Quote:
Originally posted by fasttimes
FYI guys... this information comes from the author of Virtual Dub, and was confirmed by the author of the BTWinCap drivers:
"The BT878A used in the WinTVs sample the NTSC signal at 28MHz, filter that down to 14MHz, and then scale it down via a multitap filter. According to the docs, the 14MHz rate gives you a limit of 910 pixels per line, which then drops to 780 pixels at a square aspect ratio. The WinTV thus does not capture 640 and then stretch it to 720 -- it captures 910 and drops it to 720, or whatever you require."
So there you go.
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Sorry to bring up old posts, but I was trying to really understand Xesdeeni (who I believe has this topic 100% right).
Howerver, I wanted to correct the above quote because it is linked from the FAQ.
The quote in this context is misleading. The numbers 910 and 780 express the full NTSC 63.555 usec line at different sample rates. 720 is what you expect to be the active portion of this line (about 52.666 usec). The active protion is determined by the driver. It 'crops' off the front and back sync by seting the HDelay and HActive registers of the BT8x8 chip. The BTwincap driver sets these values to return a 754 14.3 sample portion as the active line. This number relates to a cropped 910 full 14.3 line. The chip then drops this to what you ask based upon what the driver sets for the HScale register.
The trouble is that all of the prior disucssions were with regard to a 13.5 sample. If HScale does not scale 14.3 to 13.5 your picture is indead streatched because it is not dropped enough. 712 is the number for 13.5 using the BTwincap driver. 720 is not dropped enough.
Bottom Line:
BT8x8 cards sample at a high enough frequency to easily meet the nyquist requirement. 14.3 vs 4.2 for NTSC. The trick is then to get them to not drop the horizontal frame size below what would be required to represent the analog bandwidth, and to get the aspect ratio correct.
Edit
It seems the only 'mistake' in Xesdeeni's logic is applying kell. Kell applys to what the camera does (to capture a real image) and what a TV does (to show a real image) with regard to the vertical only. It should be ignored for horizontal (except maybe for an LCD screen or a CCD camera).
TVLh should not consider kell. TVLv are all the same 480 or 576. So what's the point of adjusting them for kell any ways?
VHS NTSC is 240x480 TVLh. or 320x480 pixels for a 4/3 image. 352x480 is thus a good pixel frame size for VHS.
Full (4.2) Broadcast NTSC is 330x480 TVLh or 440x480 pixels. 352x480 is not enough.
Of course the bandwidth of your VHS machine may not be 3MHz and your CATV may not be 4.2MHz. If not, you don't even need that much.