Broadcast Transmission Systems: Modulation, Links, and Transposers

Modulation Techniques in Broadcast Channels

Television transmission utilizes various combinations of modulation techniques, including:

• Amplitude Modulation (AM) combined with Vestigial Side Band (VSB) and Double Sideband Modulation (DSB)
• Quadrature Amplitude Modulation (QAM)
• Frequency Modulation (FM)
• Phase Shift Keying (PSK)
• Coded Orthogonal Frequency-Division Multiplexing (COFDM) for digital transmission

Pre-emphasis: Usage and Purpose

Where and for what purpose is pre-emphasis used in modulation?

In the path of the sound signal, and prior to modulation, a high-pass filter must be applied. This process, known as pre-emphasis, raises the level of high audio frequency components to significantly improve the signal-to-noise ratio (SNR) of the transmission.

Types of Broadcast Diffusion and Characteristics

Broadcast diffusion refers to how the program signal is distributed geographically:

• Local Diffusion: The program is received only within the coverage area of the transmitter that broadcasts the signal, usually limited to the locality or region where the station is located.
• Regional Distribution: Used when covering a wider region, typically achieved by utilizing repeaters or radio links.
• Broadcast Network: Used when the goal is to reach more than one region. This utilizes a relay system through which the program is distributed to various regional centers.

Function of the RF Transmitter Output Filter

The output filter of an RF transmitter serves to prevent mixing products between signals, and to ensure that a large number of harmonics do not pass through the transmitting antenna. This band-pass filter blocks these unwanted components, preventing them from raising their level and causing interference.

Definition of Radio Link Types

Out-of-Band Links

These links are transmitted in the area immediately above the UHF Band V, typically utilizing frequencies between 870 MHz and 1 GHz.

Microwave Links

These systems operate at much higher frequencies, starting around 2.4 GHz and increasingly utilizing ranges from 10.3 GHz to 10.7 GHz, and beyond.

Transmitter vs. Television Transposer: Technical Differences

The primary difference between a transmitter and a transposer (or relay transmitter) lies in the format of the input signal:

• A Transmitter receives separate input signals, typically composite video and audio baseband signals.
• A Transposer receives the already modulated signal via an antenna, similar to a conventional television receiver, and then re-broadcasts it.

Purpose of Associated Radiant Panels

Why are radiant panels associated with a television transmitter?

Radiant panels are associated with a television transmitter to expand the coverage angle, increase the overall gain, or maximize the effective radiated power.

Conditions for Transposer Location

For optimal performance, a transposer system must meet specific installation criteria:

1. The system must be installed in a high location to ensure line-of-sight contact with the largest possible area of land.
2. There must be no physical obstructions between the primary transmitter providing the signal and the transposer location.
3. The original signal level received by the transposer's antenna must fall within the established rules for the TV service contour.
4. The chosen site should have reliable access via road or highway, and it is highly advisable to have readily available electricity.