1 | Buchla 700 Hardware Emulator
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2 | ----------------------------
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3 |
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4 | This repository, buchla-emu.git, contains a software emulation of the
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5 | Buchla 700's hardware.
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6 |
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7 | It is minimalistic; it emulates just enough of the hardware to be able
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8 | to run the firmware from the companion repository, buchla-68k.git.
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9 |
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10 | We don't have access to original hardware, so this is our best guess
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11 | based on the firmware source code published by Lynx Crowe - the
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12 | firmware's developer - via Aaron Lanterman:
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13 |
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14 | http://lanterman.ece.gatech.edu/buchla700/
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15 |
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16 | See the buchla-68k.git repository for the firmware source code.
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17 |
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18 |
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19 | Building the emulator
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20 | ---------------------
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21 |
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22 | The emulator uses SDL2, an abstraction layer for low-level machine
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23 | access on Linux, OS X, and Windows. It can be obtained from the
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24 | project's website:
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25 |
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26 | https://libsdl.org/
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27 |
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28 | The SDL2 website also hosts the SDL2_net and SDL2_ttf projects, which
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29 | add support for networking and TrueType fonts to SDL2
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30 |
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31 | SDL2_ttf, in turn, requires the FreeType library, which is available
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32 | from the FreeType website:
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33 |
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34 | https://www.freetype.org/
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35 |
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36 | Currently, we build the emulator natively on Linux and OS X. The
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37 | Windows version is cross-compiled on Linux using a x86_64-w64-mingw32
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38 | cross-toolchain.
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39 |
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40 | For Linux and OS X, our Makefile expects all of the above libraries to
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41 | reside in /opt/sdl2. This is how we typically install them:
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42 |
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43 | # Build and install FreeType first
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44 |
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45 | tar zxvf freetype-2.7.1.tar.gz
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46 | cd freetype-2.7.1
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47 | mkdir build
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48 | cd build
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49 |
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50 | # Skip the optional features (compressed fonts, etc.) that would
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51 | # create more dependencies
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52 |
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53 | ../configure --prefix=/opt/sdl2 \
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54 | --without-zlib --without-bzip2 --without-png --without-harfbuzz
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55 |
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56 | make
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57 | make install
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58 |
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59 | # Then build and install SDL2
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60 |
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61 | tar zxvf SDL2-2.0.5.tar.gz
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62 | cd SDL2-2.0.5
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63 | mkdir build
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64 | cd build
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65 |
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66 | ../configure --prefix=/opt/sdl2
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67 |
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68 | make
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69 | make install
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70 |
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71 | # Build and install SDL2_ttf, now that we have FreeType and SDL2
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72 |
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73 | tar zxvf SDL2_ttf-2.0.14.tar.gz
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74 | cd SDL2_ttf-2.0.14
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75 | mkdir build
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76 | cd build
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77 |
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78 | ../configure --prefix=/opt/sdl2 \
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79 | --with-sdl-prefix=/opt/sdl2 --with-freetype-prefix=/opt/sdl2
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80 |
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81 | make
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82 | make install
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83 |
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84 | # Build and install SDL2_net last
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85 |
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86 | tar zxvf SDL2_net-2.0.1.tar.gz
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87 | cd SDL2_net-2.0.1
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88 | mkdir build
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89 | cd build
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90 |
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91 | ../configure --prefix=/opt/sdl2 --with-sdl-prefix=/opt/sdl2
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92 |
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93 | make
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94 | make install
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95 |
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96 | Now that we have everything in place, invoke
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97 |
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98 | make buchla
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99 |
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100 | from the top-level directory of this repository to build the emulator.
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101 |
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102 | The cross-build for Windows is done similarly, with the following
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103 | differences when configuring the libraries:
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104 |
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105 | * We use "--prefix=/opt/sdl2-win" instead of "--prefix=/opt/sdl2",
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106 | so that the Windows versions of the libraries go to a different
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107 | directory. That's where our Makefile expects to find them when
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108 | cross-building.
