EMMA 02

Cidelsa

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Sound

The CDP1869 address and sound generator (which is part of the Video Interface System) is used for audio. 2 channels are available, one for tone generation with a span of 8 octaves, and 1 for white noise with a span of 8 frequencies. Volume of both channels is programmable independently in 16 steps.

The VIS is using 2 CDP 1802 output instructions, OUT 7 to select the register and OUT 4 for actual data output. Note that for VIS register 4 and 5 the CDP1802 address lines are used and NOT the data bus. As such 16 bits are available in one output instruction.

VIS register 4 is used to control the tone generation:

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

0

Tone

Off

Range

Tone volume

Within each tone frequency range the 'input frequency' (see table 1) is divided by the 'Tone+1' value, producing up to 128 different frequencies. This frequency is then divided by two providing a square-wave signal that is turned on or off by bit 7. A high on bit 7 turns the tone output off.

Tone frequency range

Input frequency (KHz)

CPU clock divider

Bit 6

Bit 5

Bit 4

3,579

0

0

0

6,9902344

512

0

0

1

13,980469

256

0

1

0

27,960938

128

0

1

1

55,921875

64

1

0

0

111,843750

32

1

0

1

223,68750

16

1

1

0

447,37500

8

1

1

1

894,75000

4

Table 1: Tone frequency range selection

VIS register 5 is used to control the noise generation, only bit 8 to 15 are used for noise generation the lower bits are used for video formatting functionality:

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Off

Range

Noise volume

Video bits...

For noise frequency range eight ranges are available (see table 2). The white noise output is turned on or off by bit 15. A high on bit 15 turns the white noise off.

Noise frequency range

Top of range frequency (Khz)

CPU Clock divider

Bit 14

Bit 13

Bit 12

3,579

0

0

0

0,8737793

4096

0

0

1

1,747559

2048

0

1

0

3,495117

1024

0

1

1

6,990234

512

1

0

0

13,980469

256

1

0

1

27,96094

128

1

1

0

55,92188

64

1

1

1

111,84375

32

Table 2: Noise frequency range selection