You've heard of high definition, you know about standard definition, but what about 'near HD'?

It's a term coined by manufacturers of systems designed to increase the quality of SD images to make them look closer to hi-def. Fans of near high definition say it really can improve the viewing experience, but can you make standard-definition pictures look so good, and why are some manufacturers pushing this quality?

The answer lies in something called the content gap. HD Ready TVs have been flying out of stores and into our homes at a super-fast rate. But the amount of HD content has been struggling to catch up.

Make the most of HD Ready TVs

You can watch HD quality broadcasts from services such as Sky and Freesat and the number of programmes is growing almost on a weekly basis. But a quick glance at a TV listing guide shows that HD broadcasts are but a small fraction of the total television output.

The same goes for Blu-ray titles, which are vastly outnumbered by the number of DVDs on offer.

So, the argument goes, if you want to make the most of your HD Ready set (and don't want to buy your movie collection all over again on Blu-ray), you need near HD.

Pixels, lines and frames

To understand how near HD works, we need to delve a little into the workings of TV technology. Most of Europe uses the PAL standard, with each picture or frame composed of 625 lines (although only 575 of them actually make up the content).

Each frame is split into two halves called fields and displayed at the rate of 50 images per second (50Hz) to reduce flicker. On screen, two fields are combined or interlaced to form a complete frame. Each picture line consists of a series of picture elements or pixels, which are like the dots that make up a photograph.

In a PAL display, there are 720 pixels on each picture line, so the total number of pixels displayed is 720 x 575, or a little over 400,000 pixels. As a rule of thumb, the more lines and pixels a display has, the sharper the image.

Many HD Ready sets have panels composed of 1,920 x 1,080 pixels, which means they can display around five times more picture information than a standard-definition set. That's why HD pictures look so stunning.

So with all this in mind, what technical tricks are used for creating near HD images, and can you really make the televisual equivalent of a silk purse out of a sow's ear?

Upscaling TVs

The answer lies in the upscaling process (also known as upconverting). Let's go back to our HD Ready telly with a 1,920 x 1,080-pixel display. If you connect a DVD player or feed an analogue or Freeview signal, it will receive a standard-definition signal, with each interlaced picture composed of just 720 x 575 pixels.

If this was displayed on an HD Ready screen, there would be large areas of black, because many of the screen pixels wouldn't be used for the smaller-sized image. HD Ready sets, therefore, have a video processing chip, which is used for upscaling the image. Upscaling simply means matching the incoming picture signal with the native resolution of the display.

The video processing chip does two things. First, it deinterlaces the picture so that it can be processed, and then the upscaler gets to work, using smart algorithms and filters to add pixels and lines so that the output matches the resolution of the TV. The algorithms analyse the standard-definition image and then use a process called interpolation to generate extra lines and pixels.

So does this mean we can get high-definition images from standard-definition signals? Sadly not, because upscaling cannot add detail that wasn't there in the first place.