Quote:
Originally Posted by pcspecialist
Question: Why use a lowpass filter when low frequencies take up so very little space.
Answer: Low frequencies are extremely unforgiving and music sounds better with the low frequencies discarded rather than applying much lossy compression.
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I don't get what you're trying to say here. A lowpass filter does not discard low frequencies. It keeps them.
If a considerable reduction in file size could be achieved by leaving out the lowest frequencies (which is not the case), the designers of lossy codecs would certainly have dropped them at low bitrates. For practically all normal music they are just a nice extra, but non-essential. If you'd need to reduce a song to its most essential description, you certainly wouldn't say "a 32Hz tone there", you would focus on frequencies that make up actual musical notes and voices. Many songs don't even contain anything significant below 50Hz.
The fact is, MP3 doesn't even know about ultra-low frequencies, because its frequency resolution is too small. The window size (1152 samples/channel) is too small for even a single cycle of anything below 38Hz to fit in. So those frequencies are actually described by combinations of higher-frequency components and a DC offset. This is another reason why there's little to be gained by dropping the ultra-low frequencies. But, it also means that for the MP3 encoder, each frame containing such frequencies will appear to contain a bunch of higher frequencies instead. If it's impossible to encode all those frequencies accurately, extra noise will be introduced. But the amount of distortion would need to be huge before the amplitude or frequency of the low frequency wave itself will be significantly altered.
So what I'm trying to say here is that even when encoding at low bitrates, sub-bass sounds will not be easily dropped, they will mostly be augmented with noise in the higher bass range. This could give a different sensation when using a subwoofer, though. But if you have never heard (or felt) the original song, you will most likely not notice anything unusual about the bass, whereas the typical jangling 'glass shards' distortion on the trebles is mostly immediately recognizable even on songs you've never heard before.
If you're still convinced that low bitrate MP3s drop the lowest frequencies, then you should download
this sound. It's a sweep going from 16Hz to 1600Hz, and it's encoded at a measily 48kbps (mono). Even at that bitrate, the difference between the encoding and the uncompressed sound is practically zero. The only noise (audible after amplifying the difference by at least 20dB) is in the part around 1kHz. The low frequency part does get polluted with extra frequencies when the same sound is encoded with extra noise added. But the low frequencies themselves are still present even at the lowest possible bitrates.