What is the difference from capturing at 352x240 or 640x480?greenspun.com : LUSENET : Video CD : One Thread
Can anyone tell me what the difference is between capturing at the two resolutions? Will both be full screen on a TV or just one? Thanks, Cale
-- Cale Ferguson (email@example.com), March 17, 2000
If your VCD creation was successful, whether you captured at one or the other resolution, both will fill up the TV screen from corner to corner. This is as simple as it gets, though. You have to wade through a lot of other little things and put some perspective into the flotsam before it becomes clear. For starters, your video capture device will specify one format capture. If it's the one to used for the PC, where pixels are square, then you can take it 640x480 will produce a 4:3 display in the correct proportions when you finally watch your VCD. Pinnacle Systems vidcap devices (MP10, DC10, DC40, etc.) use this. You might be allowed to capture at more or less than 640x480, but the proportion between WxH remains 4:3. Higher capture resolutions are expected (but not always) to produce better looking video than lower res. Then there is the common image (CIF) format that specifies maximum or full resolution is 704x480 (NTSC) or 704x576 (PAL). White Book VCD requirements fall neatly into quarter CIF resolution at 352x240 (NTSC) and 352x288 (PAL). DV-codec as well as MPEG-2 used in the .vobs of DVDs still yet use another full res of 720x480 (NTSC) and 720x576 (PAL). Some complications arise when, as in our case, our final target format is VCD and our input files do not match the required resolutions. In most cases, some rules about resolution figures being a factor of 4, etc being followed produce no problems for, say Panasonic taking in a 640x480 AVI file for input and duly truncating it down to the required resolutions in the produced MPEG files for VCD creation. In other instances, cropping or resampling in a video or image editor may be required otherwise the picture looks stretched vertically or horizontally on our TV screen on playing our newly created VCD. You also have to consider the quality of the original video to be captured. If it is analogue video from a high-quality source (DVD, DSS, Hi8, S-VHS) capture at CIF quarter-res (352x240, NTSC) is usually enough, bearing in mind that capturing at higher resolutions eats up more HDD space quickly and doesn't produce better results than quarter-res capture. If the source is a wretched VHS tape, however, then capture at half-res CIF (352x480, NTSC) or even all the way full-res will indeed gobble up HDD space quick but we need the extra resolution so that the results after encoding to MPEG are still acceptable. Any better now?? You look at some past entries in this forum and Ross McCl will have made some very useful tips on the subject.
-- EMartinez (firstname.lastname@example.org), March 19, 2000.
Yes I agree with all of that but would say this:
If your capturing in analogue 4:3, use ONLY the full frame of 640 x 480 as the capture size. The 720 or 704 x 480 is not an analogue size, in DVD playback that 720 or 704 is compressed back to the analogue 640. In PAL its stretched 720 to 768.
I came through a 4 year period of analogue capturing where you spent most of the time fighting the computer to stop dropped frames and it was common to capture what is called a cropped TV frame (from memory in NTSC this was 608 x 456) because that gave a better quality image at the best data rate one could squeeze from the early computers. That was fine on a analogue system because the system put it back to the SAME SIZE with the frame edge being just hidden under the TV surround but way inside the full tube size.
When I went to producing VCD's which use the dig 525 or 625 TV system I tried in ignorance to use the same trick but did not realise that the 608 was actually stretched to the full 640 in the VCD process and DVD playback. There was a quality loss because of that. My subtiles disappeared off the screen, letters were lost and one finds the learning curve had to climb quickly to get around the problem having even noticed it as a fault in the first place. In this digital era that sort of thing is called a "pixel ratio" problem.
If you do not get the process correct you will find "circles aint circles" when you see them on the TV from a VCD played in a DVD player.
Laugh if you wish, or disbelieve if you wish also, but with DV firewire source material where I am forced to use a resize to get a reasonable video track length, I actually resize to the 4:3 analogue aspect size first and then to the vcd format in the Panasonic encoder having found that process gives me better "circles" which are actually round. It gets around (pun) which width is correct, the 704 v's 720 debate. So currently I capture at PAL 720 x 576 (DV) resize to 512 x 384 in another codec because the encoder must see a frame size that can be exactly divided by 16 and then down to 352 x 288.
-- Ross McL (email@example.com), March 20, 2000.