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| [[Category:iNES Mappers]]
| | #REDIRECT [[VRC7]] |
| | | {{DEFAULTSORT:085}}[[Category:iNES Mappers]][[Category:in NesCartDB]][[Category:Mappers with cycle IRQs]] |
| [[iNES Mapper 085]] is used to represent the Konami [[VRC7]] mapper.
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| | |
| Here are Disch's original notes:
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| ========================
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| = Mapper 085 =
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| ========================
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|
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| aka
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| --------------------------
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| VRC7
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|
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|
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| Example Games:
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| --------------------------
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| Lagrange Point
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| Tiny Toon Adventures 2 (J)
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|
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|
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|
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| VRC7a vs. VRC7b
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| --------------------------
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| Lagrange Point ('VRC7a') and Tiny Toon Adventures 2 ('VRC7b') both operate exactly the same, but are wired a
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| bit differently. VRC7a uses $x010 for regs, and VRC7b uses $x008. Registers below are listed as they exist
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| on VRC7a. For VRC7b, make the appropriate adjustments
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|
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| Also, only Lagrange Point seems to use the extra sound. It's unknown whether or not the sound hardware
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| exists on VRC7b, as Tiny Toon doesn't use it.
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|
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|
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| CHR-RAM note:
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| --------------------------
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| Lagrange Point, for some reason I still don't understand, swaps its 8k CHR-RAM around. How this offers any
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| functionality is beyond me, but the game does it, so your emu must support it.
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|
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|
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|
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| Registers:
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| --------------------------
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|
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| $8000: PRG Reg 0 (8k @ $8000)
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| $8010: PRG Reg 1 (8k @ $A000)
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| $9000: PRG Reg 2 (8k @ $C000)
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|
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| $9010: Sound Address Reg (see below)
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| $9030: Sound Data Port (see below)
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|
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| $A000-$D010: CHR Regs
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|
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| $E000: [.... ..MM] Mirroring:
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| %00 = Vert
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| %01 = Horz
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| %10 = 1ScA
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| %11 = 1ScB
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|
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| $E010: [IIII IIII] IRQ Reload value
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| $F000: [.... .MEA] IRQ Control
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| $F010: [.... ....] IRQ Acknowledge
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|
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|
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| PRG Setup:
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| --------------------------
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|
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| $8000 $A000 $C000 $E000
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| +-------+-------+-------+-------+
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| | $8000 | $8010 | $9000 | { -1} |
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| +-------+-------+-------+-------+
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|
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|
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| CHR Setup:
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| --------------------------
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|
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| $0000 $0400 $0800 $0C00 $1000 $1400 $1800 $1C00
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| +-------+-------+-------+-------+-------+-------+-------+-------+
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| | $A000 | $A010 | $B000 | $B010 | $C000 | $C010 | $D000 | $D010 |
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| +-------+-------+-------+-------+-------+-------+-------+-------+
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|
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|
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| IRQs:
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| --------------------------
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|
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| VRC7 uses the "VRC IRQ" setup shared by several VRCs. It uses the following registers:
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|
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|
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| $E010: [IIII IIII] IRQ Reload
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| $F000: [.... .MEA] IRQ Control
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| $F010: [.... ....] IRQ Acknowledge
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|
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| For info on how these IRQs work, see the "VRC IRQs" section in mapper 021
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|
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|
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|
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| Sound:
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| --------------------------
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|
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| VRC7 has additional sound channels! It is a slightly dumbed down version of the YM2413 (aka OPLL). There
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| are only 6 harmony channels and no rhythmic channels. Note that some older docs claim it's an OPL2 -- but it
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| is, in fact, OPLL.
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|
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| Due to the complexity of FM-synth ... details will not be covered here. For details, refer to a YM2413 data
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| sheet or technical doc.
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|
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| $9010 is the address port
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| $9030 is the data port
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|
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| They behave just like the address/data ports on the YM2413. Though remember that there's no rhythm, and
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| only 6 channels.
