Opus

Opus Codec

This is the Opus encoder/decoder adapted for the Nuclei CPU.

The origin source code is available here, current version is v1.5.2.

We did not compile the code into a library as in the original repository, but instead compiled the executable directly from the source code. opus_demo is a test program we designed, which is suitable for running directly on the bare-metal Nuclei CPU.

In opus_demo, we prepare a piece of audio data and first encode it by calling opus_encode, and then decode the encoded data by calling opus_decode to obtain the processed audio data.

• We analyze the processed audio data to ensure the correctness of the opus encoding and decoding functionality;
• We measure the execution time of opus_encode and opus_decode to assess their performance.

File Structure

Directory	Description
celt	celt source files which is a part of Opus
silk	silk source files which is a part of Opus
include	Opus header files
src	Opus source files
data	data manipulation source files and some test results
reference	same test code but run on operating system as a reference

Prerequests

Please refer to the Prerequests section in the README.md of parent directory.

Build

Opus has both floating point and fixed-point implementation. Nuclei CPU support some extensions, such as B (Bitmanip) extension and P extension, which can enhance the performance of the codec. So there are some different build options for both floating point and fixed-point version.

First, change to the directory where Makefile is located. We take Nuclei N300 CPU as an example.

To build fixed-point version without extension:

make CORE=n300 ARCH_EXT= FIXED_POINT=1 all

To build floating-point version without extension:

make CORE=n300fd ARCH_EXT= FIXED_POINT=0 all

To build fixed-point version with B and P extension:

make CORE=n300 ARCH_EXT=_zba_zbb_zbc_zbs_xxldspn3x FIXED_POINT=1 all

To build floating-point version with B and P extension:

make CORE=n300fd ARCH_EXT=_zba_zbb_zbc_zbs_xxldspn3x FIXED_POINT=0 all

For more information about Nuclei CPU Architecture extension, please refer to ARCH_EXT section in Nuclei SDK documentation.

Functional Test

We have two opus_demo.c files, one opus_demo.c is for Nulcei CPU, and the other reference/opus_demo.c is for running on operating system with File I/O.

The test audio is in_1s.wav, which is a 1-second duration, 16k sample rate, PCM_S16LE format audio file. We extract the data from in_1s.wav to obtain in_1s.raw.

For Nuclei CPU baremetal environment, we use xxd tool to convert the raw format file in_1s.raw into data stored in data/in_1s.h.

When run on Nuclei CPU, we print the decode output data to console when uncomment the DUMP_DEC_RESULT macro in opus_demo.c. Then we save these data into a log file data/test/n300_fixed.txt and data/test/n300_float.txt manually. And you can convert the data to raw format file by to_raw.py. The raw files can be easily loaded by Audacity to show waveforms as below:

Although these four audio are not exactly same, but they are very close to each other. And people can hardly tell the difference between them by ear.

Performance Test

The encoder process 20 ms of audio data each time, so the 1s duration of audio data is divided into 50 frames. The decoder should follow inverse order, so the decoder should decode the frames for 50 times. We record the CPU cycles consumed to process each frame, and caclulate the average cycles as shown in the following table.

To show the performance of Nuclei CPU extensions, we compare the cpu cycles consumed between w/ and w/o extension. For w/o extension, the build option is ARCH_EXT=, for w/ extension, the build option is ARCH_EXT=_zba_zbb_zbc_zbs_xxldspn3x.

Test bitstream: n300_dual_best_config_ku060_16M_7cd945994_18d811786_202408191002.bit

These results can be easily calculated by data/bench/cmp.py.

fixed-point

case	w/o ext (avg cycles)	w/ ext (avg cycles)	speedup ratio
encode	6951502.84	5716036.98	1.22
decode	112640.22	106151.44	1.06

float-point

case	w/o ext (avg cycles)	w/ ext (avg cycles)	speedup ratio
encode	2889317.46	2861932.38	1.01
decode	133390.06	126119.08	1.06

Changelog

operator/function	description	file
SATURATE16	using SCLI32 to replace	fixed_riscv.h:11
MAX32	using MAXW to replace	fixed_riscv.h:15
MIN32	using MINW to replace	fixed_riscv.h:18
VSHR32	using KSLRAW to replace	fixed_riscv.h:21
MULT16_16_Q15	using KHMBB to replace	fixed_riscv.h:24
PSHR32	using SRA_U to replace	fixed_riscv.h:27