baluba skrev:
Hvor lav samplingsrate er elendig?
32khz f.eks
Generelt har du frekvensområde opptil halve samplingfrekvensen, det følger av Nyquists samplingteorem. Så 32khz gir deg et frekvensområde opptil 16khz, over det intet meningsfullt. Det er derfor 44.1khz ble valgt for CD, det gir det høbare område (antatt opp til 20kHz) + litt headroom.
Det er ikke slik at det normale frekvensområdet blir mer nøyaktig jo høyere samplingfrekvens, digital lyd blir gjerne fremstillt som "trappetrinn" som blir mindre og mindre jo høyere samplingfrekvens som brukes, men det er ikke veldig presist. Halve samplingraten gir deg øvre cutoff-frekvens.
Bitdybde gir dynamikkområde, ca 6dB pr. bit.
Hadde CD blitt oppfunnet i dag, hadde nok både bitdybde og samplingfrekvens sikkert vært større, men da den kom var både lagringsplass og prossesseringskraft et mye. mye knappere gode enn hva det er i dag.
Edit:
Grunnen til at akkurat 44.1kHz er valg (det er jo et merkelig tall - hvorfor ikke 44 eller 45 eller 48?) kommer visstnok fra videoverdenen har jeg lest.
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From John Watkinson, The Art of Digital Audio, 2nd edition, pg. 104:
In the early days of digital audio research, the necessary bandwidth of about 1 Mbps per audio channel was difficult to store. Disk drives had the bandwidth but not the capacity for long recording time, so attention turned to video recorders. These were adapted to store audio samples by creating a pseudo-video waveform which would convey binary as black and white levels. The sampling rate of such a system is constrained to relate simply to the field rate and field structure of the television standard used, so that an integer number of samples can be stored on each usable TV line in the field. Such a recording can be made on a monochrome recorder, and these recording are made in two standards, 525 lines at 60 Hz and 625 lines at 50 Hz. Thus it is possible to find a frequency which is a common multiple of the two and is also suitable for use as a sampling rate.
The allowable sampling rates in a pseudo-video system can be deduced by multiplying the field rate by the number of active lines in a field (blanking lines cannot be used) and again by the number of samples in a line. By careful choice of parameters it is possible to use either 525/60 or 625/50 video with a sampling rate of 44.1KHz.
In 60 Hz video, there are 35 blanked lines, leaving 490 lines per frame or 245 lines per field, so the sampling rate is given by :
60 X 245 X 3 = 44.1 KHz
In 50 Hz video, there are 37 lines of blanking, leaving 588 active lines per frame, or 294 per field, so the same sampling rate is given by
50 X 294 X3 = 44.1 Khz.
The sampling rate of 44.1 KHz came to be that of the Compact Disc. Even though CD has no video circuitry, the equipment used to make CD masters is video based and determines the sampling rate.
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http://www.cs.columbia.edu/~hgs/audio/44.1.html