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|
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| Channels can choose from 16 instruments. One is customizable by the game, the other 15 are fixed with the
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| following values:
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|
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| {
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| 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00, // custom instrument
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|
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| 0x03,0x21,0x04,0x06,0x8D,0xF2,0x42,0x17, // begin fixed instruments
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| 0x13,0x41,0x05,0x0E,0x99,0x96,0x63,0x12,
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| 0x31,0x11,0x10,0x0A,0xF0,0x9C,0x32,0x02,
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| 0x21,0x61,0x1D,0x07,0x9F,0x64,0x20,0x27,
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| 0x22,0x21,0x1E,0x06,0xF0,0x76,0x08,0x28,
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| 0x02,0x01,0x06,0x00,0xF0,0xF2,0x03,0x95,
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| 0x21,0x61,0x1C,0x07,0x82,0x81,0x16,0x07,
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| 0x23,0x21,0x1A,0x17,0xEF,0x82,0x25,0x15,
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| 0x25,0x11,0x1F,0x00,0x86,0x41,0x20,0x11,
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| 0x85,0x01,0x1F,0x0F,0xE4,0xA2,0x11,0x12,
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| 0x07,0xC1,0x2B,0x45,0xB4,0xF1,0x24,0xF4,
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| 0x61,0x23,0x11,0x06,0x96,0x96,0x13,0x16,
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| 0x01,0x02,0xD3,0x05,0x82,0xA2,0x31,0x51,
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| 0x61,0x22,0x0D,0x02,0xC3,0x7F,0x24,0x05,
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| 0x21,0x62,0x0E,0x00,0xA1,0xA0,0x44,0x17
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| };
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| | |
| And here are Disch's [http://nesdev.parodius.com/bbs/viewtopic.php?p=96856#96856 extended notes]:
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| <pre>
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| ----------------------------------------------------------------------------
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| ----------------------------------------------------------------------------
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| ----- VRC7 Sound -------------------------------------------------
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| ----------------------------------------------------------------------------
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| ----------------------------------------------------------------------------
| |
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| VRC7 has additional sound channels! It is a slightly dumbed down version of the YM2413 (aka OPLL). There
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| are only 6 harmony channels and no rhythmic channels.
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| | |
| Strap yourself in. FM-Synth is a beast.
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| | |
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| ---------------------------------------------
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| Disclaimers:
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| ---------------------------------------------
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| | |
| Information here is pieced together from the Yamaha YM2413 Application Manual ("YM2413.pdf"), and Mitsutaka
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| Okazaki's "emu2413.c" emulator. Anyone whose looked at those sources know they are not the easiest things
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| to comprehend without prior experience with FM synth, so here I attempt to explain things in a more
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| traditional form.
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| I don't really care about YM2413 (I hate FM synth... I find it extremely ugly), so I only cover items on the
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| VRC7 here (ie: no rhythmic information). If you want details about a full YM2413, you'll have to look elsewhere.
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| | |
| I am NOT confident about this information being 100% accurate. I made every effort to be as accurate as
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| possible, and my implementation based on the below info sounds *very close* to recordings of the real thing,
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| but I do hear some subtle differences. I graciously welcome any corrections anyone can offer.
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| | |
| Bitwidths of various counters are kind of an educated guess. With the exception of the phase accumulator,
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| which is the only counter whose size is hinted at in the documentation... so I'm fairly certain it is in
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| fact 18 bits wide.
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| I mention the use of various lookup tables. I do not know if these lookup tables actually exist on the
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| hardware, or if the values are calculated at runtime. Likewise the actual size of these lookup tables is
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| entirely unknown to me. You can choose your own size in your implementation.
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| | |
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| ---------------------------------------------
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| FM-Synth basics & other fundamental concepts:
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| ---------------------------------------------
| |
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| The basic idea of FM-Synth is you have 2 sine waves (aka, "slots"), a "modulator" and a "carrier". The
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| output of the carrier is what you actually hear. The output of the modulator alters the frequency of the
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| carrier, effectively acting like a supersonic vibrato. This bends and twists the carrier's waveform into
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| a myriad of different shapes, producing all kinds of different sounds.
