Why It Works | televisionmachine

Why Low Definition Television Works

When I had first learned of mechanically scanned television, I found it very hard to believe that anything recognizable could come over using such low resolution. The lowest definition images as used in America were made up of 24 horizontally-scanned lines, with roughly 20 dots per line when using the typical aspect ratio of five across to six down. That makes an image of only 480 dots, or less than 0.0005 megapixels! (Modern HDTV has almost 2.1 megapixels, and UHDTV has over 8, all in full color.) This was drawn only 15 times per second, compared to today's 30 or 60 frames per second in the US, and 25 or 50 frames per second in Europe.

To put that into a more modern perspective, imagine watching an animated GIF image, 20x24 pixels, in black and white, in half hour blocks for entertainment. Not only that, but it would also be dim and flickering noticeably. Did I mention it was silent? No wonder this format didn't last long! Truthfully, this particular system was only used for experimental purposes, for reception tests and such, but even the best commonly-used 60-line mechanical pictures only contained about 3000 elements, or a whopping 0.003 megapixels. The 30-line Baird system could only display a maximum of 2100 elements, though in reality it was much less due to high frequency losses throughout the transmission chain. Yet people were regularly tuning in to these broadcasts for entertainment. How is this possible?

Unlike most mathematical descriptions of things regarding video, the numbers here are really not as bad as they seem. One critical domain is missing from the numbers: time. When these images begin to move, they suddenly become alive and clear(er). The key to all low definition television is movement. The reason is this: our brains are excellent at interpreting imagery of familiar objects. We are so good at this that it's easy to see a shape in the clouds or a man in the moon or a face on the surface of Mars. Its also why we can recognize humans and animals in hand-drawn cartoons in a wide variety of styles. We can even see things that aren't really there, and many optical illusions take advantage of this phenomenon.

Comparison of the sizes of modern television formats with the 30-line Baird format

This is why this system works. A transmitted picture of a wide scene such as a park or backyard becomes very difficult if not impossible to recognize, even more so if it has never been seen by the viewer before. On the other hand, a person's face can be seen clearly, because our brain recognizes it as a face.

This same principle is true for sound. Early phonographs, especially those designed for speech such as Dictaphones, produced what should be unrecognizable sounds if you measure their performance strictly analytically, taking into account surface noise and frequency response. But in practice, it's not hard to understand what is being said. Modern telephone lines are digitally filtered to constrain their frequency response to 400-3400 Hz for efficient transmission (3 kHz is not a very wide bandwidth) yet speech and even music come across clearly.

There is also something less mysterious at play. This was an age where radio was still something new and exciting. The idea of being able to hear a person talk or a musician play live from dozens to hundreds of miles away over an invisible medium was nothing short of astonishing to most people. Then, only a few short years later, suddenly adding the ability to see someone perform a song, or dance in a studio far away was magical. In other words, television was a novelty.