average with a window of 1334 samples (very close to 31 periods). Ideally, an average over an integer number of periods is zero. Instead, we have this:
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WTF? How does this happen? Leaving that mystery unsolved, we shall instead take a look at the spectrum. This being one of those times when the FFT can't be exactly matched to the signal, we use a Dolph-Chebyshev window. The result:
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WTF, again. Horrible harmonics, strange spurious tones, and a smattering of noise at the low end of the spectrum. The THD is -59 dB, THD+N -57 dB. This is much worse than just about any CD player. What exactly is this track supposed to test?
For those who prefer frequency on a log scale:
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Is that 60 Hz power line hum?
This is clearly a recording of an analogue signal. Didn't they have computers back in '88? Were tone generators and ADCs really this poor? Why would anyone pay money for this?