Fundamentals - The MPEG - page 19

Chapter 2: Fundamentals
2.1 What is an audio signal?
Actual sounds are converted to electrical signals for convenience of handling, recording and conveying from one
place to another. This is the job of the microphone. There are two basic types of microphone, those which measure
the variations in air pressure due to sound, and those which measure the air velocity due to sound, although there
are numerous practical types which are a combination of both.
The sound pressure or velocity varies with time and so does the output voltage of the microphone, in proportion.
The output voltage of the microphone is thus an analog of the sound pressure or velocity.
As sound causes no overall air movement, the average velocity of all sounds is zero, which corresponds to silence.
As a result the bi-directional air movement gives rise to bipolar signals from the microphone, where silence is in the
centre of the voltage range, and instantaneously negative or positive voltages are possible. Clearly the average
voltage of all audio signals is also zero, and so when level is measured, it is necessary to take the modulus of the
voltage, which is the job of the rectifier in the level meter. When this is done, the greater the amplitude of the audio
signal, the greater the modulus becomes, and so a higher level is displayed.
Whilst the nature of an audio signal is very simple, there are many applications of audio, each requiring different
bandwidth and dynamic range.
2.2 What is a video signal?
The goal of television is to allow a moving picture to be seen at a remote place. The picture is a two-dimensional
image, which changes as a function of time. This is a three-dimensional information source where the dimensions
are distance across the screen, distance down the screen and time. Whilst telescopes convey these three
dimensions directly, this cannot be done with electrical signals or radio transmissions, which are restricted to a
single parameter varying with time.
The solution in film and television is to convert the three-dimensional moving image into a series of still pictures,
taken at the frame rate, and then, in television only, the two-dimensional images are scanned as a series of lines [ 1 ]
to produce a single voltage varying with time which can be digitized, recorded or transmitted. Europe, the Middle
East and the former Soviet Union use the scanning standard of 625/50, whereas the USA and Japan use
525/59.94.
[ 1 ] Watkinson, J.R., Television Fundamentals , Oxford: Focal Press (1998)
2.3 Types of video
Figure 2.1 shows some of the basic types of analog colour video. Each of these types can, of course, exist in a
variety of line standards. Since practical colour cameras generally have three separate sensors, one for each
primary colour, an RGB component system will exist at some stage in the internal workings of the camera, even if it
does not emerge in that form. RGB consists of three parallel signals each having the same spectrum, and is used
where the highest accuracy is needed, often for production of still pictures. Examples of this are paint systems and
in computer aided design (CAD) displays. RGB is seldom used for real-time video recording.