This is a programmable IMA-ADPCM decoder / mixer. It can be configured with up to 16 ADPCM channels (or "voices") with independent volumes and playback rates. Sample data is loaded from a shared 16MByte address space. The output mix is signed 16bit stereo PCM.

The aim is to minimize resource use and support slower, low power devices such as the Lattice iCE40 UP5K (logic cost is about 1000LCs, or less than 20%).

• Minimize resource use
  • Register files are implemented in a way that can be inferred as block RAMs instead of using FFs. On iCE40, 2x 512byte RAMs are used.
  • Only one multiplier is used. On iCE40 UP5K, 1 "DSP Block" is used.
• Favor fmax over performance
  • One target for this project is >35MHz on an UP5K
  • Other device shouldn't have much trouble meeting timing at >100MHz

• Playback of up to 16 concurrent, independently configurable ADPCM channels.
• ADPCM data loaded from a shared 16MByte address space using a ready/valid interface to also support slower memories i.e. flash.
• Per-channel signed 8bit stereo volume control.
• Per-channel Q4.12 16bit playback rate. This gives a maximum playback rate of approx. 16.0 the output rate and a minimum of (1/4096 = 0.0002).

• No hard filters / EG / interpolators. The channel registers can be modified during playback which allows an external gateware / soft CPU to emulate some of these effects.
• ADPCM decoding is always enabled. LPCM for cleaner single-cycle waveforms may be an optional extra later. High playback rates help mask the shortcomings of ADPCM here but isn't ideal.
• ADPCM start/end/loop addresses must be 1kbyte aligned.

• yosys
• nextpnr-ecp5 if the included /demo is to be used
• Verilator if the included /sim tests are to be run
• SymbiYosys if the included /formal tests are to be run

The IMA-ADPCM data used is compatible with what ffmpeg generates with the following command:

ffmpeg -i input.wav -ac 1 -f s16le -acodec adpcm_ima_wav output.adpcm

ffmpeg can be used to encode samples for playback but depending on the source material, a lookahead encoder such as adpcm-xq may produce better results. The expected header, which ffmpeg creates by default, is:

• 2 bytes: Initial predictor
• 2 bytes: Initial step index (high byte ignored)

The block size in 4bit nybbles can be configured using the ADPCM_BLOCK_SIZE but the default is to use what the above encoders (and many others) default to, which is 1kbyte.

IMA-ADPCM encoded data is read using the ports prefixed with pcm_. pcm_read_address and pcm_address_valid are used for reading and the module will wait indefinitely for pcm_data_ready to be asserted with valid data on pcm_read_data. The address is 16bit, not 8bit, the LSB shouldn't be disregarded.

Channels are configured using the ports prefixed with ch_write_. During a write, the inputs must be held stable until ch_write_ready is asserted. ch_write_ready is asserted for one cycle only. Writes take at least 2 cycles but potentially more if there is contention during a write. ch_write_byte_mask can be used to do 8bit writes on the lower/upper bits of the VOLUMES register but can be set to 2'b11 if this isn't needed.

Each channel has 8x 16bit registers arranged in an array-of-structs layout in a register file. Only 6 of the 8 regsiters of each channel are used but the 2 unused ones remain as padding.

The start address for each channel is (channel_id * 8 + offset).

START, END and LOOP are effectively 1kbyte block indexes since all addresses are 1kbyte aligned. All addresses must point to the start of an ADPCM block header.

Channels are started and stopped using a separate interface using ports prefixed with gb_write_. There is 1 bit per configured channel, so the width of these registers depends on the value of the CHANNELS parameter. These registers serve as the key-on / key-off control.

gb_write_busy is asserted immediately after a write to these registers and remains asserted until the write is committed. Attempting a write while busy will overwrite the previously staged value. Writes are commited once per OUTPUT_INTERVAL cycles. The busy flag is also asserted on reset to mute all channels.

status_read_address and status_read_data can be used to read certain channel state. May be useful if the module is driven by a CPU.

These may be useful if the module is driven by an FSM rather than a CPU:

• gb_playing outputs the value of PLAYING.
• gb_ended outputs the value of ENDED.

Signed 16bit stereo output is updated once per OUTPUT_INTERVAL cycles. output_valid is asserted for 1 cycle at which point output_l / output_r have valid data to be registered. The outputs are undefined at all other times.

• The /sim directory contains Verilator-driven tests that assert that the ADPCM decoding and PCM output work as exepcted for different test cases. Note git-lfs is required to checkout the test reference files.
• The /formal directory contains SymbiYosys-driven tests for parts of the control interface. The PCM decoding / mixing / output is ignored which is covered by the above tests instead.

The /demo directory contains a ULX3S project that allows the user to play different notes with a switchable sample bank. It also includes a simple tracker for automatic playback.