关于avi录制的问题

AVISTREAMINFO strmInfoCaption;
BITMAPINFOHEADER bihCaption;
HRESULT hr; ZeroMemory(&bihCaption, sizeof(bihCaption));
bihCaption.biSize = 4;
ZeroMemory(&strmInfoCaption, sizeof(strmInfoCaption));
strmInfoCaption.fccType  = streamtypeTEXT;
strmInfoCaption.fccHandler = 0x2e565247;
strmInfoCaption.dwScale  = 2;
strmInfoCaption.dwRate = uRate;
strmInfoCaption.dwStart  = 0;
strmInfoCaption.dwLength = 0;
strmInfoCaption.dwInitialFrames  = 0;
strmInfoCaption.dwSuggestedBufferSize = 0;
strmInfoCaption.dwQuality = -1;
strmInfoCaption.dwSampleSize = 0;
strmInfoCaption.dwEditCount  = 0;
strmInfoCaption.dwFormatChangeCount  = 0;
strmInfoCaption.dwSampleSize = 0;以上是我的参数设置，strmInfoCaption.dwScale = 2;strmInfoCaption.dwRate我给它传递的参数是25，当dwScale设置为1时，生成的录像在播放时会比实际摄像头录制时快，如果设置为2，则又会比实际摄像头录制时慢，会长出三分之二时间来，这个该怎么改啊