How does TiVo insert its graphics into ongoing digital playback over HDMI?

[rlcarr]

When a S3 or TiVoHD is playing back a recording from a digital channel over an HDMI connection, how does it insert the TiVo graphics over the ongoing playback (like the info screen, or the time bar, etc.)?

Since (as I understand it) what is coming off the drive is bit-for-bit identical with the broadcast that was recorded, how does the TiVo get its graphics into the stream -- which is after all being sent digitally to the TV? Does it have to do a decode/reencode (I didn't think it had the horsepower for that)? Or does MPEG-2/MPEG-4 have a layers concept and the TiVo graphics are a separate stream that's in another layer? Or something else?

[Stephen Tu]

HDMI is sending uncompressed data, it's not compressed MPEG-2 anymore. The Tivo has already decoded the original frames, it just has to overlay graphics on top of the decoded frames.

[rlcarr]

Ah, thanks! I thought the compressed stuff went to the HDTV and it decoded it. But with a second of thought I would have realized that was impossible (how could the TiVo put in the letter/pillarboxing, for example, without decoding)