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109 |
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110 | * We additionally specify "--host=x86_64-w64-mingw32" to enable
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111 | cross-compilation.
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112 |
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113 | Then, to cross-build the emulator, invoke
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114 |
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115 | make buchla WIN=1
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116 |
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117 | from the top-level directory of this repository. Defining the "WIN"
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118 | variable selects the cross-toolchain and "/opt/sdl2-win" as the
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119 | library directory.
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120 |
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121 |
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122 | Emulated hardware
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123 | -----------------
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124 |
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125 | Here's what we emulate:
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126 |
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127 | * Motorola 68000 CPU. This is actually the Musashi CPU emulator by
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128 | Karl Stenerud:
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129 |
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130 | https://github.com/kstenerud/Musashi
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131 |
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132 | * Motorola MC6840: Timers.
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133 |
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134 | * Motorola MC6850: Serial console and MIDI ports.
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135 |
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136 | * Epson SED1335: LCD controller.
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137 |
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138 | * Intel 82716: Video chip.
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139 |
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140 | * National Semiconductor LMC835: Equalizer.
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141 |
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142 | * General Instrument AY-3-8910: A sound chip, which is not used for
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143 | sound generation, but only for its I/O ports. It connects the CPU
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144 | to the above equalizer chip.
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145 |
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146 | * Western Digital WD1772: Floppy disk controller.
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147 |
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148 | * A few LEDs.
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149 |
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150 | * Item X: A program running on a microcontroller. It converts the
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151 | analog signals from the Buchla's controller pads to digital
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152 | values.
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153 |
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154 | Neither the program, nor the microcontroller are known, but the
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155 | protocol (known from the firmware source code) is pretty simple
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156 | and self-explanatory.
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157 |
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158 | * Item Y: The actual sound generator, referred to by the firmware
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159 | source code as "the FPU." This could actually be two chips:
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160 |
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161 | 1. One chip, maybe a DSP, for generating the 15 different
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162 | parameter envelopes for each of the 12 voices:
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163 |
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164 | - 4x FM modulator (oscillator) frequency.
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165 |
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166 | - 6x FM modulator (oscillator) envelope.
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167 |
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168 | - 1x Output signal amplitude envelope.
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169 |
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170 | - 1x Output signal filter envelope.
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171 |
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172 | - 1x Output signal filter resonance envelope.
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173 |
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174 | - 1x Output signal stereo location.
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175 |
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176 | - 1x "Dynamics." (TBD - currently not emulated.)
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177 |
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178 | Over time, the chip interpolates between the points of the
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179 | envelopes drawn in the MIDAS VII instrument editor.
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180 |
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181 | 2. A second chip for the actual sound generation. This is likely
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182 | a DSP, possibly a Hitachi HD61810, which supports a 16-bit
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183 | floating-point format that's also found in the firmware
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184 | source code (12-bit mantissa, 4-bit exponent).
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185 |
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186 | This chip takes in the current levels of a voice's envelopes
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187 | and, based on them, performs the FM synthesis for this voice
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188 | by modulating the user-drawn carrier waves A and B according
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189 | to the selected FM configuration (algorithm).
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190 |
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191 | We don't know how many of the envelopes not related to FM
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192 | (e.g., the filter) are actually used digitally. At least some
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193 | of the envelopes probably control analog circuits.
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194 |
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195 | Obviously, the emulator does everything digitally.
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196 |
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197 | This "two chip" hypothesis would be in line with the "four
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198 | computers" marketing claim from the Buchla 700 marketing copy. The
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199 | four "computers" would be the Motorola 68000, the microcontroller
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200 | that does the A/D conversion of the pad inputs, plus the two CPUs
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201 | that constitute "the FPU."
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202 |
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203 | If you have access to an actual Buchla 700, please do contact us. It
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204 | would be great to be able to compare the emulation to real hardware.
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205 |
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206 | If it's non-functional, this is also fine. We might be able to gain
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207 | some insights from reading the FPU microcode PROMs.
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