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Comment by ethereon | Hacker News Reader

I believe you're messing up the units here.

- The sampling rate, measured in kHz, for CD tracks is 44.1 kHz (not kbps), which determines the frequency range. Nyquist-Shannon applies to this.

- The bit-depth, number of bits/sample, determines the SNR of the signal, but has no effect on the frequency range. A typical audio CD has a bit depth of 16.

- The bit-rate, measured in kbps, is (sampling_rate * bit_depth * channel_count). So for an audio CD, this would be 441000 * 16 * 2 = 1411200 bps = 1411.2 kbps.

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> The sampling rate, measured in kHz, for CD tracks is 44.1 kHz (not kbps) ...

Yes, corrected, thank you. The original point stands -- the higher resampling rates are increasingly pointless.

> The bit-depth, number of bits/sample, determines the SNR of the signal ...

Yes, and is thought to be the source of the much-remarked, subtle difference between vinyl album music and CD music. But no one knows for sure.

> The bit-rate, measured in kbps, is (sampling_rate * bit_depth * channel_count). So for an audio CD, this would be 441000 * 16 * 2 = 1411200 bps = 1411.2 kbps.

True. Unfortunately, these numbers may motivate people to argue for higher and higher MP3 sampling rates, ignoring the fact that the original temporal sampling rate severely limits the usefulness of these efforts.

>The original point stands -- the higher resampling rates are increasingly pointless.

If we were talking about sampling rates it would. But we're not, we're talking about bitrates.

Any MP3 of a CD, whether 128kbps, 192kbps, 320kbps or something else, will still have a sample rate of 44.1kHz. Resampling at 128kHz would indeed be stupid and pointless, and would also result in a much larger file than the original CD. That is not what mp3 does; it takes your 16bits@44.1kHz CD bitstream, and gives you a compressed bitstream that will, when decompressed, give you another 16bits@44.1kHz bitstream which sounds similar. The bitrate measures the size of this compressed bitstream, and therefore gives an indication of how much information (in the technical sense) you must have discarded to form it. But it's entirely unrelated to the sampling rate - you can't even say bitrate = sampling rate * bit depth when we're talking about a compressed bitstream.

> The original point stands -- the higher resampling rates are increasingly pointless.

But the mp3 tracks are not resampled. They use the same sampling rate of 44.1kHz, with a variable bit-depth. So, for instance, a 320kbps mp3 file still has the original sampling rate of 44.1kHz.

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