|
|
原帖由 jjx01 于 2007-12-26 17:59 发表 5 V! W+ M9 `5 l+ y
》大家看上面这张图,2倍的采样率完好的重现了源信号: t! Y+ P, v' U; [% o2 W
. N. E1 J! i. W8 A+ @- e( I8 b正弦线最后变成了三角形组合……
- E+ N! P1 Y& h$ A9 j- b+ l) ? L4 s# D0 f4 L
44。1也不是刚好是20的两倍……
; D5 ?- r4 [" b8 b' O9 Y- w! v x; V Q% H, P
Why 44100? Why not make it a round decimal value like 44000, or a round binary quantity like 44032? Why not 32KHz or 48KHz? In general, the human ear can hear tones out to about 20KHz. According to a smart fellow named Nyquist, you have to sample at twice that rate. Because of imperfections in filtering, you actually want to be a little above 40KHz. According to John Watkinson's _The Art of Digital Audio_, 2nd edition, page 104, the choice of frequency is an artifact of the equipment used during early digital audio research. Storing digital audio on a hard drive was impractical, because the capacity needed for significant amounts of 1 Mbps audio was expensive. Instead, they used video recorders, storing samples as black and white levels. If you take the number of 16-bit stereo samples you can get on a line, and multiply it by the number of recorded lines in a field and the number of fields per second, you get the sampling rate. It turned out that both NTSC and PAL formats (the video standards used in US/Japan and Europe, respectively) could handle a rate of 44100 samples per second. This rate was carried over into the definition of the compact disc. ( g' z+ y5 N3 ]6 K- C1 b
另外,虽然不是正好的两倍,但采样出来也是和零差不多了。 |
|