OK, I understand now.
The improvement with component YUV over S-video Y/C is marginal if the original was recorded in any of the color under tape formats such as 3/4U, VHS, SVHS, Hi8, etc. JVC did add component video I/O to their SVHS industrial decks some years ago but the improvement was, IMO, marginal over simple Y/C.
On the otherhand, since DV is not colorunder, but actually a component tape format to begin with, having component I/O while not as good as native DV with 1394, will show significant improvement using YUV interconnects. I use a DVCAM DSR60 feeder deck here for getting DV video into my component analog edit system and the difference between YUV and Y/C is dramatic. Not so, as I said, with the SVHS player.
You just need to tune to a channel that has, a DD audio signal as that will be a requirement to record since the only way to get modulator output is to have DD audio on the SD channel. Once you select a channel with DD audio but with SD video, just record like you would any HDTV channel. The picture will be in SD 4:3 AR and the audio will be on DD, either 5.1 or 2.0 or what ever flavor they may decide to metadata you. If you tune to a channel that does not have DD audio you will not get a picture either. I have recorded this so I know it works! I have not tried to feed a conventional RF channel 3 output to the DST50 and record that on DVHS. That is a simple thing to test so I would think someone here has tested it. I just have not had the reason to try it.