On Tue, Sep 21, 2004 at 04:56:04PM +0200, Bernhard Praschinger wrote: > Hallo > > > If you're in a 625line (usually "PAL") video country then VHS has > > 576 active (vertical) lines in 2 fields just like TV and DVDs. > > > > VHS does lack resolution though since it effectively only has 200 > > or so horizontal lines. > I still have not found any reliable resource which tells about the > limitations and real resolutions of a VCR.
That's because a real VCR is a pure analog device, so there is not really a "resolution" concept as we think of "resolution" in the digital world. > The best thing I have found by now is that the video bandwith of a > VCR is limited to 2MHz. Normal PAL has 5MHz. So calcualting back > this would mean that VCD "samples" every 0,125us one point, which > is close to the halfe amount of samples that PAL (every 0,067us) > has. Except that for a VCR, there's no sampling either. > So that would mean that VCR should be able to recognize and play > back 360 different points per video line. The number you derived is a number that was sometimes quoted for theoretical resolution on a VCR, so if it were a digital device, that would be a close approximation. However, other factors, such as head wear, circuit drift, tape quality, can all impact the amount of signal that is recorded on, and read from, the tape. > The other thing is how exact they sample every point (6 or 8 Bit). No sampling, no 6 or 8 bits. They don't work that way (well, at least all the analog one's do not). > And how they store every point. I have no information about that. > And that is very important for the quality too. It's not stored as "points", but as analog waveforms. The processing cirucity in the vcr splits the broadcast signal into color and luma signal components. The color components are low pass filtered (in the digital world this would be a resolution reduction), and then downconverted to a lower frequency carrier and recorded directly onto the tape. The luma component is low pass filtered (another resolution reduction), and the filtered result modulates an FM carrier that is chosen so that when recorded with lower vestigal sideband there's a limited amount of controlled interference between the luma component and the color subcarrier on the tape. Then the two signals are mixed together and recorded directly onto the tape. What determines the "resolution" of the vcr is the amount of FM deviation allowed for the FM carrier recorded on the tape, The more deviation allowed, the more of the original signal can be recorded onto the tape. The physics of the tape, tape oxide, recording heads, and head-to-tape velocity place an upper limit on how much FM deviation is possible, further limiting the amount of quality that can be recorded. > So still no final VCR reference, and guidlines. But I think > recording at a resolution of 720x576 (PAL) is charming for a VCD, > but not for a S-VCD. Recording at 352x576 or even 352x288 would be > something that is really worth thinking about, unless you have too > much free space on your HD's ;) In any case, the simple answer becomes that if he's coming off a consumer grade VCR, either VHS or Beta, his digital capture board will be capable of capturing far more signal quality than his VCR is capable of reproducing. Sadly, he will also capture all the noise that results from the recording process, and may even have sync issues because VCR's play fast and loose with the timing specs because TV's are very forgiving and playing fast and loose with the timing allowed for a much simpler design (read as "an affordable design"). The best bet, if he does have HD space to spare, is to capture at the higher resolution, and then use y4mscaler to shrink the resolution down if we wants to store more time in less space, because the downscale will also reduce noise. ------------------------------------------------------- This SF.Net email is sponsored by: YOU BE THE JUDGE. Be one of 170 Project Admins to receive an Apple iPod Mini FREE for your judgement on who ports your project to Linux PPC the best. Sponsored by IBM. Deadline: Sept. 24. Go here: http://sf.net/ppc_contest.php _______________________________________________ Mjpeg-users mailing list [EMAIL PROTECTED] https://lists.sourceforge.net/lists/listinfo/mjpeg-users