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| Each of the 6 channels have 2 slots (a Carrier and a Modulator). Each slot behaves independently and has
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| its own settings and counters. Note that I will refer to "slots" often in these docs. Do not confuse
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| a slot for the whole channel.
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| "ADSR" stands for Attack/Decay/Sustain/Release. These represent 4 phases of amplitude (volume) changes in
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| synthesized audio. This is a common technique in all synth audio (not just FM-Synth).
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| - Attack is when the tone begins, and you have a rapid increase in volume, increasing to *above* the
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| desired output level.
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| - Decay is when attack has reached its maximum, and the volume starts to decline to the desired
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| output level.
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| - Sustain is when the volume has reached the desired level. It holds the volume at that level for as
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| long as the tone is to be played. Although sometimes the volume might slowly drop.
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| - Release is when the tone is done, and volume gradually decreases until it's completely silent.
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| "Key on" / "Key off" represents the entry and exit into ADSR. You can think of it like a piano or a keyboard...
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| when you "key on", you are pressing a key, and when you "key off" you are releasing a key. Effectively,
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| this means that when you key on, you enter "Attack", and when you key off, you enter "Release".
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| | |
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| ---------------------------------------------
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| Volume and Attenuation:
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| ---------------------------------------------
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| VRC7 doesn't really have a concept of an output volume. Instead, it does everything with "attenuation",
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| which is basically the opposite of volume. Attenuation is like a forced reduction -- so high attenuation
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| means low output. Zero attenuation means the output is as high as possible.
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| All attenuation levels are expressed in decibels (dB), which is a logarithmic (non-linear) scale. VRC7's
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| threshhold or maximum attenuation is 48 dB. This means that at 48 dB, output is zero.
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| Note that even though dB are non-linear, you can still work with them as if they were linear. That is,
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| 10dB + 10dB is still 20dB. The only thing is that when converted to linear units, 20dB is MUCH
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| MUCH more than 2x 10dB.
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| Since VRC7 handles all its output levels in terms of dB, this means you will only need to convert from
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| dB to linear units in exactly one place: when determining the linear output of the "slot".
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| Converting dB <-> Linear can be accomplished with the below formulas:
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| dB = -20 * log10( Linear ) * scale (if Linear = 0, dB = +inf)
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| Linear = 10 ^ (dB / -20 / scale)
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| 'scale' is an optional factor you can use to scale up dB so that they're in an easier to use base.
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| I recommend using (1<<23)/48 for a scale (this would mean that 1<<23 would represent 48 dB). This will
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| make envelope calculations much easier (see Envelope Generation section for details).
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| Remember the threshhold is 48 dB. So if you have 48 dB or higher, Linear=0.
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| | |
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| ---------------------------------------------
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| Clock rate:
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| ---------------------------------------------
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| VRC7 has its own oscillator to drive the clock rate. It's clocked at 3.6 MHz (exactly 2x the NTSC
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| NES CPU clock rate), but those clocks are divided by 72, effectively making the rate at which each individual
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| unit is clocked 49715.90909 Hz.
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| I find it very likely that clocking each individual unit is done serially across the 72 cycles, but the
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| effect that detail has on the generated audio is tiny to the point of being insignificant.
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| | |
| To think of this in terms of CPU cycles, you could say that all units are clocked once every 36 CPU
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| cycles on NTSC. However, this is techncially inaccurate, as the NES clock does not drive the VRC7.
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| And on PAL systems, the clock rate doesn't sync up like that.
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| | |
| ---------------------------------------------
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| Registers:
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| ---------------------------------------------
| |
| | |
| Register descriptions to follow. Details as to what each field actually does will not be covered here
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| but will be explained in future sections.
