
	ColecoVision Sound Generation Hardware

 The ColecoVision uses the Texas Instruments SN76489A sound
generator chip. It contains three programmable tone generators,
each with its own programmable attenuator, and a noise source
with its own attenuator.

1. Tone Generators

 Each tone generator consists of a frequency synthesis section
requiring ten bits of information to define half the period of
the desired frequency (n). F0 is the most significant bit and
F9 is the least significant bit. The information is loaded into
a ten stage tone counter, which is decremented at a N/16 rate
where N (3.579MHz) is the input clock frequency. When the tone
counter decrements to zero, a borrow signal is produced. This
borrow signal toggles the frequency flip-flop and also reloads
the tone counter. Thus, the period of the desired frequency is
twice the value of the period register.

The frequency can be calculated by the following:

   f = 3.579MHz/(32n)

The output of the frequency flip-flop feeds into a four stage
attenuator. The attenuator values, along with their bit position
in the data word, are shown below. Multiple attenuation control
bits may be true simultaneously. THus, the maximum attenuation
is 28 db.

	A0	A1	A2	A3		Weight

	0	0	0	1		 2 db
	0	0	1	0		 4 db
	0	1	0	0		 8 db
	1	0	0	0		16 db
	1	1	1	1		 Off

2. Noise Generator

 The noise generator consists of a noise source that is a
shift register with an exclusive OR feedback network. The
feedback network has provisions to protect the shift register
from being locked in the zero state.

	Noise Feedback Control

		FB		Configuration

 		0		"Periodic" Noise
		1		"White" Noise


	Noise Generator Frequency Control

	NF0	NF1		Shift Rate

	0	0		N/512
	0	1		N/1024
	1	0		N/2048
	1	1		Tone gen #3 output

3. Control Registers

 The SN76489A has eight internal registers which are used
to control the three tone generators and the noise source.
During all data transfers to the SN76489A, the first byte
contains a three bit field which determines the destination
control register. The register address codes are shown here.



	Register Address Field

	R0	R1	R2	Destination Control Register

	0	0	0	Tone 1 Frequency
	0	0	1	Tone 1 Attenuation
	0	1	0	Tone 2 Frequency
	0	1	1	Tone 2 Attenuation
	1	0	0	Tone 3 Frequency
	1	0	1	Tone 3 Attenuation
	1	1	0	Noise Control
	1	1	1	Noise Attenuation

4. Data Formats

 The formats required to transfer data are shown below.

	Frequency
 LSB                  MSB    LSB                   MSB
+---+--------+-----------+  +---+---+-----------------+
|   |Reg Addr|    Data   |  |   |   |       Data      |
+ 1 +--+--+--+--+--+--+--+  | 0 | X +--+--+--+--+--+--+
|   |R0|R1|R2|F6|F7|F8|F9|  |   |   |F0|F1|F2|F3|F4|F5|
+---+--+--+--+--+--+--+--+  +---+---+--+--+--+--+--+--+

	Noise Control
 LSB                          MSB
+---+---+---+---+---+----+-------+
|   |  Reg Addr |   |    | Shift |
+ 1 +---+---+---+ X | FB +---+---+
|   | 1 | 1 | 0 |   |    |NF0|NF1|
+---+---+---+---+---+----+---+---+

	Attenuator
 LSB                  MSB    
+---+--------+-----------+
|   |Reg Addr|    Data   |
+ 1 +--+--+--+--+--+--+--+
|   |R0|R1|R2|A0|A1|A2|A3|
+---+--+--+--+--+--+--+--+

