PhotonTV

Quick Links

The TV

Love it or hate it, television is really an incredible invention. Broadcasting color television over a limited bandwidth is one impressive thing. Broadcasting color television, and retaining compatibility with black and white television sets (meaning that color broadcasts could be viewed in black and white on black and white sets) was really amazing. Sure, some compromises were made to make it all work, but consider that broadcasts started back in 1939 at the New York World’s Fair, and the same fundamental system was used tonight to send American Idol to a 20 million people. OK, like we said – love it or hate it, the technology is impressive.

Impressive, but with some limitations. And as a photographer using the TV as your medium, these are the things that are helpful to understand.

Scan Lines
Pictures are drawn on the TV in horizontal lines. The top line is drawn (from left to right), then the third, then the fifth, etc. When the bottom line is drawn, the second line is drawn, then the fourth, then the sixth. Once all the lines are drawn (525 in all on an NTSC television), you’ve got a complete picture.

This whole process of drawing lines on the screen takes about 0.03 seconds for a single picture or frame. That means you see about 30 frames drawn per second, which is almost fast enough to make the image appear continuous and not appear as if it’s constantly being redrawn.

Why draw the odd and even lines separately? It would seem easier to just start at the top, draw each line in order, and finish at the bottom. It would, but this weaving or interlacing of the lines creates a steadier image with less flicker. By weaving the lines together in an even/odd manner we’re less sensitive to the changing frames. Even though you’re only seeing 30 frames per second, the interlace gives the appearance of twice this rate, or a picture changing – at least half of it – at 60 frames per second.

This method to reduce flicker works reasonably well. However, consider a very thin horizontal line being displayed on the TV. Instead of odd and even scan lines weaving together to create a visually smooth image, you’ll see flicker as the scan line redraws that single line at 30 frames per second. Basically, you don’t get the benefit of interlacing with a very thin horizontal line.
So, you might ask…”instead of this interlace thing, why not just draw the screen faster?” Bandwidth. Using the interlace method television could be transmitted on a 6 MHz channel. Drawing the screen faster would have required more bandwidth, and more bandwidth for each channel would have meant less television channels. Interesting compromise....worse looking channels but more of them.

Anyhow, dealing with interlacing isn’t that much trouble for you as a digital photographer. Products like PhotonTV provide “anti-flicker” filters which smooth the image and allow the interlacing to better perform its trick.

Non-Square Pixels
On a computer monitor, the pixels are square. The tiny dots that make up the picture have equal widths and heights – tiny little squares. That’s not the case on a television, though. Without getting into the math, an NTSC television warps or stretches the pixels by about 10% (a PAL television squeezes it by about 10%). This means that what looks like a square on your monitor is going to be stretched to look like a rectangle on a television.

Fortunately, this isn’t a difficult thing to correct. Products like PhotonTV display your photos as square pixels when creating the VCD, but before burning the VCD disc they apply the appropriate stretch factor to the images. So what looked like a square on your PC monitor still looks like a square on your TV – the software stretches it to compensate for the ‘non-square’ pixels on the TV.

Here’s the question though – if PhotonTV corrects for non-square pixels, what happens if you play the disc on your PC? Things should look wrong, because the image is corrected for a television set, but then you fooled it by playing it on your PC. Don’t worry. PhotonTV also stores a little information in the MPEG file to tell players that “this movie has has been non-square pixel scaled”. PC-based DVD players check that information and know to rescale for the PC. That way, the same disc can look “square” when played on either televisions or PCs.

Overscan
Overscan refers to a resizing of the image so that it fills the entire screen. In most televisions, the image the television receives is actually overscanned so that the edges are not visible – they’re resized past the edges of the television.

Why? It’s a way to hide the edge distortion or warping at the very corners of the television. Tough to fix, but easy to hide. The TV just enlarges the display so you can’t see the edges. Of course, it’s hiding the problem by cutting off the edges of your image, though.

Fixing this is also relatively easy – PhotonTV provides a means to reduce the size of the image so that when it is overscanned or enlarged on the TV the entire image is still visible. In general, a 5% correction is enough to ensure your entire image can be seen on most TVs, and on newer televisions you might not need any overscan correction at all. However, in addition to the 5% correction to see your entire image, an additional 5% may be needed to make sure that text isn’t warped at the edges. The general rule of thumb therefore is that “video safe” (being able to see the whole image) requires a 5% correction, but allow another 5% for the “title safe” area so that text isn’t strangely warped. Or put simply – don’t put text right up to the edge of the image.

Color Safe
The origin of the color safe issue lies with the addition of color TV. Color TV was introduced in the mid 40s, with the amazing ability to preserve backwards compatibility with black and white television. That is, if you had a black and white television, you could still view color broadcasts (although they were obviously seen in black and white on your black and white set).

To achieve this compatibility, color needed to be laid on top of the existing signal, and there wasn’t much bandwidth or space available to do that. In general, the color signal could only go 20% above white and 20% below black. Enough to transmit and display color, but not highly saturated colors. Or put simply, not enough to broadcast the full range of color.

The obvious thought is…”well, I’m not broadcasting my pictures, though. I’m just playing them off a DVD player”. True, but the broadcast circuit (decoder) on your TV that you’re plugging the DVD player into doesn’t know that, and it’s unlikely that it will display highly saturated colors very well.
You can try this quick test – make a VCD with a bright yellow rectangle next to a pure red rectangle. Play it on your TV, and chances are the edge between the red and yellow rectangles will pulse or shimmer when viewed on your TV. This is a form of “chroma crawl”, where the colors are so hot that a form of distortion is seen and lines crawl around the boundary of the colors.

Correcting for this is relatively easy. Products like PhotonTV have intelligent color correction, which will bring the saturation of individual colors into safe limits. Your reds might not appear as red anymore, but they probably weren’t going to appear that vivid on the television anyhow. Correct it yourself before the TV decides to try to do it for you.

Another option is to bypass the RF/composite connection to your TV and connect your DVD player through an S-Video or component (YUV or RGB) connection. These connections support broader ranges of frequencies, which prevents chroma crawl and color distortion. If you’ve got this type of connector on your TV you probably don’t need to color safe filter your images before displaying them.

NTSC and PAL
We’ve referred to NTSC a few times in this guide so far. NTSC, or “National Television System Committee” format is just one of the television formats worldwide. If you live in the US, you watch NTSC television. If you live in Europe, though, you watch PAL or “Phase Alternating Line” television. Similar in concept, design and quality but different enough that a VCD made for an NTSC television won’t play on a PAL television, and vice versa.

Just a quick glance at the differences in image display on the two systems:

1) PAL has more lines of resolution per frame – 625 lines compared to 525 for NTSC. But NTSC displays more frames per second – 29.97 compared to PAL’s 25 frames per second. So overall, the quality related to lines per second are similar in both formats.

2) NTSC sometimes suffers from color distortions, where PAL is less prone to color distortion at the expense of displaying weaker color.

Most VCD software can encode discs for either format, so as a digital photographer you typically don’t need to know much more than which television format is used in your region. To that end, major countries that use NTSC discs are the US, Canada, Mexico, most of Central America, Japan, Taiwan and South Korea. PAL is used in most other regions, including Europe, South America, Australia, China, and Hong Kong.

A third format – SECAM – originated in France and is used by a few countries worldwide. For the purposes of this guide, it’s essentially the same as PAL in terms of compatibility.

A complete list of which countries support which formats can be found here
http://www.acvl.org/manual/chap10.htm

Next: Applying it to VCD Menus