Biblio5

Source: Refs. 7, 8.

2. SECAM—Sequential color and memory (Europe); and 3. PAL—Phase alternation line (Europe).

The systems are similar in that they separate the luminance and chrominance information and transmit the chrominance information in the form of two color difference signals, which modulate a color subcarrier transmitted within the video band of the luminance signal. The systems vary in the processing of chrominance information.

In the NTSC system, the color difference signals I and Q amplitude-modulate subcarriers that are displaced in phase by p/ 2, giving a suppressed carrier output. A burst of the subcarrier frequency is transmitted during the horizontal back porch to synchronize the color demodulator.

In the PAL system, the phase of the subcarrier is changed from line to line, which requires the transmission of a switching signal as well as a color burst.

Note: VSB transmission is used, similar to North American practice.

In the SECAM system, the color subcarrier is frequency modulated alternately by the color difference signals. This is accomplished by an electronic line-to-line switch. The switching information is transmitted as a line-switching signal.

14.5.2 Standardized Transmission Parameters (Point-to-Point TV)

These parameters are provided in Table 14.2a for the basic electrical interface at video frequencies (baseband) and Table 14.2b for the interface at IF for radio systems. Signal- to-weighted noise ratio at output of final receiver is 53 dB.

Table 14.2a Interconnection at Video Frequencies (Baseband)

424 TELEVISION TRANSMISSION

Table 14.2b Interconnection at Intermediate Frequency (IF)

14.6 METHODS OF PROGRAM CHANNEL TRANSMISSION

A program channel carries the accompanying audio. If feeding a stereo system, there will, of course, be two audio channels. It may also imply what are called cue and coordination channels. A cue channel is used by a program director or producer to manage her/ his people at the distant end. It is just one more audio channel. A coordination channel is a service channel among TV technicians.

Composite transmission normally is used on TV broadcast and community antenna television (CATV or cable TV) systems. Video and audio carriers are “combined” before being fed to the radiating antenna for broadcast. The audio subcarrier is illustrated in Figure 14.6 in the composite mode.

For point-to-point transmission on coaxial cable, radiolink, and earth station systems, the audio program channel is generally transmitted separately from its companion video providing the following advantages:

•Allows for individual channel level control;

•Provides greater control over crosstalk;

•Increases guardband between video and audio;

•Saves separation at broadcast transmitter;

•Leaves TV studio at separate channel; and

•Permits individual program channel preemphasis.

14.7 TRANSMISSION OF VIDEO OVER LOS MICROWAVE

Video/ TV transmission over line-of-sight (LOS) microwave has two basic applications:

1. For studio-to-transmitter link, connecting a TV studio to its broadcast transmitter; and

2. To extend CATV systems to increase local programming content.

As covered earlier in the chapter, video transmission requires special consideration. The following paragraphs summarize the special considerations a planner must take

into account for video transmission over radiolinks.

Raw video baseband modulates the radiolink transmitter. The aural channel is transmitted on a subcarrier well above the video portion. The overall subcarriers are themselves frequency modulated. Recommended subcarrier frequencies may be found in CCIR Rec. 402-2 (Ref. 9) and Rep. 289-4 (Ref. 10).

14.7.1 Bandwidth of the Baseband and Baseband Response

One of the most important specifications in any radiolink system transmitting video is frequency response. A system with cascaded hops should have essentially a flat bandpass in each hop. For example, if a single hop is 3 dB down at 6 MHz in the resulting baseband, a system of five such hops would be 15 dB down. A good single hop should be ±0.25 dB or less out to 8 MHz. The most critical area in the baseband for video frequency response is in the low-frequency area of 15 kHz and below. Cascaded radiolink systems used in transmitting video must consider response down to 10 Hz.

Modern radiolink equipment used to transport video operates in the 2-GHz band and above. The 525-line video requires a baseband in excess of 4.2 MHz plus available baseband above the video for the aural channel. Desirable characteristics for 525-line video then would be a baseband at least 6 MHz wide. For 625-line TV, 8 MHz would be required, assuming that the aural channel would follow the channelization recommended by CCIR Rec. 402-2 (Ref. 9).

14.7.2 Preemphasis

Preemphasis is commonly used on wideband radio systems that employ FM modulation. After demodulation in an FM receiver, thermal noise has a ramplike characteristic with increasing noise per unit bandwidth toward band edges and decreasing noise toward band center. Preemphasis and its companion, deemphasis, makes thermal noise more uniform across the demodulated baseband. Preemphasis-deemphasis characteristics for television are described in CCIR Rec. 405-1 (Ref. 11).

14.7.3 Differential Gain

Differential gain is the difference in gain of the radio relay system as measured by a low-amplitude, high-frequency (chrominance) signal at any two levels of a low-fre- quency (luminance) signal on which it is superimposed. It is expressed in percentage of maximum gain. Differential gain shall not exceed the amounts indicated below at any value of APL (average picture level) between 10% and 90%:

Based on ANSI/ EIA/ TIA-250C (Ref. 6). Also see CCIR Rec. 567-3 (Ref. 12).

14.7.4 Differential Phase

Differential phase is the difference in phase shift through the radio relay system exhibited by a low-amplitude, high-frequency (chrominance) signal at any two levels of a low-frequency (luminance) signal on which it is superimposed. Differential phase is expressed as the maximum phase change between any two levels. Differential phase, expressed in degrees of the high-frequency sine wave, shall not exceed the amounts indicated below at any value of APL (average picture level) between 10% and 90%:

(0.15–6 MHz)

aConnecting earth station to national technical control center.

bIn cases where the receive earth station is colocated with the broadcaster’s premises, a relaxation of up to 3 dB in video signal-to-weighted-noise ratio may be permissible. In this context, the term colocated is intended to represent the situation where the noise contribution of the local connection is negligible.

cLaw of addition not specified in CCIR. Rec. 567.

dHighest frequency: 5 MHz.

