PAL resolution IS...VHS resolution IS...

Hello, somehow your posting reminds me of Nicky Page's theories: BTW, he changed his old "VHS is 320x240" statement to the above (read the article). The best explanation I found is here, altough I never bothered to check those infos (sadly, german only): Also, there a quite some links on the web that should technicly explain it. IIRC, through Jukka Aho's excellent links you should find such infos: . Altough, I personally, have seen with my own eyes that there is a big difference if I capture a VHS at a low resolution or full PAL. But, everyone should use what works best for him (or her). And the "more infos for filters etc." argument has been said more than once (but, some people say that everything above xxx x xxx :) is giving you only more noise...). So, we are again, where we were. ^^-+I4004+-^^, thanks for the great links, especially the third one seems just great. Cheers, Mijo.

I think that two different things are being intermixed - NTSC Broadcast Standard and consumer grade VHS recording. For NTSC Broadcast, its 485 (525 - 40 service lines) lines correspond 340 "pixels" of subjective resolution (Kell factor) because TV signal is an interlaced one, and human eye somehow can detects the drop of resolution. As for horisontal resolution, according to my book sources, the bandwidth is chosen such a way that it makes a square "pixel", which correponds to 453 x 340 effective resolution. Resolution of the consumer-grade VHS VCR (perhaps used by 99% of the people reading this forum) must be substantially lower. Just record some high-quality TV broadcast and play it back. The degradation in signal quality is noticable. How bad is the resolution of your VCR depends on particular model. But 352 x 240 effective "pixels" sound reasonable. Let me enphasize what I mean by effective "pixels". An average person looking at the signal played by consumer-grade VCR on standard TV and at 352x240 resolution picture played on progressive PC monitor will make a conclision that both pictures have the same resolution The fact that VHS signal has some particular effective resolution doesn't mean that your capturing resolution should be the same. In vertical direction the choice is clear. Either you capture at full (480 for NTSC) or half. In horizontal direction your signal is purely analog and the capturing process represents the convolution of video signal with the sampling voltage of the capture card. For example, if you capture your analog 352 x 240 signal with 352 x 240 capture card resolution, your effective horizontal resolution will be only 352/Sqrt(2)= 250. To achieve a reasonable quality you have to capture at least at 352*1.41= 496 pixels of horizontal resolution. Aldonix

Yummy... Great thread. This will make it into the FAQ. Thanks to everyone for their nice discussion. All the best, Mijo.

THE TEST! 

The experiment I described in my previous post was quite similar. But instead of recording an artificial pattern I captured a 2 min movie fragment with horizontal and vertical elongated contrast images (like contrast wall corners). It is amazing that in horizontal resolution our results are very similar - < 264 in my test and 240 in yours. As for huge descrepancy in vertical resolution, it can probably be explained by the fact that you captured a coherent test pattern (which is important in the direction parallel to scan lines) and I captured a "natural object" from a movie. Actually I was right in horizontal resolution where all that statistical stuff works and Xesdeeni was right in vertical. Aldonix

So for purposes of getting all on the same page, see if we can all agree on this: 1. NTSC VHS can reproduce approximately 350x350 lines of (analog) resolution (note that this is technically NOT "lines of resolution," which must be specified for a square display--that would be about 240x350). 2. Broadcast NTSC can reproduce approximately 420x350 lines of (analog) resolution (note that this is technially NOT "lines of resolution," which must be specified for a square display--that would be about 320x350). 3. To digitize VHS without losing effective resolution, use 500x480 (or greater horizontally). 4. To digitize broadcast NTSC without losing effective resolution, use 600x480 (or greater horizontally). Xesdeeni

VS, It looks like PAL has a different set of bandwidths for over-the-air: Broadcast PAL: 5.0MHz ~= 384x408 lor => 720x480 digital res [but the 4.43MHz color modulation throws a wrench into this] Broadcast PAL-N/M: 4.2MHz ~= 322x408 lor -> 606x480 digital res VHS PAL appears to be the same bandwidth (I'm basing this on internet research, since I don't have access to PAL equipment), so yes, 500x576 would be best. aldonix, What are you talking about "decrease of horizontal resolution?" Decrease from what? If the analog signal is being bandwidth limited, as the broadcast and VHS formats are, then there is a physical (analog) limit to the number of lines of resolution which it can contain. The pixel numbers I gave are an attempt to indicate what digital resolution capture will not lose any of the available information (assuming negligible loss through the equipment itself). I'm most confused by you saying that 720 pixels of digital resolution will lose ANY of the analog data. I'm pretty sure from all my tests and from the math based on the research I've done that 720 will pretty much cover every analog format we've talked about. I think it would take an analog format with more than 5MHz of bandwidth to contain more information than can be captured by this digital resolution. Xesdeeni

Just a quick note, if you're intersted in the ITU recommandations (without having to search dozens of webpages etc.),take a look at der_Karl's PDF "Der Karl's Capture Karten aspect ratio fuer Dummies ;-)" ( ; original home: ). The original discussion can be found here (again, german only): . Even the document is in german, on the end you can find the 'EBU Technical Recommendation R92-1999'. Cheers, Mijo.

There is a theorem (forgot the mane of the author) which says that in order to digitize an analog signal with bandwidth f, you have to use at least 2f sampling frequency. Practical implementation of that theorem is used in digitizing audio for CDs. To cover the whole 20KHz frequency range they use 44.1kHz sampling frequency. So in order to get 100% accuracy you have to use at least 700 samples per line for NTSC VHS and 840 samples per line for NTSC Broadcast (I used Xesdeeni data from September 26th post). That's a theoretical minimum. Also I think that a direct time-domain sampling is not the best algorithm to come to that minimum. So, if you analog signal bandwidth is twice less than your capture card bandwidth, you are in good shape. If not - you will lose some information ;). Aldonix