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| $9010: [..AA AAAA]
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| A = Address for use with $9030
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|
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| $9030: [DDDD DDDD] -- data port
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| R:00-R:07 -> Custom instrument settings (see below)
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|
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| R:1x: [FFFF FFFF] (where x=0-5, selecting the channel)
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| F = low 8 bits of F-Num (frequency control)
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|
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| R:2x: [..SK BBBF] (where x=0-5, selecting the channel)
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| F = high bit of F-Num
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| B = Block select (or octave)
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| K = Key on (1=key on, 0=key off)
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| S = Sustain On (poorly named, has no impact on Sustain mode -- actually affects Release)
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|
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| R:3x: [IIII VVVV]
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| I = Instrument select
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| V = 'Volume' (poorly named, it's more like "Carrier Base Attenuation Level")
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| (regs R:1x, 2x, and 3x apply to both Carrier and Modulator
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| regs R:0x apply differently to each )
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|
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| R:00: [AFPK MMMM] (applies to Modulator)
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| R:01: [AFPK MMMM] (applies to Carrier)
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| A = Enable Amplitude Modulation (AM)
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| F = Enable Frequency Modulation (FM)
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| P = Disable Percussive Mode (0=percussive, 1=normal)
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| K = Key Scale Rate (KSR)
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| M = 'MULTI' Freqency multiplier
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|
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| R:02: [KKLL LLLL]
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| K = Modulator Key Scale Level (KSL)
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| L = Modulator base attenuation level
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| R:03: [KK.C MFFF]
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| K = Carrier Key Scale Level (KSL)
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| C = Carrier rectify sine wave (0=full sine wave, 1=half sine wave)
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| M = Modulator rectify sine wave
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| F = Modulator Feedback level
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|
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| R:04: [AAAA DDDD] (Modulator)
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| R:05: [AAAA DDDD] (Carrier)
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| A = Attack Rate
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| D = Decay Rate
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|
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| R:06: [SSSS RRRR] (Modulator)
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| R:07: [SSSS RRRR] (Carrier)
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| S = Sustain Level
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| R = Release Rate
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|
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| There are 16 selectable instruments (selected via R:3x). Instrument 0 is configurable via regs
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| R:00 through R:07. The other instruments are fixed at the below values:
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|
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| 0x03,0x21,0x04,0x06,0x8D,0xF2,0x42,0x17 // instrument 1
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| 0x13,0x41,0x05,0x0E,0x99,0x96,0x63,0x12 // instrument 2
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| 0x31,0x11,0x10,0x0A,0xF0,0x9C,0x32,0x02 // instrument 3
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| 0x21,0x61,0x1D,0x07,0x9F,0x64,0x20,0x27 // instrument 4
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| 0x22,0x21,0x1E,0x06,0xF0,0x76,0x08,0x28 // instrument 5
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| 0x02,0x01,0x06,0x00,0xF0,0xF2,0x03,0x95 // instrument 6
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| 0x21,0x61,0x1C,0x07,0x82,0x81,0x16,0x07 // instrument 7
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| 0x23,0x21,0x1A,0x17,0xEF,0x82,0x25,0x15 // instrument 8
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| 0x25,0x11,0x1F,0x00,0x86,0x41,0x20,0x11 // instrument 9
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| 0x85,0x01,0x1F,0x0F,0xE4,0xA2,0x11,0x12 // instrument A
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| 0x07,0xC1,0x2B,0x45,0xB4,0xF1,0x24,0xF4 // instrument B
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| 0x61,0x23,0x11,0x06,0x96,0x96,0x13,0x16 // instrument C
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| 0x01,0x02,0xD3,0x05,0x82,0xA2,0x31,0x51 // instrument D
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| 0x61,0x22,0x0D,0x02,0xC3,0x7F,0x24,0x05 // instrument E
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| 0x21,0x62,0x0E,0x00,0xA1,0xA0,0x44,0x17 // instrument F
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|
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| | |
| **** SIDE NOTE ****
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| Writing to Regs R:00 through R:07 do NOT seem to have an immediate effect on channels using
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| instrument 0. Lagrange Point (Track 2 of the NSF) will write to these regs while a channel
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| using instrument 0 is still keyed on and audible, resulting in an ugly and very noticable
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| "blurp" noise at the end of a note. This is not heard on the real hardware, so instrument
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| data must be cached somehow. Perhaps it only takes effect when the channel is keyed on,
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| or when R:3x is written to? Don't know exactly.
| |
| | |
| | |
| | |
| | |
| ---------------------------------------------
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| Phase / Frequency Calculation:
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| ---------------------------------------------
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| | |
| Each slot has an 18-bit up counter which determines the current phase (position in the sine wave).
