Well said. It's amazing how many people are confused as to the real meaning behind "capture resoltion." Much of it comes from the term "lines of resolution" which is a term from long before digital video. It describes how many lines can be resolved on the continuous (analog) horizontal scan you described. So, *vertical* lines are drawn on the screen closer and closer together until they blur together. These lines are counted across a width of screen equal to the height. For a 4:3 aspect ratio then, the "240 lines" for VHS equate to 240*4/3 resolvable dots across the screen, or roughly 352x480. For broadcast, the "80 lines/MHz" rule of thumb and , 4.2MHz luminance cutoff equate to 330 lines. That's 330*4/3, or roughly 480x480 for broadcast. This whole mess is also why a DVD is described as 540 "lines of resolution"... since if you draw a 4:3 picture with 720 horizontal dots, you only count 3/4 of them since "lines" is defined as per-picture height.It most definitely is not. I don't have any links on hand, but vertical resolution (because of how the signal is transmitted -- the analog signal is a continous wave with points at regular intervals at certain points marking the point where fields begin/end) is ALWAYS bang-on 480 regardless of source.
Horizontal resolution is variable (for the aforementioned reason), but (assuming your capture card doesn't suck; bt878's do) in theory, 352x480 (on a good card) is enough for VHS and ~540x480 (on a good card) is enough for broadcast.
If the source is from a digital source, such as satellite -- Dish, DirecTV, Starchoice and ExpressVu all transmit a signal at 480x480. There will be some lossage of course, in the A>D>A>D process that occurs when you capture the output.
However, if you are viewing on a PC, and the deinterlace filter is not very good (i.e. it drops a field to deinterlace), there MAY NOT be a visible difference between 240 and 480 vertically.
Certainly you will see it on a TV.
(Nevermind the MythTV FAQ, the bits about capture size are not entirely accurate)
There... I feel better. Bottom line is the number of scanlines is *FIXED* by the TV standard (visible NTSC 486, PAL 576). The number of dots to cut up the horizontal scan is somewhat arbitrary, but that's what "lines of resolution" refer to. It pains me to hear people talking about capturing TV at 320x240 since it's 1:1 square pixels, and "there's only 240 lines of resolution anyway." Tragic... :)
-Cory
************************************************************************* * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * *************************************************************************
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