Why Only 30 Lines?
One of the primary criticisms leveled against the Baird system (and mechanically scanned television in general) is the very low resolution it provides, and rightfully so. How can an image made up of no more than 30 vertical strips and an absolute maximum of 70 dots per line have any entertainment value? The answer is, not much outside that of a novelty. But why did Baird settle on such a restrictive format? To answer that question, it's important to know a few things about Baird's situation.
By the time Baird achieved television, he already had experience as an entrepreneur. Unfortunately for him, he suffered from a weak constitution and was frequently stricken with colds and the flu, which would often lay him up for days to weeks. This would eventually result in him being forced to sell his portion of the business he built to others or just abandoning the effort altogether. Discouragements like this would directly effect the way he would run his future enterprises. It was during one of these low points that he decided to return to television after experimenting with it briefly as a boy.
Without a reliable income to fund his research, he was required incorporate whatever he could find to make his contraptions, giving his early apparatus a distinctly "home made" look. He was also well aware of the importance of television, and knew that once his invention was announced to the world, all the well funded and professionally staffed radio corporations of the day would come in and try to sweep his invention out from under his feet. Consequently, he kept many of the key details of his work secret and rarely documented anything, lest his contemporaries obtain this information and threaten his future.
His secrecy and poor documenting would later go on to harm his legacy however. With only his earliest devices cobbled together from cardboard, wood, ceiling wax and string surviving to this day, he has unfairly been accused of being no more than a showman. Not only that, but some of his early business practices and impressive "firsts" were designed primarily to keep his shareholders satisfied by proving that he was still the dominant name in television, rather than to announce the development of marketable products.
Baird was determined to succeed with television, and in order to do so, he had to ensure he had a market presence before anyone else.
At this time in Britain, radio was still state of the art, and the fledgling BBC held the monopoly on broadcasting. Therefore the quickest way to get television to the masses was to convince the BBC to broadcast his television signal over their medium wave AM transmitter. This required the signal to fit within a communications channel originally designed to be just adequate for speech.
With these restrictions, Baird had to design his television system to be as efficient as possible with a picture containing the least amount of wasted space. Several compromises had to be struck. The maximum number of lines was limited to 30, with a frame rate of 12.5 frames per second, progressively scanned. This was the best trade-off between detail and flicker. He settled on a vertically scanned, portrait-oriented image three units wide by seven units tall, about the same shape as a modern door frame. His commercially produced disc
30-line television picture as seen on an original Televisor
A working Baird Televisor
receivers, however, had the outer three lines on each side of the picture slightly widened, and that along with the short blanking period at the start of each line (more on that later) gave an aspect ratio of almost one to two. This unusual shape would allow for a head and shoulders view of a person, showing the face, shirt and hands, and also a long shot of a single figure dancing or performing. (You can see these attributes demonstrated on the restored 30-line TV recordings by Don McLean.)
The 12.5 Hertz frame rate also happened to be a multiple of the 50 Hz frequency used for the AC power grid. This provided two benefits: firstly, synchronous motors could be used to drive the transmitting and receiving disc, locking the picture signal to the power grid, and enabling simple synchronization (in practice this method was not used in Baird's commercial receivers). Secondly, any interference patterning from mains hum would remain stationary on the screen, rather than moving slowly to the left or right.
There was another benefit of the tall picture. All of the original 30-line camera-scanners shared one issue from a production standpoint: they were all difficult if not impossible to tilt during use, making height adjustments on-the-air difficult. The reason for this was due to the use of vertical scanning. Let's take a mirror drum scanner as an example (it also happens to be the only type of scanner that was able to follow the action during the 30-line era). Television requires extremely precise timing in order to create a stable picture, and either rotating the rapidly spinning drum on its axis or moving the image mask up or down during a performance would disrupt the sensitive timing at the receiver, resulting in a wobbling of the image (called "hunting") or a complete loss of synchronization at the receiver. Therefore it made sense to adopt a picture shape that minimized the effect of height differences between performers.
There was one major drawback to this simple television system. Unlike practically all modern analog video formats, there were no distinct timing signals sent along with the picture, and the synchronization equipment needed to compare the line rate of the video signal to the line rate (and thus the speed) of the scanning disc. This was obtained by placing the aforementioned black bar at the start of each line. The synchronization gear would react to the bar in that part of the image and keep the picture in sync. Consequently, there always had to be some kind of picture on the screen. Under no circumstances was a fade to black allowed! Any scene changes were done by sliding a checkerboard pattern in front of the scanner, faded to white, or, later on at the BBC, faded to a caption scanner.
Mechanically scanned television is certainly capable of higher resolutions and frame rates than Baird's system. In fact, the eastern United States commonly used a 60 line, 20 fps standard broadcast over short waves which allowed for more detail, with 80, 96, and even 120 line systems experimentally tried. The Baird company would even go on to build a 240 line 25 fps system, although this was greatly pushing the limits of the technology.