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| Each clock, this counter is incremented:
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| phase += F * (1 << B) * M * V / 2
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| | |
| where:
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| F = 9-bit F-num of the channel
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| B = 3-bit Block of the channel
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| M = see below
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| V = vibrato (FM) output
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| | |
| R:00 or R:01 specify a 4 bit 'MULTI' value. That MULTI value is run through the below LUT to get 'M':
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| | |
| MULTI: 0 1 2 3 4 5 6 7 8 9 A B C D E F (hex)
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| M: 1 2 4 6 8 10 12 14 16 18 20 20 24 24 30 30 (dec)
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|
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| If FM is enabled for the slot (see R:00 or R:01 for the enable bit), 'V' is the output of the
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| FM unit. See AM/FM section for details.
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| | |
| If FM is disabled, 'V' = 1
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| | |
| | |
| Another 'phase_secondary' value is used to actually generate the phase:
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| | |
| phase_secondary = phase + adj
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|
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| For the Carrier:
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| adj = the output of the Modulator. Note that slot output is 20-bits wide, but the phase is only 18
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| bits wide... this means that the high 2 bits of the modulator output are effectively dropped.
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| | |
| For the Modulator:
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| R:03 has a 3-bit 'F' value specifying the feedback level.
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| if F=0: adj = 0
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| otherwise: adj = previous_output_of_modulator >> (8 - F)
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|
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|
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| | |
| The bits of the phase_secondary value are extracted and used to generate the sine wave:
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| | |
| phase_secondary: [RI IIII III. .... ....]
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| R: rectification bit
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| I: index to half-sine lookup table
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| ** 'I' may be more or less bits depending on how big your half-sine lookup table is **
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| | |
| 'R' determines what to do with output after it's been converted to a linear level. See next section
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| for details of this bit, and details of the half-sine table.
| |
| | |
| | |
| ---------------------------------------------
| |
| Attenuation / Output calculation:
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| ---------------------------------------------
| |
| | |
| The attenuation level determines the slot output on each clock. Attenuation level is determined
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| as follows:
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| | |
| TOTAL = half_sine_table[I] + base + key_scale + envelope + AM
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| | |
|
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| half_sine_table[I]:
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| --------------
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| The half-sine table mentioned in the previous section does not actually hold the output of the sine
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| function. Rather, it holds the attenuation level of the sine function. Example:
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| | |
| sin(pi/2) = 1 ~~~> I='0100 0000' ~~~> half_sine_table[ I ] = 0 dB
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| sin(0) = 0 ~~~> I='0000 0000' ~~~> half_sine_table[ I ] = +inf dB
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| | |
| This table is effectively:
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| half_sine_table[I] = Convert_Linear_To_dB( sin( pi * I / (1 << bitwidth_of_I) ) )
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| | |
|
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|
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| base:
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| --------------
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| For Modulator: base = (0.75 * L), where L is the 6-bit base level (see register R:02)
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| For Carrier: base = (3.00 * L), where L is the 4-bit 'volume' (see register R:3x)
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| | |
| | |
| | |
| key_scale:
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| --------------
| |
| Key Scale Level, 'K', is a 2-bit value (see regs R:02, R:03) that adds attenuation as the pitch
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| of the tone increases (ie: higher pitches = quieter).
