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> It is impossible for the audio to drift ahead of or behind the video, regardless of the video framerate or audio samplerate.

How is this "impossible" if the video clock and audio clock are on different physical devices?

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The video clock (I assume you mean pixel clock) isn't relevant. The synchronization of video and audio happens on a per-frame basis. And since the frames are clocked out on the audio-clock, there cannot be any drift.

Having said that, the CGA card does not actually have its own clock. It takes the NTSC base frequency of 14.31818 MHz from the OSC-pin of the ISA bus. On early PCs, such as the IBM 5150/5160, the CPU frequency is also derived from this same base clock (which is why you get the oddly specific speed of 4.77 MHz).

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