hifi.slovanet.sk

[dustin]

Zajímavé

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.

[velky bobo]

Jen tak pro zajímavost, 44100=2.2.3.3.5.5.7.7
... což může mít v některých situacích docela zajímavé důsledky.

[dustin]

Jistě, je to vše důsledek počátečních možností technologií.

http://en.wikipedia.org/wiki/NTSC

The figure of 525 lines was chosen as a consequence of the limitations of the vacuum-tube-based technologies of the day. In early TV systems, a master voltage-controlled oscillator was run at twice the horizontal line frequency, and this frequency was divided down by the number of lines used (in this case 525) to give the field frequency (60 Hz in this case). This frequency was then compared with the 60 Hz power-line frequency and any discrepancy corrected by adjusting the frequency of the master oscillator.

The only practical method of frequency division available at the time was the use of multivibrators, which could only divide by small numbers. For interlaced scanning an odd number of lines per frame was required in order to make the vertical retrace distance identical for the odd and even fields; an extra odd line means that the same distance is covered in retracing from the final odd line to the first even line as from the final even line to the first odd line, so simplifying the retrace circuitry. This meant that a chain of multivibrators was needed, each of which had to divide by a small, odd number. (Note that an odd number is never divisible by any even number). The closest practical sequence to 500 was 3 × 5 × 5 × 7 = 525. Similarly, the British 405-line system used 3 × 3 × 3 × 3 × 5 and 625-line PAL used 5 × 5 × 5 × 5. Although other values were theoretically possible, all of them involved division by unacceptably large numbers like 13 or 17, which produced reliability problems.

[blueneon81]

Len sranda je tá, že PCM procesory pre NTSC poskytli 44,056 kHz a PCM procesory pre PAL práve tých 44,1 kHz