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48, 44.1 khz, why no and/or not 40 khz, Only for 2 channel audio...........
post Dec 18 2002, 04:11
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Look at this Poll

How high can you hear (with music & lowpass)

The poll starter didn't put frequency above 20 khz in the poll
So why don't just use a 40 khz PCM instead of 44.1 khz as human
can't listen to frequency above 20 khz?

16 x 44100 x2 = 1141.2 kbit/s

16 x 40000 x 2 = 1280 kbit/s

It does help to save some space!
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post Dec 18 2002, 08:24
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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.  

The 44.1 Khz sample rate is used in Mp3s and such because most of them are ripped from CDs and resampling is was not as good as it is today (Not too good still)..

Sampling vs Frequency Response:
   The sampling frequency must be at least twice as high as the highest frequency that you wish to reproduce because you must have at least 1 data point for each half cycle of the audio waveform. The highest frequency that you can record with a sampling rate of 8k is 4000hz. At a sampling rate of 44k, you can record up to 22khz but the filters used in the D/A conversion process have a very high rate slope at 20,000hz which will allow nothing higher than 20khz to get through. The newer D/A converters with high rates of oversampling are able to use low pass filters with a slower roll off.

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