READERS’ LETTERS: Hi Rez and oversampled DACs

GARY L writes ...

One thing to mention about Nyquist Theorem, is the “at least” part, which is often overlooked. If your at the limit (half the frequency of the sample rate) and the sampling lines up nicely with the phase of the recorded frequency, then yeah you get the full amplitude of the sine wave… however if you phase shift that along by 90deg (quarter a wavelength) rather than comparing peak to trough, you are comparing mid point to mid point and get 0 difference in amplitude. This means the top frequencies are shot. When you go down to a quarter of the sample rate (~11kHz for 44.1kHz) you can be upto 1.5dB down. Note this is not a fixed amount down it will be waving about all over the place so you can’t EQ it out. To get the sound correct to under 0.35dB at 20kHz you need to sample at 160kHz, by doing this I wouldn’t expect my ears to magically hear 80kHz, but I would expect my ears to perceive an improvement in the 5kHz-20kHz region.

Paul – given there is music out there we like, other than the great stuff Octave Records produce, what should we be looking out for when buying hi-res, what should give us comfort and what should we look for as red flags?

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