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| | |
| If K=0: key_scale=0
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| Otherwise:
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| F = high 4 bits of the current F-Num
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| B = 3-bit Block (Octave)
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| A = table[ F ] - 6 * (7-B)
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| | |
| if A < 0: key_scale = 0
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| otherwise: key_scale = A >> (3-K)
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|
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| table:
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| F: $0 $1 $2 $3 $4 $5 $6 $7 $8 $9 $A $B $C $D $E $F
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| A: 0.00 18.00 24.00 27.75 30.00 32.25 33.75 35.25 36.00 37.50 38.25 39.00 39.75 40.50 41.25 42.00
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|
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|
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| envelope:
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| --------------
| |
| Output of the envelope generator. See Envelope Generation section for details.
| |
| | |
| | |
| AM:
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| --------------
| |
| If Amplitude modulation is enabled for the slot (see R:00, R:01), AM is the output of the amplitude
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| modulation unit. Otherwise, AM=0.
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| | |
| See AM/FM section for details.
| |
| | |
| | |
| | |
| | |
| Finally... after all that, we have our 'TOTAL'. This is the total attenuation for the slot.
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| | |
| 1) This attenutation is then converted to linear units to get the preliminary output. This is
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| scaled up to a 20-bit value
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| | |
| 2) If the high bit ('R') of the 18-bit 'phase_secondary' value (see previous section) is set, this
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| means we are in the negative portion of the sine wave, which means output needs to be negated.
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| However, if we are rectifying to a half sine wave (see R:03), output is zero'd instead.
| |
| | |
| 3) Output is then run through a filter which averages this output with the previous clock's output
| |
| | |
| 4) The result is the FINAL, actual output.
| |
| | |
| Pseudo-code to clarify:
| |
| | |
| total = half_sine_table[I] + base + key_scale + envelope + AM
| |
| prevoutput = output
| |
|
| |
| // 1)
| |
| output = convert_dB_to_Linear( total ) * (1<<20)
| |
|
| |
| // 2)
| |
| if R:
| |
| if halfsine: output = 0
| |
| else: output = -output
| |
|
| |
| // 3)
| |
| FINAL = (output + prevoutput) / 2
| |
|
| |
|
| |
| 'FINAL' is what the slot actually outputs. This is a 20-bit value. The modulator's output will
| |
| be sent to the carrier, and the carrier's output will be audible (though you will want to scale it
| |
| down... 20-bit audio is crazy loud when ouputting 16-bit samples).
| |
| | |
| 'FINAL' is also the value used when calculating the modulator's feedback (see prev section).
| |
| | |
| | |
| | |
| ---------------------------------------------
| |
| Envelope Generation:
| |
| ---------------------------------------------
| |
| | |
| Each slot has a 23-bit up counter (hereon 'EGC') for envelope generation, very similar to the
| |
| 18-bit phase counter. It determines the output of the envelope generator... which adds attenuation
| |
| to the output (see previous section).
| |
| | |
| | |
| The envelope generator operates as an ADSR unit. When the channel is keyed on, both the Carrier
| |
| and the Modulator enter the Attack Phase. When keyed off, they enter Release phase.
| |
| | |
| When the ADSR unit completes a full ADSR cycle, it enters a 5th 'Idle' phase.
| |
| | |
| EGC is incremented every clock. The value by which it's incremented depends on which phase of ADSR
| |
| we're in. Those rates are then adjusted by a 'Key Scale Rate' factor (see R:00, R:01).
| |
| | |
| EGC also serves as the direct output of the envelope generator (except in the Attack phase).
| |
| When EGC=0, output is 0 dB, and whdn EGC=(1<<23), output is 48 dB. Because of this, I
| |
| recommend scaling all units in your emulator to work with dB in this (1<<23)/48 base. Doing
| |
| so results in minimal unit conversion.
| |
| | |
| Formula for determining the rate to increase EGC:
| |
| BF = (3-bit Channel Block << 1) + high bit of F-Num... forming a 4-bit value
| |
| K = Key Scale Rate bit (see R:00, R:01)
| |
| if K: KB = BF
| |
| otherwise: KB = BF >> 2
| |
| | |
| R = base rate (see subsections below)
| |
| RKS = R*4 + KB
| |
| RH = RKS >> 2 (if RH > 15, use RH=15)
| |
| RL = RKS & 3
| |
|
| |
|
| |
| The subsections below will provide a value for R, then will use RH and RL to determine
| |
| the rate by which EGC is incremented.
