noise, dithering, compression and high-end amps
2004-03-10 09:43:30
I have some understanding of digital processing, dithering and noise shaping, although not very deep. But there is something that caught me hard and I wanted to ask for clarification. Once upon a time at Creative site JohnV wrote:Lossy audio encoding is based basically on the concept of "adding" distortion/noise. The more noise is added, the higher is the compression ratio. The noise is called quantization noise. Now, quantization noise, which is the difference between the original and encoded can be divided into audible quantization noise and inaudible quantization noise. Together those form the global quantization noise. Lossy audio is based on the idea that as much inaudible quantization noise will be introduced as possible, and of as little as possible audible quantizatio noise. The psychoacoustics model is used to analyse the audio and to form a masking threshold which is used in the quantization phase. Very simplified, the masking threshold defines when something is masked (not audible) or non-masked. The idea is to introduce as much quantization noise as allowed by the masking threshold. If this works, and the masking threshold is totally perfect (not in real life) only inaudible quantization noise will be introduced. Now, of course some psychoacoustic models are better than others, and allow more precise masking threshold which in turn allows more inaudible quantization noise -> more compression/better quality. It was part of discussions that waveform preservation is completely useless goal and better sound is achieved by more clever distortions. What I'd like to understand better, is (1) how does adding noise help compression ratio? I thought that increasing entropy makes it harder to compress data. What kind of trick is here? Is it simply implied that bit reduction is equal to adding quantization noise? (2) What kind of distortions can be considered equivalent to quantization noise? I assume that one of main issues is related with least significant bit, and dithering helps there. What kind of other imprecisions could be attributed as quantization noise? Does DAC clock jitter fall here? Does nonlinear distortions of amps belong here? Seems so, as both can be seen as timing imprecisions. (3) What I'm wondering is if its possible to overcome lonlinearities of amps by intentionally adding noise with shaping and accounting for psychoacoustic models?