The DVX100a will have perfectly matched sound and video. In the DVX100, the sound will precede the video by about 2 frames in 24pA mode. However, the 2 frame offset will be constant so you can easily align the sound and image. FCP can be set to automatically do the offset.
Crystal sync is a system where a crystal oscillator lays a constant 60Hz signal on a separate sync track of an analog audio tape. The crystal oscillator doesn't control the tape recorder motor--it's used only as a reference. A crystal synced camera has it's motor controlled by a crystal oscillator so it runs at a very constant speed. Later, the tape can be played back and transferred at a constant speed using the sync track as a reference. Digital recording systems (video and audio) are inherently very stable in their recording rates.
