将Wav格式压缩成Mp3

　　Compressing Wav file to mp3

● 1. IntrodUCtion
First, I don't meant to give you informations about how to understand the mp3 algorithm. My goal is to eXPlain how to use an already existing encoder with BCB.

● 2. Choosing the mp3 encoder
There are tons of mp3 encoders. Some of them are free others are not. Some are fast but produce an awful result. Others are slow but with Excellent result and give a high audio quality. The ideal would be a free, reasonably fast encoder giving a high audio quality, all at the same time.
Enjoy! This pearl exists. But we have to look at it in the GNU world. There is a GNU project, called LAME, for Lame Ain't a Mp3 Encoder, under the GPL license. The official web site of the LAME project is http://www.mp3dev.org/mp3/
Moreover, as it is a GNU project, we have Access to the source and there is a version compiled for Win32 in a DLL.
Among all the other encoders, I want to quote two of them. The first, FRAUNHOFER, because it is a fast and excellent encoder : http://www.iis.fhg.de/ but it's not  free though.
The second because it's a very fast encoder but the audio result is awful. So don't use it except if you are looking for a fast encoder. It's the encoder from Xing Tech : http://www.xingtech.com/
Note : The Lame encoder has a limitation. The sample rate must be 32000, 44100 or 48000.?

● 3. Some informations about the WAV format
A wav file is just a collection of chunks. There is a format chunk wich contains all the informations about the samples. For instance, the bitrate, the number of channels, if it's stereo or mono... There is also a chunk containing the data. In other Words, this chunk contains all the samples. In front of the file, there are 12 characters indicating that the file is a wav file.
The two chunks given above must be present in the file.
There could be other chunk but we just ignore them. They are not needed for our purpose. If you want to know more about wav file, take a look at http://www.wotsit.org/ for a complete description.
The format chunk :

struct FormatChunk
{
    char            chunkID[4];
    long            chunkSize;

    short           wFormatTag;
    unsigned short  wChannels;
    unsigned long   dwSamplesPerSec;
    unsigned long   dwAvgBytesPerSec;
    unsigned short  wBlockAlign;
    unsigned short  wBitsPerSample;
    // Note: there may be additional fields here, depending upon wFormatTag.
};

Above, you can see the struct representing the format chunk. The chunkID is always "fmt " with an ending space (4 characters). It's the identification of the chunk. All other chunk have such an ID. The chunkSize parameter contains the number of bytes of the chunk, the ID and chunkSize excluded.
The format chunk must be the first chunk in the file.

The data chunk :
struct Chunk
{
    char chunkID[4];
    long chunkSize;
};
In the case of the data chunk, the chunkID contains "data". The chunkSize parameters contains the size of the raw data (samples). The data begins just after chunkSize.

In the case of the data chunk, the chunkID contains "data". The chunkSize parameters contains the size of the raw data (samples). The data begins just after chunkSize.
Dans le cas du bloc de données, chunkID contient "data". Le paramètre chunkSize contient la taille du bloc de données proprement dites. Celles-ci commencent juste après chunkSize.
So, when we read a wav file, all we have to do is :
- read the first 12 characters to check if it's a real wav file.
- read the format chunk in a struct similar to the formatChunk struct.
- skip the extra parameters in the format chunk, if any.
- find the data chunk, read the raw data and carry out with the encoding.
-skip all other chunks.
Donc, ce que nous devons faire est :
- lire les 12 premiers caractères pour déterminer si on est bien en présence d'un fichier wav.
- lire le bloc de format dans une structure similaire à la structure formatChunk.
- ignorer les caractères supplémentaires dans le bloc de format, s'il y en a.
- ignorer tous les blocs qui ne sont pas le bloc de données.
- trouver le bloc de données, lire ces données et lancer l'encodage.

● 4. Importing the DLL
The DLL used for the encoding  is called lame_enc.dll.
Unfortunately, this DLL was build with VC 6 from Microsoft. If we just create a lib file from the DLL and try to import the library in BCB, we'll get an 'Unresolved external error' at link time for each function we'll try to use. Due to the declaration type, BCB is expecting a function name with a leading underscore and the function names doesn't have such a leading underscore.

To resolve this issue, we must, first, create a def file from our DLL. Open a console windows and type :

IMPDEF lame_enc.def lame_enc.dll

Open the lame_enc.def file with an editor (Notepad for instance) and modify it like this. This will create aliases for the functions :
    LIBRARY     LAME_ENC.DLL
    EXPORTS
    _beCloseStream = beCloseStream
    _beDeinitStream = beDeinitStream
    _beEncodeChunk = beEncodeChunk
    _beInitStream = beInitStream
    _beVersion = beVersion
    _beWriteVBRHeader = beWriteVBRHeader
    beCloseStream                  @4
    beDeinitStream                 @3
    beEncodeChunk                  @2
    beInitStream                   @1
    beVersion                      @5
    beWriteVBRHeader               @6

Now, we can create the lib file from our def file. We'll import that lib file in our project. To create the lib file, type :
implib lame_enc.lib lame_enc.def

● 5. The code
First, you have to import the libary in your project. Next, include the header file of the DLL into your unit. In the DLL header file, you have to add extern "C" in front of all exported function.
Here is the header with the moifications (lame_enc.h) :
/*  bladedll.h
    +++++++++++++++++++++++++++
    +   Blade's Encoder DLL  &n