C++ Algorithm compression ratio calculation - c++

Question

I am stuck since 2 days with a seemingly simple calculation.
But I just don't get it.
I am encoding an audio file with a compressing algorithm.
The entire audio file is separated into "chunks" of 960 bytes.
Each chunk is compressed to 60 bytes.
My uncompressed file is 1480320 bytes long.
My encoded file is 46320 bytes long.
Something seems to be wrong.
I tried to calculate the theoretic uncompressed file size from the file size of the encoded audio.
Here is how the file is encoded:
short *m_in;
short *m_out;
unsigned char *m_data;
unsigned char *m_fbytes;
int m_max_frame_size;
int m_frame_size;
int m_sampling_rate;
int m_max_payload_bytes;
int m_bitrate_bps;
int m_iByteLen1FrameEncoded;
int m_iByteLen1FrameDecoded;
m_sampling_rate=48000;
m_max_frame_size = 960*6;
m_max_payload_bytes=1500;
m_bitrate_bps= 24000;
m_iByteLen1FrameEncoded=60;
m_iByteLen1FrameDecoded=960;
m_in = (short*)malloc(m_max_frame_size*sizeof(short));
m_out = (short*)malloc(m_max_frame_size*sizeof(short));
m_data = (unsigned char*)calloc(m_max_payload_bytes,sizeof(char));
m_fbytes = (unsigned char*)malloc(m_iByteLen1FrameDecoded*sizeof(short));
FILE *fin= fopen(uPathInput.c_str(), "rb");
FILE *fout=fopen(uPathOutput.c_str(), "wb");
int curr_read=0;
int stop=0;
while (!stop)
{
int err;
err = fread(m_fbytes, sizeof(short), 960, fin);
curr_read = err;
for(int i=0;i<curr_read;i++)
{
opus_int32 s;
s=m_fbytes[2*i+1]<<8|m_fbytes[2*i];
s=((s&0xFFFF)^0x8000)-0x8000;
m_in[i]=s;
}
if (curr_read < 960)
{
for (int i=curr_read;i<960;i++)
{
m_in[i] = 0;
}
stop = 1;
}
//iLen will always return 60, so I guess the 960 bytes are compressed to 60 bytes, right?
int iLen = opus_encode(m_enc, m_in, m_iByteLen1FrameDecoded, m_data, m_max_payload_bytes);
if (fwrite(m_data, 1, iLen, fout) !=iLen)
{
fprintf(stderr, "Error writing.\n");
}
}
fclose(fin);
fclose(fout);
}
The compression ratio seems to be
960/60 = 16
So I calculated 46320 bytes * 16.
But that gets me to 741120 bytes.
And that doesn't fit. I expected it to be 1480320 bytes.
I am trying to find the error in my calculation, but I just don't manage.
Does anybody see where I went wrong?
Thank you very much for any help!

Answer 1

Right, to expand on my comments. The problem is found here:
fread(m_fbytes, sizeof(short), 960, fin);
You're reading 960 shorts, which should be 2 bytes wide, so you're really reading 1920 bytes. If opus_encode() returns the compressed size in bytes, that would make the compression ration 32 as Robert observed.
I'd also simplify the code processing the chunks:
size_t ITEM_SIZE = sizeof(short);
int ITEM_COUNT = 960;
// fread should first return a short item count, then zero
size_t shorts_read = 0;
while (shorts_read = fread(m_fbytes, ITEM_SIZE, ITEM_COUNT, fin)) {
size_t i = 0;
for (; i<read; i++) {
opus_int32 s;
// etc.
}
for (; i < ITEM_COUNT; i++) {
m_in[i] = 0;
}
// opus_encode() etc
}
You get rid of the useless stop flag and a level of nesting, and the construct is idiomatic for "read until you can't." (See this SO question.)
I retract what I mentioned about the code being hokey, I thought fread returns the bytes read, not the items read.