| |
| | |
| Note that if R=0, then EGC is not incremented at all.
| |
| | |
| Attack:
| |
| -------
| |
| R = slot attack rate (4-bits as written to R:04, R:05)
| |
| EGC += (12 * (RL+4)) << RH
| |
|
| |
| Once EGC wraps, reset EGC to zero and enter Decay phase
| |
|
| |
| Decay:
| |
| -------
| |
| R = slot decay rate (4-bits as written to R:04, R:05)
| |
| EGC += (RL+4) << (RH-1)
| |
|
| |
| Once output level reaches the slot sustain level (see R:06, R:07), set EGC to the sustain
| |
| level (do not reset it to 0!), and enter Sustain phase.
| |
|
| |
| The sustain level is (3 dB * L), where L is the 4-bit value written to the register.
| |
| This means you enter Sustain when EGC >= (3 * L * (1<<23) / 48)
| |
|
| |
| Sustain:
| |
| -------
| |
| If slot is percussive (see R:00, R:01): R = slot RELEASE rate (R:06, R:07, low bits)
| |
| otherwise: R = 0
| |
|
| |
| When EGC reaches (1<<23), output is fixed at 48 dB and enter Idle phase
| |
|
| |
| Release:
| |
| -------
| |
| If channel has "Sustain On" set (see R:2x), R = 5
| |
| otherwise, if slot is percussive: R = slot release rate (R:06, R:07)
| |
| otherwise: R = 7
| |
|
| |
| When EGC reaches (1<<23), output is fixed at 48 dB and enter Idle phase
| |
|
| |
| Idle:
| |
| -------
| |
| R=0
| |
| Output fixed at 48 dB
| |
|
| |
|
| |
| As previously mentioned, the output of the envelope generator is EGC, except in Attack phase.
| |
| In Attack, the actual rate of attack is logarithmic (it also decreases attenuation, rather than
| |
| increasing it).
| |
| | |
| attack_output = 48 dB - (48 dB * ln(EGC) / ln(1<<23))
| |
| (ln = natural log)
| |
| | |
|
| |
| ---------------------------------------------
| |
| Key On / Key Off:
| |
| ---------------------------------------------
| |
| | |
| R:2x has the Key On bit for the channel. This bit only has an impact when its state transitions.
| |
| Upon transition, do the following for both Carrier and Modulator:
| |
| | |
| When being set (0->1): (key on)
| |
| - Reset EGC to zero
| |
| - Reset 18-bit phase counter to zero
| |
| - Enter Attack phase
| |
|
| |
| When being clear (1->0): (key off)
| |
| - If currently in attack, EGC must be set to the current output level
| |
| - Enter Release phase
| |
| | |
|
| |
| ---------------------------------------------
| |
| AM/FM:
| |
| ---------------------------------------------
| |
| | |
| There is one AM unit and one FM unit. The output of these units are shared across all slots.
| |
| | |
| Both units have a 20-bit counter that is increased by 'rate' every clock.
| |
| | |
| sinx = sin(2 * pi * counter / (1<<20))
| |
| | |
| AM unit:
| |
| 'rate' = 78
| |
| AM_output = (1.0 + sinx) * 0.6 dB (emu2413 uses 1.2 dB instead of 0.6, but that sounds way too steep to me)
| |
|
| |
| See the "Attenuation / Output calculation" section for how this output is applied
| |
| | |
|
| |
| FM unit:
| |
| 'rate' = 105
| |
| FM_output = 2 ^ (13.75 / 1200 * sinx)
| |
| (note: '^' is exponent, not xor)
| |
|
| |
| See the "Phase / Frequency Calculation" section for how this output is applied
| |
| </pre>
| |
