如何使用fread和fwrite从文件读取pcm样本？

Question

I want to read pcm samples from a file using fread and want to determine the signal strength of the samples.How do I go about it?

For reading, how many bytes constitute 1 pcm sample? Can I read more than 1 pcm sample at a time? This is for WAV and AAC files.

Answer 1

You have to understand that WAV-files (and even more so AAC-files) are not all the same. I will only explain about WAV-files, you'll hopefully understand how it is with AAC-files then. As you pointed out, a WAV-file has PCM-encoded data. However that can be: 8-bit, 16-bit, 32-bit, ...Mono, Stereo, 5.1, ...,8kHz, 16kHz, 44.1kHz, etc. Depending on these values you have to interpret the data (eg when reading it with the fread()-function) differently. Therefore WAV-files have a header. You have to read that header first, in the standard way (I do not know the details). Then you know how to read the actual data. Since it is not that easy, I suggest you use on of the libraries out there, that read WAV-files for you, eg http://www.mega-nerd.com/libsndfile/ . Of course you can also google or use SO to find others. Or you do it the hard way and find out how WAV-file headers look like and decode that data first, then move on to the actual PCM-encoded data.

Answer 2

I have no experience tackling with WAV file, but once read data from mp3 file. As to the mp3 file, each 576 pcm samples are encoded into a frame. All the frames are stored directly into a file alone with some side information. When processing encoded data, I read binary data from the mp3 file and stored in a buffer, decoding buffered data and extract what is meaningful to me.

I think processing wav file(which stores pcm samples per my understand) is not quite different. You can read the binary data from file directly and perform some transformation according to wav encoding specification.

The file itself does not know what kind of data even what format of the data is in it. You can take everything in a file as bytes(even plain text), read byte from file interpreting the binary data yourself.