[mythtv-users] Re: Best video capture resolution for output to TV?

[Bruce Markey]

Ray Olszewski wrote:
> At 11:06 AM 5/16/2003 -0400, Joseph A. Caputo wrote:
> [...]
> 
>> > It all depends on your personal preference.  Some people think VHS
>> > quality is good enough, while some people won't settle for anything 
>> less
>> > than DVD quality (full NTSC resolution).  If it were me, I would be
>> > recording at 720x480, but I want the best quality I can get.
>>
>> Why capture at 720x480 when NTSC resolution is not that high?  Anything
>> beyond 640x480 is a waste (actually, even 480x480 is just fine... see 
>> below)
> 
> 
> Please forgive my confusion here ...but in what sense does NTSC even 
> *have* a horizontal "resolution"? I understand that the vertical 
> resolution -- 480 real, out of 525 theoretical, lines, in two interlaced 
> frames -- is well defined. But I thought the horizontal signal was 
> continuous, not discrete, making the relevant question the appropriate 
> rate at which to *sample* it for digitizing.

Exactly. Each scan line is an analog wave. The faster the
clock is set to latch on samples for the capture, the more
samples per scan line. The higher the sample rate, the more
detailed the digital representation of the analog signal.
However, there is the law of diminishing returns and there
is a trade off for CPU and disk space. 


Perceived Quality
10

9                                      *   *  *    *
                               *   *
8                      *   *
                   *
7              *

6          *

5
      *
4

3  *

2

1
  240 280 320 360 400 440 480 520 560 600 640 680 720
Horizontal resolution

If recordings look okay at 352x480 but better at 720x480
does it look enough better to justify files twice as big
and only half the maximum recording time? TiVo maxes out 
at 544x480.

Signal quality is also a factor. A DVD player or digital
channel from a digital cable box over s-video may look
slightly better at 640 or 720 but for broadcast TV,
anything over 480 looks pretty much the same.

>  (There is probably a theoretical answer to the sampling-
> rate question too, but I've never seen it explained.)

This doesn't cover digital sampling rates but has gads
of authoritative information about how TV signals work:
http://www.ntsc-tv.com/ . One relevant section is:

   http://www.ntsc-tv.com/ntsc-index-04.htm

which show test patterns for approximating the effective
horizontal resolution of a display device and how Kell
factor limits the maximum horizontal resolution of a 
display.

--  bjm