Audio Delay From Audio Capture Source Filter

Dear All:
I designed a AVstream driver with video and audio streams.
Then, using the graphedit to play and capture it.
The audio stream is offered from Audio Capture Source (qcap.dll).
The video stream is offered from Video Capture Source filter's capture pin.
However, the audio sound has a serious dealy (about 1 ~ 2 sec) to video in
previewing.
But, its captured avi file is AV-Sync well.
This is my driver's bug?

I had tried other video devices (USB2.0 Video Grabber).
Their preview and capture are AV-Sync both.
The USB device is a standard USB Audio Class device.
It uses Microsoft's USBAUDIO.sys driver.
Why cannot MS's driver cause the preview delay?

Best Regards,
H.P.