This is simplified implementation of Karplus-Strong (KS) string synthesis based on papers, Digital Synthesis of Plucked-String and Drum Timbres and Extensions of the Karplus-Strong Plucked-String Algorithm.
A register map controls and configures the KS synthesis module. This register map is accessed through a SPI interface. Synthesized sound samples are sent out through the I2S transmitter interface.
SPI Mode: CPOL = 0, CPHA = 1
The 16-bit SPI frame is defined as,
| Read=1/Write=0 | Address[6:0] | Data[7:0] |
The Register Map has 16 Registers of 8-bits each. It is divided into configuration and status registers,
| Register[7:0] | Configuration Registers |
| Register[11:8 ] | Status Registers |
Each register is mapped as follows,
| Register/Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|
| 0 | i2s_noise_sel | ks_freeze | freeze_prbs_7 | freeze_prbs_15 | ~rst_n_ks_string | ~rst_n_prbs_7 | ~rst_n_prbs_15 | |
| 1 | ~lfsr_init_15[7:0] | |||||||
| 2 | load_prbs_15 | ~lfsr_init_15[14:8] | ||||||
| 3 | load_prbs_7 | lfsr_init_7[6:0] | ||||||
| 4 | clip_noise | dynamics_en | fine_tune_n | drum_string_n | toggle_pattern_prbs_n | round_en | pluck | |
| 5 | fine_tune_C[7:0] | |||||||
| 6 | dynamics_R[7:0] | |||||||
| 7 | ~ks_period[7:0] | |||||||
| 9 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
| 10 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 |
| 11 | ui_in[7] | ui_in[6] | ui_in[5] | ui_in[4] | ui_in[3] | ui_in[2] | ui_in[1] | ui_in[0] |
| 12 |
The 8-bit signed sound samples are sent out at f_sck = 256 kHz through this interface.
Connect a clock with frequency f_clk = 256 kHz and apply a reset cycle to initialize the design, this sets the audio sample rate at fs = 16 kHz. Use the spi register map or the ui_in to futher configure the design. The synthesized samples are sent continuously through the I2S transmitter interface.
inputs:
- ~rst_n_prbs_15, ~rst_n_prbs_7
- load_prbs_15, load_prbs_7
- freeze_prbs_15
- freeze_prbs_7
- i2s_noise_sel
- ~rst_n_ks_string
- pluck
- NOT CONNECTED
outputs:
- segment a: rstn_n
- segment b: rst_n_prbs_15
- segment c: rst_n_prbs_7
- segment d: rst_n_ks_string
- segment e: freeze_prbs_15
- segment f: freeze_prbs_15
- segment g: i2s_noise_sel
- dot: pluck
bidirectional:
- sck_i
- sdi_i
- sdo_o
- cs_ni
- i2s_sck_o
- i2s_ws_o
- i2s_sd_o
- prbs_15
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