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What is an Audio-Visual Video?
In distant communication, the transmitted data or message are basically of three types. Those are described below.
Audio data: Transmissible dialogue or any other sound is audio data. Each information is generated as sound wave with frequency between 20 Hz to 20,000 Hz (Audio Frequency or AF). The entire range of this frequency is called audio frequency range.
With microphone or similar arrangements, these sound waves are converted into similar alternating voltages. To transmit it to receiving end, a transmitting antenna is used, which transmits the alternating voltage as electromagnetic waves. Obviously, the electromagnetic waves should be exactly similar to the original sound, which requires the frequency of original sound wave and electromagnetic wave to be identical.
The changes in the amplitude of these two waves also have to be identical. This electromagnetic wave is the audio signal. As the audio message is a sound wave, its speed in air at 20°C is approximately 330 m ᐧ s-1. But the audio signal being an electromagnetic wave, its speed in air is 3 × 108 m ᐧ s-1.
Let frequency of an audio message be 3 kHz. As the frequency of the generated audio signal has to be the same, the wavelength of the signal is
λ \(=\frac{\text { velocity }}{\text { frequency }}\) = \(\frac{3 \times 10^8 \mathrm{~m} \cdot \mathrm{s}^{-1}}{3 \times 10^3 \mathrm{~s}^{-1}}\) = 105m = 100 km
To transmit this signal directly through a transmitting antenna, an antenna of the same size as the wavelength is required, which is practically impossible. For this reason, no audio signal can be transmitted directly from transmitter end to receiver end. This constraint applies not only to audio signal, but to the following two message (data) signals also.
Audio-visual or video data: Picture, hand-written or typed matter, still or video photograph, everything in the field of transmission is called video data. In TV and similar transmissions, audio data is superimposed on video data and these are called audio-visual data. But generally, the term ‘video data’ itself covers audio and visual data combined.
As microphone is used for converting an audio data to an audio signal, a camera scanner is likewise the necessary setup used for converting video data into video signal. This camera can scan a picture by a special process and at the same time, can generate similar alternating electromagnetic video signal conforming to the brightness and colour of the picture.
Digital data: Digital data comprises a sequence of layers of low and high potentials, where the magnitudes of the potentials are not important. Rather, the duration of the alternating voltages in different layers play the decisive part [Fig]. In digital transmission, the original audio and visual data are not converted into analogue alternating voltage. With the advancement of digital electronics, today many types of data, including computer data, are generated as digital data. By applying a special process to this digital data, electromagnetic digital signals are generated.
In practice, however, no data signal is suitable for direct distant communication. There are two reasons for this.
i) From the technical point of view, it needs a huge sized transmitter and receiver antenna and that is impossible to manufacture.
ii) Voluminous transmitted data from many transmitting centres could crowd the receiving station at the same time. Their superposition may generate a fuzzy and unrecognisable signal.
If the data signal contains a single frequency, then the waveform is sinusoidal. But in reality, no waveform of a signal is purely sinusoidal. Hence whatever form it may have, it is treated as a wave generated form superposition of many sinusoidal waves.