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59 lines
3.2 KiB
Markdown
59 lines
3.2 KiB
Markdown
# Commodore Amiga
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a computer with a desktop OS, lifelike graphics and 4 channels of PCM sound in 1985? no way!
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in this very computer music trackers were born...
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imported MOD files use this chip, and will set A-4 tuning to 436.
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## amplitude/period modulation
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Amiga has support for (rather primitive) amplitude and period (frequency) modulation.
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however, nobody has used this feature as it is rather useless, not well-documented and works in a complicated way.
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Amiga sample playback is done by two chips: Paula (the one that you probably know) and Agnus (the one that actually feeds Paula with samples).
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Agnus has several DMA (direct memory access) units which read from chip memory independent of the CPU. four of these DMA units are used for samples.
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when DMA is enabled, Paula requests sample data from Agnus, and then plays these samples back.
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there's a catch though. since the data bus is 16-bit, Paula requests **two** 8-bit samples at once! this explains why:
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- the sample length registers are in words rather than bytes (thereby allowing samples up to 131070 in length)
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- the maximum playback rate (31250Hz PAL; ~31469Hz NTSC) is two times the HBlank rate (Agnus fetches samples on HBlank, around 15625Hz on PAL or ~15734Hz on NTSC)
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during normal sample playback, the first sample is output and then the second. afterwards, two more samples are fetched, and so on.
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now, when amplitude or period modulation are enabled, things work differently.
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the channel is silenced, and the two 8-bit samples are **treated as a big-endian 16-bit number**, which is then written to the next channel's volume or period.
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in the case of amplitude modulation, only the second sample is significant because the volume register uses 7 bits (to represent 0 to 64 (65 to 127 are treated as 64)) and the other bits are ignored.
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in the case of period modulation, both samples are significant. the first sample is the upper byte, and the second is the lower byte.
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## effects
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- `10xx`: **toggle low-pass filter.** `0` turns it off and `1` turns it on.
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- `11xx`: **toggle amplitude modulation with the next channel.**
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- does not work on the last channel.
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- `12xx`: **toggle period (frequency) modulation with the next channel.**
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- does not work on the last channel.
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- `13xx`: **change wave.**
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- only works when "Mode" is set to "Wavetable" in the instrument.
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## info
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this chip uses the [Generic Sample](../4-instrument/sample.md) instrument editor.
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- the maximum rate for sample playback is technically 31469Hz but anything higher than 28867Hz will sound glitchy on hardware.
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- sample lengths and loop will be set to an even number.
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- samples can't be longer than 131070.
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## chip config
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the following options are available in the Chip Manager window:
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- **Stereo separation**: sets the amount of left/right separation.
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- **Model**: allows you to change the chipset.
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- Amiga 500 (OCS): has a low-pass filter on top of the user-selectable filter.
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- Amiga 1200 (AGA): doesn't have the aforementioned low-pass filter.
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- **Chip memory**: more chip memory means more space for samples.
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- **PAL**: run the chip at PAL clock (3.54MHz) instead of NTSC (3.58MHz).
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- **Bypass frequency limits**: when enabled, the ~31KHz frequency limit is disabled, allowing you to play higher notes.
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