SENG440

A .wav file audio compressor and decompressor written in C and optimized for an ARM-based embedded system. Created for UVic SENG 440: Embedded Systems.

Usage

ssh <netlinkID>@seng440.ece.uvic.ca

mkdir /tmp/<your-name>

git clone https://github.com/brettjanz/SENG440.git /tmp/<your-name>

cd /tmp/<your-name>

make

Makefile Options

• make : Build main.exe, deploy it to the arm machine, and run it with test.wav
• make opt : Build main_optimized.exe, deploy it to the arm machine, and run it with test.wav
• make all : Build both mains, deploy them to the arm machine, run both with test.wav, and compare their outputs
• make clean : Clean up any files on the arm machine

Optimizations

Global Variables

Global variables will never be placed in a register. We can change this by making variables local to the functions that use them

• FILE* input_file, FILE* output_file, Wave* wave, and uint32_t num_samples have been made local to main()
• uint16_t bytes_per_sample has been made local to read_wav()
• CompressedWave compressedWave is now uint16_t* samples local to main() and compress_data()

If/Else Optimization

Since smaller values occur more often, we can change the order in which we check for different magnitudes in order to speed up execution

• get_codeword() now uses a switch case that checks lower values first
• compressed_magnitude() now uses a switch case that checks lower values first

File Read/Write Minimization

We can save on overhead by reducing the amount of times we perform file I/O with fread() and fwrite()

• read_wav() has been reduced to two read operations, one for the header and one for the sample data
• write_wav() has been reduced to two write operations, one for the header and one for the sample data

Function Inlining

We can save on overhead with simple but often-used functions by inlining their return value instead of making a function call

• signum() has been removed and had all its instances inlined
• magnitude() has been removed and had all its instances inlined
• compressed_signum() has been removed and had all its instances inlined
• write_wav() has been removed and had all its instances inlined

Loop Unrolling

We can save on overhead by reducing the number of iterations performed in for loops

• compress_data() now operates in blocks of 8 samples with a switch case handling the modulo remainder
• compress_data() now uses a single-statement while loop instead of a three-statement for loop
• decompress_data() now operates in blocks of 8 samples with a switch case handling the modulo remainder
• decompress_data() now uses a single-statement while loop instead of a three-statement for loop

Print Statements

Calls to printf() are slow and unnecessary in an embedded system

• All non-error, non-timer printf() calls have been removed