contains 3 projects

• passthru: for checking stm32f4-discovery hardware, microphone and speaker interface. Requires only one discovery board. Useful for anybody wishing to use this board for audio purpose.
• vocoder_passthru: adds vocoder to passthru test using single discovery board: For checking codec2 from microphone encoding directly decoding out to speaker. This doubles CPU power required since both encoding and decoduing must be done for each frame.
• lora_codec2: adds SX1262 radio, half duplex transceiver. Requires two discovery boards, each with its own sx1262. PTT is blue button (push to talk)

QuickStart

After cloning this repository:

$ cd codec2
$ git submodule init
$ git submodule update

Follow tool install instructions here: steps 2 and 4 for toolchain and peripheral library

(from root directory of this project)
$ mkdir build
$ cd build
$ cmake .. -DCMAKE_TOOLCHAIN_FILE=../codec2/stm32/cmake/STM32_Toolchain.cmake -DPERIPHLIBDIR=/opt/STM32F4xx_DSP_StdPeriph_Lib_V1.8.0/
$ make

Select codec2 bit-rate: uncomment the line in CMakeLists.txt: #target_compile_definitions(lora_codec2 PRIVATE -DCODEC2_MODE=CODEC2_MODE_<bitrate>) Select desired LoRa bandwidth on the line defining LORA_BW_KHZ in CMakelists.txt. Spreading factor will be selected appropriate for the vocoder bit-rate and LoRa bandwidth selected. Spreading factor functional range is 5 to 12. See file loRa_codec2/main.h where spreading factor is defined according to codec2 bitrate vs LoRa bandwidth.

Flashing stm32f4-discovery https://github.com/texane/stlink providing address isnt needed when using st-flash with .hex files i.e: st-flash --format ihex write lora_codec2.hex

codec2 mode	samples per frame	encoded bytes per frame	frames per lora packet	LoRa packet length bytes	LoRa BW KHz	LoRa SF	air-time used percent	packet duration (ms)
3200	160	8	16	128	500	10	96
2400	160	6	16	96	500	10	77	237
1600	320	8	8	64	500	10	56
1600	320	8	16	128	500	11	86	555
1400	320	7	16	112	500	11	77	493
1300	320	6.5	8	52	500	11	90	288
1200	320	6	16	96	500	11	71	452
1200	320	6	32	192	500	11	62	1281
700C	320	3.5	16	56	250	11	96	576
450	320	2.25	16	36	250	11	71	453

LoRa data rate selection is only possible in steps by a factor of two. Codec2 bit-rate change will affect LoRa packet duty cycle. When duty cycle is under 50%, the LoRa data-rate can be reduced (bandwidth reduced or SF increased). If packet duty cycle is over 100%, then LoRa data-rate must be increased to faster. Typical 2.5 to 2.7dB change in link budget for each step of LoRa data-rate.

Latency across the radio link is due to LoRa packet duration.

SX1272 or SX1276 shouldn't be used for this, because SX1261/SX1262 receives large LoRa packets with less errors.

Wiring to LoRa transceiver

pin function	port/pin	discovery connector	sx1262 shield pin
Vdd		P1-3	J3-4
Gnd		P1-1	J2-7
NSS	PA10	P2-41	J1-8
MOSI	PA7	P1-17	J2-4
MISO	PA6	P1-18	J2-5
SCLK	PA5	P1-15	J2-6
DIO1	PA2	P1-14	J1-6
BUSY	PB1	P1-21	J1-4
AntSwPwr	PB3	P2-28	J2-1
Other modules could be also used, such as dorji module

implementation details

DMA is used for microphone interface because PDM microphone operates at 16x the required sample rate. In the case of codec2, 8Ksps is required, with the PDM microphone operating at 64ksps. 16 samples are collected using DMA, needing an interrupt only when at least 16 samples are ready to send to PDM filter.

credits

• PDM filter for on-board microphone.
• USB CDC device for stm32f4-discovery.