115 lines
3.1 KiB
Plaintext
115 lines
3.1 KiB
Plaintext
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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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