Question

Just out of curiosity while learning both wave file format and C++, I stumbled on this page that details the canonical layout of wave format

Now, I tried to write a C++ program to read out the sample rate

char buffer[4];
// already read other values before reading sample rate
inFile.read(buffer, 4);
// conversion using little endian
sampleRate = (long) buffer[0] + ((long) buffer[1] << 8) + ((long) buffer[2] << 16) + ((long) buffer[3] << 24);

In the hex editor, I can see my sample wave file has sample rate as 44 ac 00 00, which translates into 44100 rate. However, with the above code, I always end up with -21436

After some debugging, it turns out that the raw hex value “ac” should be translated into 172 in dec, but C++ assumed the hex value as “ffffffac”, which translates to -84 in dec. If C++ just assumes it is plain “ac”, I would have ended up with the correct sample rate value.

So the questions is, how do I read out the data from wave file and actually get the correct sample rate? Do I have to do some magic to strip away the leading “f”s ?

Thanks a bunch in advance!

Answer 1

There are two ways to fix your code. The first is to ensure that unsigned arithmetic is used throughout by making everything unsigned. The second is to use a bit mask to remove the sign extensions.

sampleRate = ((long) buffer[0] & 0xff) + (((long) buffer[1] << 8) & 0xff00) + (((long) buffer[2] << 16) & 0xff0000) + ((long) buffer[3] << 24);