I don't know about "broadcast quality" but a 4.7GB DVD-RAM (and also a 4.7 GB DVD-R) can hold:
<ul type="square">[*] 1 hour in XP.
[*] 2 hours in SP.
[*] 4 hours in LP.
[*] 6 hours in EP. I find this speed unacceptable. Quality is too low.
[*] FP speed will squeeze the best possible bit rate into the amount of time you have. e.g. When recording the 2nd half of the Farscape marathon, I used FP on a blank disc to record the last 5 hours.
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I find LP quite acceptable (better than VHS SP). The only problem is that on LP, the freeze frames show some grain. So, if frame grabs are needed, it's better to use SP, or even XP. When the picture is moving, LP looks perfectly fine.
I only use XP for very special things (e.g. Crusade, Brimstone, "Thirdspace" or "A Call to Arms" if they were broadcast again, now). I used to use SP on DVD-RAM for Buffy and Angel, but now I'm doing everything on LP (on DVD-RAM).
The sound, even on LP, is vastly better than tape. The clarity is incredible.