ap9
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PCM/WAV file format information, please.
Hello! I'm currently writing a program that will be reading in and working with PCM/WAV files. I already know the header formats, gleaned from http://www.intersrv.com/~dcross/wavio.html but I need some help with regards to the meaning of the data portion of the file. I.e. how do the numbers reflect the waveform of the sound being generated at that time (in that sample), or some such relationship.
Thanks!
ap9
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ASKER
Oh, a small point -- for the data chunk, there is a string that consists of "data". Now, is there a 4 byte value after this that represents the length of the data chunk, or not? I've read conflicting reports about this. Thanks.
ap9
ap9
How do you interpret the sample frames?.. It depends on your meaning of "interpreting" a sample frame.
First of all you should base your code on the format header. In this piece of text we'll work with a simple example: A wave file with:
SampleRate 11025
BitperSample 16
Stereo
Data bytes = 1024
Before going on, you're right!... After each header there is a value representing the length in bytes of the chunk (without incluthin the chunk name string).
In our example we can see that each sample is represented by a 16 bit value, so a "Char" data type won't be enough to store the samples, so we choose "Integers" to store our values (Remember an integer needs 2 bytes in memory). We also see that our wave file is Stereo, this means that we will have 2 integer values per sample. In conclusion each sample frame is formed by 4 byes = 2 integers = 2 sample points (the first int for the left, and the second for the right channel).
Our data chunk says that our wave file has 1024 bytes, this means that we will have 1024/4 = 256 sample frames, this means that we will have 256 values for the left channel and 256 values for the right channel.
I've choosen a Stereo example here because it's a little more difficult, (but no too much). In this case, if you want to graph the wave form, you should make two graphs, one for each channel (Remember that the left speaker can have a completely different music that the right speaker)... If you want to make only one graph, you can maybe calculate the average of the right and left sample points.
Of course for Mono sound you avoid all this trouble.
It's not very difficult if you see.... You should just read values according to the Wave format.... In stereo you read: left1, right1, left2, right2, left3, right3, left4, right4, etc... In mono you read: value1, value2, value3, value4, etc... and finally you draw those values.
Trillo
First of all you should base your code on the format header. In this piece of text we'll work with a simple example: A wave file with:
SampleRate 11025
BitperSample 16
Stereo
Data bytes = 1024
Before going on, you're right!... After each header there is a value representing the length in bytes of the chunk (without incluthin the chunk name string).
In our example we can see that each sample is represented by a 16 bit value, so a "Char" data type won't be enough to store the samples, so we choose "Integers" to store our values (Remember an integer needs 2 bytes in memory). We also see that our wave file is Stereo, this means that we will have 2 integer values per sample. In conclusion each sample frame is formed by 4 byes = 2 integers = 2 sample points (the first int for the left, and the second for the right channel).
Our data chunk says that our wave file has 1024 bytes, this means that we will have 1024/4 = 256 sample frames, this means that we will have 256 values for the left channel and 256 values for the right channel.
I've choosen a Stereo example here because it's a little more difficult, (but no too much). In this case, if you want to graph the wave form, you should make two graphs, one for each channel (Remember that the left speaker can have a completely different music that the right speaker)... If you want to make only one graph, you can maybe calculate the average of the right and left sample points.
Of course for Mono sound you avoid all this trouble.
It's not very difficult if you see.... You should just read values according to the Wave format.... In stereo you read: left1, right1, left2, right2, left3, right3, left4, right4, etc... In mono you read: value1, value2, value3, value4, etc... and finally you draw those values.
Trillo
ASKER
Ah, I see! Yes, you're right, it isn't very hard -- I was thinking that there was more involved than just that. Thank you very much! I accept your answer.
ap9
ap9
ASKER
Or maybe, by way of example, suppose I have a simple wave (i.e. a sine wave -- sin(x)) sampled at 44.1kHz mono, what happens when I encode it into WAV format (not worrying about the headers, just how it is represented in the data chunk).
I've increased the point value of the question to 200, as I do appreciate your help!
ap9