Author: Tomas Rutkauskas

How to generate a wave file and play it backwards

Answer:

Here's some code that plays a *.wav file backwards. It shows how *.wav files are 
generated.


1   procedure Interchange(hpchPos1, hpchPos2: PChar; wLength: word);
2   var
3     wPlace: word;
4     bTemp: char;
5   begin
6     for wPlace := 0 to wLength - 1 do
7     begin
8       bTemp := hpchPos1[wPlace];
9       hpchPos1[wPlace] := hpchPos2[wPlace];
10      hpchPos2[wPlace] := bTemp
11    end
12  end;
13  
14  procedure ReversePlay(const szFileName: string);
15  var
16    mmioHandle: HMMIO;
17    mmckInfoParent: MMCKInfo;
18    mmckInfoSubChunk: MMCKInfo;
19    dwFmtSize, dwDataSize: DWORD;
20    pFormat: PWAVEFORMATEX;
21    wBlockSize: word;
22    hpch1, hpch2: PChar;
23    waveOutHAndle: Integer;
24    data: PChar;
25    waveHdr: PWAVEHDR;
26  begin
27    data := nil;
28    mmioHandle := mmioOpen(PChar(szFileName), nil, MMIO_READ or MMIO_ALLOCBUF);
29    if mmioHandle = 0 then
30      raise Exception.Create('Unable to open file ' + szFileName);
31    try
32      mmckInfoParent.fccType := mmioStringToFOURCC('WAVE', 0);
33      if mmioDescend(mmioHandle, @mmckinfoParent, nil,
34        MMIO_FINDRIFF) <> MMSYSERR_NOERROR then
35        raise Exception.Create(szFileName + ' is not a valid wave file');
36      mmckinfoSubchunk.ckid := mmioStringToFourCC('fmt ', 0);
37      if mmioDescend(mmioHandle, @mmckinfoSubchunk, @mmckinfoParent,
38        MMIO_FINDCHUNK) <> MMSYSERR_NOERROR then
39        raise Exception.Create(szFileName + ' is not a valid wave file');
40      dwFmtSize := mmckinfoSubchunk.cksize;
41      GetMem(pFormat, dwFmtSize);
42      try
43        if DWORD(mmioRead(mmioHandle, PChar(pFormat), dwFmtSize)) <> dwFmtSize then
44          raise Exception.Create('Error reading wave data');
45        if pFormat^.wFormatTag <> WAVE_FORMAT_PCM then
46          raise Exception.Create('Invalid wave file format');
47        if waveOutOpen(@waveOutHandle, WAVE_MAPPER, pFormat, 0, 0,
48          WAVE_FORMAT_QUERY) <> MMSYSERR_NOERROR then
49          raise Exception.Create('Can''t play format');
50        mmioAscend(mmioHandle, @mmckinfoSubchunk, 0);
51        mmckinfoSubchunk.ckid := mmioStringToFourCC('data', 0);
52        if mmioDescend(mmioHandle, @mmckinfoSubchunk, @mmckinfoParent,
53          MMIO_FINDCHUNK) <> MMSYSERR_NOERROR then
54          raise Exception.Create('No data chunk');
55        dwDataSize := mmckinfoSubchunk.cksize;
56        if dwDataSize = 0 then
57          raise Exception.Create('Chunk has no data');
58        if waveOutOpen(@waveOutHandle, WAVE_MAPPER, pFormat, 0, 0,
59          CALLBACK_NULL) <> MMSYSERR_NOERROR then
60        begin
61          waveOutHandle := 0;
62          raise Exception.Create('Failed to open output device');
63        end;
64        wBlockSize := pFormat^.nBlockAlign;
65        ReallocMem(pFormat, 0);
66        ReallocMem(data, dwDataSize);
67        if DWORD(mmioRead(mmioHandle, data, dwDataSize)) <> dwDataSize then
68          raise Exception.Create('Unable to read data chunk');
69        hpch1 := data;
70        hpch2 := data + dwDataSize - 1;
71        while hpch1 < hpch2 do
72        begin
73          Interchange(hpch1, hpch2, wBlockSize);
74          Inc(hpch1, wBlockSize);
75          Dec(hpch2, wBlockSize)
76        end;
77        GetMem(waveHdr, sizeof(WAVEHDR));
78        waveHdr^.lpData := data;
79        waveHdr^.dwBufferLength := dwDataSize;
80        waveHdr^.dwFlags := 0;
81        waveHdr^.dwLoops := 0;
82        waveHdr^.dwUser := 0;
83        if waveOutPrepareHeader(WaveOutHandle, WaveHdr,
84          sizeof(WAVEHDR)) <> MMSYSERR_NOERROR then
85          raise Exception.Create('Unable to prepare header');
86        if waveOutWrite(WaveOutHandle, WaveHdr, sizeof(WAVEHDR)) <> MMSYSERR_NOERROR 
87  then
88          raise Exception.Create('Failed to write to device');
89      finally
90        ReallocMem(pFormat, 0)
91      end;
92    finally
93      mmioClose(mmioHandle, 0)
94    end;
95  end;