A problem for video experts

[ricky1756]

I was just reading over these posts. As I'm only moderately knowledgable in video science, a lot was lost on me. I just have to say though "Damn, you guys are good"

[Yonidan]

Doesn't GB-PVR display the video while it's capturing?

well, on, it doesn't. I can't watch TV while the GB-PVR is recording. I can't see what exactly I'm recording, and it's a problem for me. what can I do?

[jagabo]

VLC may be able to stream the GBPVR output file as it is recording. Start recording with GBPVR, start VLC and select Media -> Streaming. Select the file that's currently recording and press the Stream button. In the Stream Output dialog select Play Locally. Press the Stream button. You should then see the video that's recording.

[Yonidan]

ok thanks, I'll try it.

[jagabo]

Note that in Windows a program can create a file for its own exclusive use while it's recording, or it can create the file and let other programs read it while it is recording. So whether or not you can use VLC to play the file depends on how GBPVR creates the file. Please write back and let others know if this works for you or not.

[Yonidan]

it works, thank you very much, you solved the problem.

now I need a professional program for cutting videos, for example- if I record a movie, and I want to remove the intervals of commercials, how can I do that? can you recommend me a good software for that please? thanks.

[jagabo]

TDA, Mpeg Video Wizard, Mpg2Cut2

[Yonidan]

I tried Mpg2Cut2, everything is ok, except one thing:
every movement of the slider there, is too lengthy, of almot one second.
if I want to remove a section of half sec, it's impossible.
is there a way to move the slider delicately?

[jagabo]

Mpg2Cut2 does no reencoding so it only allows you to cut on key frames. If the frame slider is too inaccurate you can use the left and right arrow keys to move one key frame at a time backwards and forwards. That's typically about 1/2 second with DVD files.

If you want frame accurate editing you will need a program that reencodes cut GOPs. TDA and Mpeg Video Wizard can do that.

[Yonidan]

does the reencoding process change something in the quality of the video?

[jagabo]

Reencoding with lossy codecs like MPEG 2 always leads to a loss of quality. So the reencoded GOPs will be of lesser quality than the rest of the video. With the "smart" editors mentioned the rest of the video will be unchanged. Other "dumb" editor will reencode the entire video.

In MPEG2 GOPs are typically about half a second long. If you use high enough a bitrate for the reencoded GOPs the difference in quality will be minimal.

GOP is an acronym for Group Of Pictures. A GOP consists of a key frame (a full picture, much like a JPG image) followed by a series of "predicted" frames. Predicted frames only include the changes from frame to frame. If you cut away the key frame there is no way for a decoder to reconstruct the entire picture. So the cut GOP needs to be reencoded with a new key frame and new predicted frames.

[Yonidan]

I hope I don't get you wrong, cause your english it too hard for me

so programs like TDA and Mpeg Video Wizard do reencoding which means- there will be a loss of quality? there won't be loss of quality with Mpg2Cut2?

[jagabo]

so programs like TDA and Mpeg Video Wizard do reencoding which means- there will be a loss of quality?

Yes these programs will reencode -- but only about 1/4 second around each cut. The rest of the video will not be reencoded.

there won't be loss of quality with Mpg2Cut2?

There will be no loss of quality at all -- but you don't get frame-accurate editing.

[Yonidan]

[quote="jagabo"]

so programs like TDA and Mpeg Video Wizard do reencoding which means- there will be a loss of quality?

Yes these programs will reencode -- but only about 1/4 second around each cut. The rest of the video will not be reencoded.

then of The rest of the video will not be reencoded, why is there a loss of quality?

[jagabo]

There is a loss of quality in those short reencoded sections. Not the rest of the video.

[Yonidan]

so if I cut short section from the beginning, and short section from the end, which part of the video will be reencoded and in lower quality?

[manono]

If you didn't cut on an I-Frame, just the part until the next I-Frame, half a second or less at each end.

[jagabo]

A GOP (Group of Pictures) is about 1/2 second in a DVD compatible MPG file. If the I frame (the first frame) of a GOP is removed that GOP has to be reencoded. On average, if you cut a GOP in the middle, the remain portion of the GOP is about 1/4 second. That remaining portion of the GOP must be reencoded. So at each cut from 0 to 1/4 second must be reencoded.

The situation is worse with high compression codecs like Xvid and h.264. GOP sizes there can be 300 frames or more. In those cases you may have to reencode 12 seconds when you cut.

[Yonidan]

well, I think I'll use Mpg2Cut2, thanks.

another question, hope it's the last one:
I've noticed that when I capture from some TV channels, I don't get a picture on full screen, I mean-there are black zones on the sides, as I marked here in red. when I watch tv live these black areas don't appear, only in records. do you know what causes it? thanks.

[poisondeathray]

another question, hope it's the last one:
I've noticed that when I capture from some TV channels, I don't get a picture on full screen, I mean-there are black zones on the sides, as I marked here in red. when I watch tv live these black areas don't appear, only in records. do you know what causes it? thanks.

one possible reason is tv overscan
https://www.videohelp.com/glossary?O#Overscan

On old CRT tv's it gets worse with age as the magnets weaken. I still have an antique laying around and 10-20% of the edges are gone!

[jagabo]

What you see in that sample is exactly what's broadcast. You don't see the black bars on TV because the TV overscans the picture (about 3 percent of the frame is not visible on all four sides).

[Yonidan]

I mean-I don't see those black bars also when I watch tv via the capture card. do you know why?

[jagabo]

The player is simulating overscan and cutting off the edges of the frame. Find a source where the picture is static for a long time (channel guide, test pattern) and has recognizable objects near the edge.

[Yonidan]

so it depends on the TV channel I record from?
in another channel (VH-1 Classic) it's even worse.
but in MTV for example, it's ok, the picture there is spread out on full screen.

[jagabo]

so it depends on the TV channel I record from?

No. Overscan is a property of the TV. The TV overscans everything. I recommend using a static image because it's hard to see how much the TV overscans with normal video where everything is changing all the time.

[Yonidan]

well, never mind.
thank you all.