[mythtv-users] PVR-350 capture test problem (strange horizontal
dan_mythtv at yahoo.com
Fri Aug 13 09:07:57 EDT 2004
Wow, thanks for taking the time to post such a thorough explanation. It's good to know that my picture quality should be okay. And 640x480 is definitely looking pretty good on my tv. It maybe doesn't look quite as good as I'd like, but like you said, I was sitting pretty close at that time. I'm not sure where one changes the bitrate in mythtv setup, but I'll look through the settings for it when I get to that stage.
"Michael T. Dean" <mtdean at thirdcontact.com> wrote:Dan Werner wrote:
> I tried width=640, height=480, and it works...perfect video!
> Now, my question becomes: am I going to lose something when I start
> using TV-out (video quality?) by not having the full NTSC width and
> height? Is it worth trying to figure out how to get 720x480?
NTSC video does not have a "width." Remember, NTSC is an *analog*
format. Therefore, it specifies 480 (visible) lines of video. Each of
those lines is a continuous (analog) line--not a bunch of dots, as a
digital line would be.
Think of it like this (if this e-mail is displayed in a fixed width
font, paste the following into an editor using a variable width font for
a better visual representation). If we have a line of video:
we can sample that line as many times as we want to create our digital
both have the same width--the width of the original line, but the first
example only sampled the line 43 times; the second example sampled the
line 72 times. Therefore, the second example has the potential to be a
more realistic digital representation of the analog signal. Because
each pixel can show a different color, the more pixels we have, the more
"continuous" the line looks because we can use smaller changes in color
than with fewer pixels.
However, when we start talking about compressed digital video (whether
we're talking about RTJPEG, MJPEG, MPEG-1, MPEG-2, MPEG-4), things get a
bit more complicated. All of these compression formats are "lossy."
They trade data accuracy for reduced storage size. Therefore, this
compression may negatively impact the data representation we have.
For example, if we're recording in MPEG format, we can specify a bitrate
at which to record. We can divide the bitrate by the number of frames
per second to find the total number of bits available for representation
of each frame--regardless of pixel resolution. (This is something of a
simplification since some frames simply specify the difference between
the previous frames.) So, if we're recording MPEG-2 at 4500kbps in NTSC
(30fps), we have 150kb per frame. Now, if we have to fit 720 x 480
pixels in that 150kb, we have 12% more pixels to represent than with 640
x 480 pixels, so we have to compress the data farther, resulting in a
greater loss of accuracy. To lessen the impact of a greater number of
pixels, we must increase the bitrate.
In other words, at a given bitrate it's possible that a higher
resolution setting can actually decrease image quality. Therefore, far
more critical than choosing a high resolution is choosing an appropriate
bitrate at which to record. Choosing the bitrate should take into
account the resolution you've decided to use.
Note, also, that your screen size/viewing distance will have an impact
on your ability to notice a difference between resolutions. For
example, when viewing my 27 inch TV from approximately 10 feet, I can't
tell any difference between 480x480, 640x480, and 720x480. I still
record at 640x480 because I can and I have the disk space, but I'd be
just as happy with 480x480. I will admit that I'm not as sensitive to
picture distortion as many people are. Having started using Myth after
several years of taping/watching everything using my VHS VCR in EP mode,
I don't even notice bad picture quality, anymore. :)
Also, if you really want 720x480, I haven't had any problems with it (or
640x480 or 480x480) on my PVR-350 with Chris Kennedy's patches to the
ivtv-0.1.10pre2 driver. (I'm currently using ck100b--mainly because I
haven't had a chance to upgrade lately (but it is very stable); however,
starting with the ck101 series, Chris began a major rewrite of the
memory handling to use scatter gather arrays, so you might want to stick
with a pre-101 patch until things stabilize a bit.) Check out the ivtv
ML's for more info ( http://sourceforge.net/mail/?group_id=73219 ).
Note that there have been some major changes to the ck versions, so it's
critical you a) forget everything you know about configuring ivtv, b)
read *all* the documentation in the doc directory of the source, and c)
keep up with the changes on the ML.
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