Digital TV (DTV) System Basics: Transmitter and Receiver Explained

A Digital TV (DTV) system consists of two main components: the digital TV transmitter and the digital TV receiver. These components work together to deliver digital television signals, offering higher quality and more efficient broadcasting compared to traditional analog TV systems. Let’s explore the basics of DTV using block diagrams of each. The evolution of Digital TV aims to replace the older NTSC system and improve picture and sound quality. It’s also often referred to as HDTV (High Definition Television).

DTV employs a scanning concept to “paint” a picture on the CRT (Cathode Ray Tube). A DTV screen is made of thousands of tiny dots of light called pixels. The more pixels there are, the better the resolution and picture quality. The format of a DTV screen is defined in terms of “(Number of pixels/horizontal line) by (Number of vertical pixels)”. DTV uses progressive line scanning rather than the interlaced scanning used in the NTSC analog TV version, a major difference between the two. This format is compliant with computer screens, allowing such monitors to display DTV formats.

Let’s delve into the DTV transmitter and receiver modules.

Digital TV Transmitter

Figure 1: Digital TV Transmitter Block Diagram

Figure 1 depicts a digital TV transmitter block diagram. The video and audio signals are processed by this DTV transmitter.

• Video Signal Processing: The video signal from the camera, consisting of R (Red), G (Green), and B (Blue) signals, are converted to luminance and chrominance form. These signals are then digitized using an Analog-to-Digital (A/D) converter.
• Serialization and Data Compression: The resulting digital signal is serialized and passed to the data compressor. The purpose of data compression is to reduce the number of bits required to represent the video data, allowing for a higher transmission rate within a limited bandwidth. MPEG2 is commonly used as a data compression method.
• Data Randomization: The signal is then passed through a data randomizer. This randomizer randomizes the data signal, which aids in clock recovery at the receiver.
• RS Encoding: The randomized signal is passed to the Reed-Solomon (RS) encoder, which adds 20 parity bytes per block of data. This helps in correcting up to 10 bytes of error per block.
• Trellis Encoding: The signal is then provided to the Trellis Encoder, which modifies the data to further assist in error correction at the receiver.
• Packetization: This processed signal is passed to the packetizer, which packages the video along with the audio according to the DTV packet format (Figure 2). The audio signal is also passed through an A/D converter and processed using AC-3 data compression before being packetized.

Figure 2: DTV Packet Format

As shown in Figure 2, a DTV packet consists of 188 bytes of data, including both video and audio, as well as 4 bytes used for synchronization and header information. The header helps in identifying the packet number, its sequence, and the video format.

Packets are then assembled into frames of data, each representing one frame of video. A frame typically consists of 626 packets, which are transmitted sequentially. The final signal is then fed to the modulator. DTV commonly uses 8-VSB (8-Vestigial Sideband) modulation. Some systems even employ 16-level VSB modulation.

Digital TV Receiver

Figure 3: Digital TV Receiver Block Diagram

Figure 3 depicts the block diagram of a digital TV (DTV) receiver. The DTV receiver essentially performs the reverse operations of the DTV transmitter. Its primary function is to pick up the composite signal, demodulate it, and decode it into the original audio and video formats. The tuning and IF (Intermediate Frequency) portions are similar to a standard TV receiver.

As previously mentioned, the 8-level VSB signal is demodulated using a synchronous detector to recover the original bit streams. After equalization, the bits are processed for error correction using the Trellis and RS decoders. The signal is then demultiplexed into audio and video formats and fed to the CRT (or other display technology) for viewing.

ATSC 3.0: The Latest Digital TV Standard

Conclusion

Digital TV systems offer numerous benefits, including high picture and sound quality, interactive features, and more efficient spectrum usage compared to analog TV systems. Digital television broadcasting is now widely used across many parts of the world.