[opus] Custom mode

[Peter Svensson]

Hi all !



If someone could give me a hint on how to proceed with the following i'd be
very happy:



I have a test setup on an nrf52832 (Cortex M4) in which I receive audio
from a PDM microphone (64 sample frame) and pass it directly to an I2S
device i.e. from ISR to ISR. With uncompressed audio this works just fine.



Now I try to insert OPUS1.3 in the path but cannot make it work. The audio
passes through but ends up “down pitched” and heavily distorted.



Due to the 32MHz base clock (+ prescalers) available it is not possible to
get the desired 16kHz sample rate. I am therefore forced to work with
15625Hz.



As I understand it the opus encoder does not mind a strange sample rate as
it will re-sample it to 48kHz (Correct?). The decoder, however, will always
output 48kHz, 24kHz, 16kHz etc (Correct?) which is incompatible with my
15625Hz rate. (However; If my assumptions are correct there could be a
possibility to make my I2S device master and, thus, be able to pull the
data at 16kHz)



Consequently I have been looking at “opus custom” as a solution but even
with a (seemingly) low complexity it does not yield the desired result.



*Setup code:*



mode = opus_custom_mode_create(15625, 64, &err);



size = opus_custom_encoder_get_size(mode, 1);

enc = malloc(size);

err = opus_custom_encoder_init(enc, mode, 1);

err = opus_custom_encoder_ctl(enc, OPUS_SET_BITRATE(16000));

err = opus_custom_encoder_ctl(enc, OPUS_SET_COMPLEXITY(0));



size = opus_custom_decoder_get_size(mode, 1);

dec = malloc(size);

err = opus_custom_decoder_init(dec, mode, 1);



*ISR code (called once every 4.096ms):*



    nbBytes = opus_custom_encode(

        enc, (opus_int16 *)lMicBuf, 64, codedMicBuf, 64);



    frame_size = opus_custom_decode(

        dec, codedMicBuf, 64, decodedMicBuf, 64);





*Build:*



C:\Program Files\SEGGER\SEGGER Embedded Studio for ARM
3.50\gcc\arm-none-eabi\bin\cc1" -fmessage-length=0
-fno-diagnostics-show-caret -mcpu=cortex-m4 -mlittle-endian
-mfloat-abi=hard -mfpu=fpv4-sp-d16 -mthumb -mtp=soft -nostdinc "-isystem



-D__SIZEOF_WCHAR_T=4 -D__ARM_ARCH_7EM__ -D__SES_ARM
-D__ARM_ARCH_FPV4_SP_D16__ -D__SES_VERSION=35000 -DARM_MATH_CM4
-D__FPU_PRESENT -DDEBUG "-D DEBUG_NRF" -DCONFIG_GPIO_AS_PINRESET
-DINITIALIZE_USER_SECTIONS -DNO_VTOR_CONFIG -DNRF52 -DNRF52832_XXAA
-DCONFIG_NFCT_PINS_AS_GPIOS -DOPUS_BUILD -DVAR_ARRAYS -DFIXED_POINT
-DDISABLE_FLOAT_API -DCUSTOM_MODES "-DARM_MATH_CM4 "
"-DOPUS_ARM_INLINE_EDSP " -DDOPUS_ARM_INLINE_ASM -DOPUS_ARM_INLINE_MEDIA
-MD



Any ideas?



Regards

Peter
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