Suplementary Reading Text 1

Computer-to-video conversion

Although the computer screen has the standard characteristics of a TV display, images are produced in a very different way. If you want to record anything from your computer to video for playback on a TV monitor, you need a print-to-tape device.

In a TV display, a tight beam of electrons scans the screen in much the same way you read a page of text – from the upper-left corner, it moves line by line to the lower right. Usually, one pass writes the entire image once. The number of passes the beam writes per second is called the vertical refresh rate and is measured in kiloHertz. Most computer systems follow the American TV standard and use a vertical refresh rate of 60Hz whereas PAL, the European TV standard, requires 50Hz.

Another difference is with bandwidth. When PAL was defined, the bandwidth available for a TV signal was very narrow. While the TV image had to be refreshed at least 50 times a second for flicker to remain unnoticeable, there was not enough bandwidth to transmit all 625 lines of one TV image in a fiftieth of a second. The developers of PAL, therefore, employed a clever trick called interlaced video. They split each frame of the image into two fields of 312.5 lines, the odd lines into field A, the even ones into field B. The fields are transmitted at a rate of 50 per second, leaving us with an effective frame rate of 25 per second while eliminating most of the flicker.

This is fine for viewing from several yards, but should you move as close to your TV as you would to your computer screen, you would end up with a headache after half an hour. Also, if any parts of the displayed image occupy only one horizontal scan line, that scan line will flicker quite-noticeably at 25Hz.

All video equipment works with PAL-standard, 50Hz-interlaced video. Computers tend to use 60Hz (or more) and non-interlaced video look more stable. To get a signal from your computer to record on a VCR, there are two possibilities:

1 Use a display adaptor that can produce PAL-standard video. You would not be able to connect such a card to a standard computer monitor, however. A video monitor or a multi-sync monitor is needed. You wouldn't want to look at such a screen for hours on end – interlaced video is not suitable for word processing.

2 Put up with the standard display signal from your computer (probably 60Hz) and use a scan converter. It can take a video signal with one refresh or scan rate, and convert it to the other. A scan converter is actually a small digital frame-grabber with asynchronous video output.

Vocabulary List

Yard – measure, of length (1 yard =0,914m)

Post-Reading Activities

Task 1

Answer the questions to the text:

1. What are the main differences in the way images are produced on a TV screen and on a computer screen.

2. Why did the developers of the PAL system invent interlaced video? What are its advantages and disadvantages?

3. Which of the two suggested ways of getting a signal from a computer to record on a VCR do you think is preferable? Why?

Task 2

Match the sentence halves to form complete sentences.

1. If you want to play back anything from your computer on a TV monitor,	a) it will have a vertical refresh rate of 60Hz.
2. If your computer system follows the American TV standard,	b) you must use a scan converter.
3. If you use a monitor with interlaced video for word processing,	c) you need a print-to-tape device.
4. If you use a display adaptor that can produce PAL-standard video,	d) you cannot use a standard computer monitor, but must use a video or multi-sync monitor instead
5. If you want to use the standard display signal from your computer,	e) you will get a headache!

Text 2. IS IT POSSIBLE TO CREATE PERFECT