That was an awesome video, resulting in some follow up questions. Is it correct to say that lowering the bits increased the ground noise level, then why, (basically is quantization on the amplitude of the wave??)?
If lowering the number bits still produces the same output wave as input wave what and where is the lower limit that changes the sound?
Finally how is this 24 bit 16 bit related to my 128kbps mp3s?
With proper dithering, lowering the bits raises the noise floor. This is because dithering is approximately equal to changing the quantization error from something regular to something random.
If you don't dither, then the quantization error will likely be something harmonic rather than the hiss you hear in that video. This is much more noticeable[1].
Lowering the number of bits produces the same wave, but with more error. Dithering ensures that the error is random noise, which is why you hear "tape hiss" in that video as the bits gets lowered. With a tone frequency chosen to maximize the harmonic quantization error and no dithering you would start to hear it.
Lastly, 24 and 16 bit are largely unrelated to mp3s. In practice, mp3s are decoded to 16 bit audio, but they don't store the audio as PCM, but (roughly) rather as frequency/amplitude pairs which could in theory be decoded at any sample size. In any event, the absolute magnitude of
error introduced by encoding a 128kbps mp3 is way, way, way larger than 16 bit quantization error, and the goal of the mp3 encoder is to find a way that is as inaudible as possible.
MP3 is an old format at this point, so it does have audible defects in some types of music at even the maximum bitrate (certain percussive instruments are an issue). Vorbis and AAC work in some ways similar to mp3 but are strict improvements (though for the love of music, don't use the FAAC decoder, it's generally considered horrible).
1: Still not noticeable at 16-bit audio, as a similar test to the one in the article was done with no dithering on the 24 -> 16 bit conversion. at somewhere between 12 to 14 bits it would start to be discernible.
Ahh, a lots been cleared up but I don't understand
"Lowering the number of bits produces the same wave, but with more error"
If the wave is the same how can there be more error? and is quantization error caused by quantizing the steps in amplitude?
If you dither properly, then if you use a small number of bits you'll have a sine wave with noise and with more bits you'll have a sine wave with less noise.
If you dither improperly, then you'll end up with a sine wave plus some noise (less than if you had dithered) plus some harmonic distortion.
When talking about audio, one typically doesn't compare in the time domain, since that's not even remotely how ears work. One compares in the frequency domain, and until your quantization error is very large compared to the signal, there will still be some of the original content in the result.
If you consider the original to have been an unquantized, but already sampled grayscale image, then essentially all of the pixels have large quantization error (since they were originally some shade of gray, but now they are all either black or white), so in one sense the original image is gone. But in another sense what you see is the original image, plus some noise.
If lowering the number bits still produces the same output wave as input wave what and where is the lower limit that changes the sound?
Finally how is this 24 bit 16 bit related to my 128kbps mp3s?