CoCalc -- CV-Sound.txt

GitHub Repository: alexbevi/BizHawk
Path: blob/master/BizHawk.Emulation.Cores/Consoles/Coleco/docs/CV-Sound.txt
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	ColecoVision Sound Generation Hardware
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 The ColecoVision uses the Texas Instruments SN76489A sound
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generator chip. It contains three programmable tone generators,
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each with its own programmable attenuator, and a noise source
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with its own attenuator.
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1. Tone Generators
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 Each tone generator consists of a frequency synthesis section
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requiring ten bits of information to define half the period of
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the desired frequency (n). F0 is the most significant bit and
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F9 is the least significant bit. The information is loaded into
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a ten stage tone counter, which is decremented at a N/16 rate
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where N (3.579MHz) is the input clock frequency. When the tone
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counter decrements to zero, a borrow signal is produced. This
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borrow signal toggles the frequency flip-flop and also reloads
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the tone counter. Thus, the period of the desired frequency is
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twice the value of the period register.
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The frequency can be calculated by the following:
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   f = 3.579MHz/(32n)
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The output of the frequency flip-flop feeds into a four stage
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attenuator. The attenuator values, along with their bit position
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in the data word, are shown below. Multiple attenuation control
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bits may be true simultaneously. THus, the maximum attenuation
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is 28 db.
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	A0	A1	A2	A3		Weight
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	0	0	0	1		 2 db
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	0	0	1	0		 4 db
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	0	1	0	0		 8 db
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	1	0	0	0		16 db
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	1	1	1	1		 Off
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2. Noise Generator
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 The noise generator consists of a noise source that is a
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shift register with an exclusive OR feedback network. The
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feedback network has provisions to protect the shift register
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from being locked in the zero state.
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	Noise Feedback Control
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		FB		Configuration
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 		0		"Periodic" Noise
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		1		"White" Noise
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	Noise Generator Frequency Control
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	NF0	NF1		Shift Rate
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	0	0		N/512
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	0	1		N/1024
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	1	0		N/2048
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	1	1		Tone gen #3 output
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3. Control Registers
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 The SN76489A has eight internal registers which are used
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to control the three tone generators and the noise source.
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During all data transfers to the SN76489A, the first byte
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contains a three bit field which determines the destination
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control register. The register address codes are shown here.
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	Register Address Field
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	R0	R1	R2	Destination Control Register
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	0	0	0	Tone 1 Frequency
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	0	0	1	Tone 1 Attenuation
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	0	1	0	Tone 2 Frequency
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	0	1	1	Tone 2 Attenuation
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	1	0	0	Tone 3 Frequency
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	1	0	1	Tone 3 Attenuation
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	1	1	0	Noise Control
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	1	1	1	Noise Attenuation
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4. Data Formats
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 The formats required to transfer data are shown below.
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	Frequency
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 LSB                  MSB    LSB                   MSB
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+---+--------+-----------+  +---+---+-----------------+
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|   |Reg Addr|    Data   |  |   |   |       Data      |
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+ 1 +--+--+--+--+--+--+--+  | 0 | X +--+--+--+--+--+--+
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|   |R0|R1|R2|F6|F7|F8|F9|  |   |   |F0|F1|F2|F3|F4|F5|
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+---+--+--+--+--+--+--+--+  +---+---+--+--+--+--+--+--+
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	Noise Control
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 LSB                          MSB
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+---+---+---+---+---+----+-------+
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|   |  Reg Addr |   |    | Shift |
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+ 1 +---+---+---+ X | FB +---+---+
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|   | 1 | 1 | 0 |   |    |NF0|NF1|
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+---+---+---+---+---+----+---+---+
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	Attenuator
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 LSB                  MSB    
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+---+--------+-----------+
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|   |Reg Addr|    Data   |
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+ 1 +--+--+--+--+--+--+--+
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|   |R0|R1|R2|A0|A1|A2|A3|
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+---+--+--+--+--+--+--+--+
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