Gilderflukeco old MACs 8 bit Digital Audio System Manuel d'utilisateur télécharger pdf (Page 95)

- Internal Details -

Because of the rather low I/O speed of an IBM compatible computer, the data is buffered into and

out of the DAS-100 through four 2 KByte deep FIFO buffers. This allows the IBM to ignore the hardware for

periods as long as it takes 1 KByte worth of audio to come or go on the DAS-100. Even at a 15 KHz

bandwidth (35.1 KHz sample rate), this allows the IBM 1/35th of a second between having to service the

DAS-100. It is during these periods that the software handles all disk I/O, screen handling, and other

functions. The disk drive controller buffers data by a certain number of blocks ahead, and dumps these

to/from the disk as these buffers are filled/emptied. When the buffers are changed, you can see the time

display update on the screen.

The format of the data which is burned into the EPROMs is in blocks of 1 KByte each. The first byte of

the audio data is normally replaced by one flag byte. This sets the operating settings for the following

1023 audio data bytes. Bits 0 to 2 set the sample rate, while bits 3 to 5 set the output filter cutoff point.

The last two bits flag the ends of samples.

bits 2 1 0 sample rate frequency cutoff

1 1 1 35,156.25 Hz 15.00 KHz

1 1 0 23,437.50 10.00 KHz

1 0 1 17,578.125 7.50 KHz

1 0 0 11,718.75 5.00 KHz

0 1 1 8,789.00 3.75 KHz

0 1 0 7,031.00 3.00 KHz

0 0 1 4,687.00 2.00 KHz

0 0 0 Delay

OFF

bits 5 4 3 filter frequency frequency cutoff

1 1 1 1.5 MHz 15.00 KHz

1 1 0 1.0 MHz 10.00 KHz

1 0 1 750.0 Khz 7.50 KHz

1 0 0 500.0 KHz 5.00 KHz

0 1 1 375.0 KHz 3.75 KHz

0 1 0 300.0 KHz 3.00 KHz

0 0 1 200.0 KHz 2.00 KHz

0 0 0 OFF

OFF

bit 6:

0 no effect

1 end of all sampled data after this 1 KByte is played.

bit 7:

0 no effect

1 stop after this 1 KByte is played.

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If the sample frequency is set to delay, then the next two bytes following the flag byte are used to

set how long the delay will be (MSB first). Each count is equivalent to one 1 KByte block of data played

when played at a 15 KHz bandwidth (this is equal to 1024/35,156.25 of a second). The normal flag byte

for the remaining 1021 bytes of data follows the two delay bytes. All four of these flag bytes take the

place of the normal flag bytes which would be found in these positions.

This condition doesn't effect the frame rate, but just shuts down the output filter.

1 2 ... 90 91 92 93 94 95 96 97 98 99 100 ... 109 110

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Gilderflukeco old MACs 8 bit Digital Audio System Manuel d'utilisateur Page 95

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