Difference between revisions of "MEA8000 Voice Generator"
From CPCWiki - THE Amstrad CPC encyclopedia!
| Line 4: | Line 4: | ||
== Overview == | == Overview == | ||
| − | |||
Voiced (Sawtooth) <-------- Digital Filters ---------> | Voiced (Sawtooth) <-------- Digital Filters ---------> | ||
________ ______ | ________ ______ | ||
| Line 14: | Line 13: | ||
| RANDOM |---| AMPL |--o | | RANDOM |---| AMPL |--o | ||
|________| |______| | |________| |______| | ||
| − | |||
| − | |||
| − | == | + | Unvoiced (Noise) (when PI=-16) |
| + | |||
| + | == Digital Filters (processed on each 8kHz sample) == | ||
| + | Each of the four filter stages works like so: | ||
| + | fm_cos = cos(+2*pi * fm_Hz/8096Hz)*2 | ||
| + | bw_exp = exp(-1*pi * bw_Hz/8096Hz) | ||
| + | _____ _____ | ||
| + | ---sample----------------->| |------------------------->| |---+---> | ||
| + | _______________ | ADD | ______________ | ADD | | | ||
| + | +-->|*-bw_exp*bw_exp|-->|_____| +-->|*fm_cos*bw_exp|-->|_____| | | ||
| + | | |_______________| _____ | |______________| _____ | | ||
| + | +-----------------------|OLDER|<--+----------------------| OLD |<--+ | ||
| − | + | |_____| |_____| | |
| − | + | ||
| − | + | ||
| − | + | ||
| − | == | + | == Pitch Generator (Sawtooth) (active when PI<>-16) == |
| + | Based on 3 registers: SAW_LEVEL is probably signed 13bit, P is probably unsigned 9bit, PI is signed 5bit. The following is processed every 125us (8kHz): | ||
| + | SAW_LEVEL=(SAW_LEVEL+P) AND 1FFFh ;sawtooth (-1000h..+0FFFh) | ||
| + | sample = SAW_LEVEL * AMPLITUDE ;volume | ||
| + | And, additionally, every 8ms (125Hz): | ||
| + | P=(P+PI) AND 1FFh ;the "AND 1FFh" is assuming that P is only 9bit wide | ||
| − | + | == Noise Generator (Random) (active when PI=-16) == | |
| − | The | + | The following is processed every 125us (8kHz): |
| − | + | NOISE_LEVEL=random(-1000h..+0FFFh) ;random | |
| + | sample = SAW_LEVEL * AMPLITUDE ;volume | ||
| + | The exact random algorithm is unknown (one information fragment implies that it is done by some ADD/MULTIPLY logic) (not sure if that is true, it may also be simple SHIFT/XOR or so). | ||
| − | + | == Linear Interpolations (AMPL/FM/BW) == | |
| − | + | Changes to AMPL,FM,BW are applied by interpolation (ie. not abruptly) (ie. it takes whatever time until the new setting is reached... the interpolation speed is unknown; it is merely described to depend on FD... Probably it's meant to be the whole FD period, ie. the new Target value isn't reached until the END of the frame). | |
| − | + | ||
| − | + | ||
| − | + | == DAC Output == | |
| − | The | + | The 8kHz samples (coming from the filters) are resampled to 64kHz samples using linear interpolation, for example, if each "_" is a 8kHz sample: |
| + | _ ______ _____ | ||
| + | 8kHz __ _____ _____ ---> 64kHz __/\____/ \____ | ||
| + | The 64kHz samples are then passed to an 8bit D/A converter. According to the french datasheet, the 8bit DAC internally consists of two 4bit DACs (one for LSBs, one for MSBs) whose outputs are merged together via 1:16 resistors or so. | ||
| − | + | == Oscillator == | |
| − | + | The device can be driven by a 3.84MHz crystal, or external TTL clock of max 4MHz). The 8kHz sample rate, timings, and pitch/filter frequencies in this doc apply for the recommended 3.84MHz. | |
| − | + | ||
| − | + | ||
Revision as of 09:03, 23 January 2010
- MEA8000
- MEA8000 I/O Ports and Pinouts
- MEA8000 Voice Generator
Contents
Overview
Voiced (Sawtooth) <-------- Digital Filters ---------> ________ ______ | | | | | PITCH |---| AMPL |--o _____ _____ _____ _____ _____ |________| |______| \ | FM1 | | FM2 | | FM3 | | FM4 | | | ________ ______ o-| BW1 |---| BW2 |---| BW3 |---| BW4 |---| DAC |---> | | | | |_____| |_____| |_____| |_____| |_____| | RANDOM |---| AMPL |--o |________| |______|
Unvoiced (Noise) (when PI=-16)
Digital Filters (processed on each 8kHz sample)
Each of the four filter stages works like so:
fm_cos = cos(+2*pi * fm_Hz/8096Hz)*2
bw_exp = exp(-1*pi * bw_Hz/8096Hz)
_____ _____
---sample----------------->| |------------------------->| |---+--->
_______________ | ADD | ______________ | ADD | |
+-->|*-bw_exp*bw_exp|-->|_____| +-->|*fm_cos*bw_exp|-->|_____| |
| |_______________| _____ | |______________| _____ |
+-----------------------|OLDER|<--+----------------------| OLD |<--+
|_____| |_____|
Pitch Generator (Sawtooth) (active when PI<>-16)
Based on 3 registers: SAW_LEVEL is probably signed 13bit, P is probably unsigned 9bit, PI is signed 5bit. The following is processed every 125us (8kHz):
SAW_LEVEL=(SAW_LEVEL+P) AND 1FFFh ;sawtooth (-1000h..+0FFFh) sample = SAW_LEVEL * AMPLITUDE ;volume
And, additionally, every 8ms (125Hz):
P=(P+PI) AND 1FFh ;the "AND 1FFh" is assuming that P is only 9bit wide
Noise Generator (Random) (active when PI=-16)
The following is processed every 125us (8kHz):
NOISE_LEVEL=random(-1000h..+0FFFh) ;random sample = SAW_LEVEL * AMPLITUDE ;volume
The exact random algorithm is unknown (one information fragment implies that it is done by some ADD/MULTIPLY logic) (not sure if that is true, it may also be simple SHIFT/XOR or so).
Linear Interpolations (AMPL/FM/BW)
Changes to AMPL,FM,BW are applied by interpolation (ie. not abruptly) (ie. it takes whatever time until the new setting is reached... the interpolation speed is unknown; it is merely described to depend on FD... Probably it's meant to be the whole FD period, ie. the new Target value isn't reached until the END of the frame).
DAC Output
The 8kHz samples (coming from the filters) are resampled to 64kHz samples using linear interpolation, for example, if each "_" is a 8kHz sample:
_ ______ _____ 8kHz __ _____ _____ ---> 64kHz __/\____/ \____
The 64kHz samples are then passed to an 8bit D/A converter. According to the french datasheet, the 8bit DAC internally consists of two 4bit DACs (one for LSBs, one for MSBs) whose outputs are merged together via 1:16 resistors or so.
Oscillator
The device can be driven by a 3.84MHz crystal, or external TTL clock of max 4MHz). The 8kHz sample rate, timings, and pitch/filter frequencies in this doc apply for the recommended 3.84MHz.