Source: CCIR Rep. 965-1, Ref. 13.

As discussed in Section 9.3, a satellite transponder is nothing more than an RF repeater. This means that the TV signal degrades but little due to the transmission medium. Whereas with LOS microwave, the signal will pass through many repeaters, each degrading the signal somewhat. With the satellite there is one repeater, possibly two for worldwide coverage. However, care must be taken with the satellite delay problem if programming is interactive when two satellites in tandem relay a TV signal. Table 14.3 summarizes TV performance through satellite relay.

14.9 DIGITAL TELEVISION

14.9.1 Introduction

Up to this point in the chapter we have covered the baseband TV signal and radio relaying of that signal via LOS microwave and satellite, both analog systems using

•The input to the circuit is provided in the composite form by the signal sources (at the studio).

•The color system used by the receiving country, in the case of an international circuit, is the same as that used by the source country.

•The transmission path consists of mixed analog and digital sections.

14.9.2.2 Development of a PCM Signal from an Analog TV Signal. As we discussed in Chapter 6, there are three stages in the development of a PCM signal format from its analog counterpart. These are sampling, quantization, and coding. These same three stages are used in the development of a PCM signal from its equivalent analog video signal. There is one difference, though—no companding is used; quantization is linear.

The calculation of the sampling rate is based on the color subcarrier frequency called f sc. For NTSC television the color subcarrier is at 3.58 MHz. In some cases the sampling rate is three times this frequency (3f sc), in other cases four times the color subcarrier frequency (4f sc). For PAL television, the color subcarrier is 4.43 MHz. Based on 8-bit PCM words, the bit rates are 3 × 3.58 × 106 × 8 c 85.92 Mbps and 4 × 3.58 × 106 × 8 c 114.56 Mbps. In the case of PAL transmission system using 4f sc, the uncompressed bit rate for the video is 4 × 4.43 × 106 × 8 c 141.76 Mbps. These values are for composite coding.

In the case of component coding there are two separate digital channels. The luminance channel is 108 Mbps and the color-difference channel is 54 Mbps (for both NTSC and PAL systems).

In any case, linear quantization is employed, and an S/ D (signal-to-distortion ratio) of better than 48 dB can be achieved with 8-bit coding. The coding is pure binary or two’s complement encoding.

14.9.3 Bit Rate Reduction and Compression Techniques

In Section 14.9.2 raw video transmitted in a digital format required from 82 Mbps to 162 Mbps per video channel (no audio). Leaving aside studio-to-transmitter links (STL) and CATV supertrunks, it is incumbent on the transmission engineer to reduce these bit rates without sacrificing picture quality if there are any long-distance requirements involved.

CCIR Rep. 646-4 (Ref. 14) covers three basic bit rate reduction methods:

1. Removal of horizontal and vertical blanking intervals;

2. Reduction of sampling frequency; and

3. Reduction of the number of bits per sample.

We will only cover this last method.

14.9.3.1 Reduction of the Number of Bits per Sample. There are three methods that may be employed for bit rate reduction of digital television by reducing the number of bits per sample. These may be used singly or in combination:

•Predictive coding, sometimes called differential PCM;

•Entropy coding; and

•Transform coding.

430 TELEVISION TRANSMISSION

Differential PCM, according to CCIR, has so far emerged as the most popular method. The prediction process required can be classified into two groups. The first one is called intraframe or intrafield, and is based only on the reduction of spatial redundancy. The second group is called interframe or interfield, and is based on the reduction of temporal redundancy as well as spatial redundancy.

14.9.3.2 Specific Bit Rate Reduction Techniques. The following specific bit rate reduction techniques are based on Refs. 14 and 15.

Intraframe coding. Intraframe coding techniques provide compression by removing redundant information within each video frame. These techniques rely on the fact that images typically contain a great deal of similar information; for example, a one-color background wall may occupy a large part of each frame. By taking advantage of this redundancy, the amount of data necessary to accurately reproduce each frame may be reduced.

Interframe coding. Interframe coding is a technique that adds the dimension of time to compression by taking advantage of the similarity between adjacent frames. Only those portions of the picture that have changed since the previous picture frame are communicated. Interframe coding systems do not transmit detailed information if it has not changed from one frame to the next. The result is a significant increase in transmission efficiency.

Intraframe and interframe coding used in combination. Intraframe and interframe coding used together provide a powerful compression technique. This is achieved by applying intraframe coding techniques to the image changes that occur from frame to frame. That is, by subtracting image elements between adjacent frames, a new image remains that contains only the differences between the frames. Intraframe coding, which removes similar information within a frame, is applied to this image to provide further reduction in redundancy.

Motion compensation coding. To improve image quality at low transmission rates, a specific type of interframe coding motion compensation is commonly used. Motion compensation applies the fact that most changes between frames of a video sequence occur because objects move. By focusing on the motion that has occurred between frames, motion compensation coding significantly reduces the amount of data that must be transmitted.

Motion compensation coding compares each frame with the preceding frame to determine the motion that has occurred between the two frames. It compensates for this motion by describing the magnitude and direction of an object’s movement (e.g., a head moving right). Rather than completely regenerating any object that moves, motion compensation coding simply commands that the existing object be moved to a new location.

Once the motion compensation techniques estimate and compensate for the motion that takes place from frame-to-frame, the differences between frames are smaller. As a result, there is less image information to be transmitted. Intraframe coding techniques are applied to this remaining image information.

14.9.4 Overview of the MPEG-2 Compression Technique

This section is based on the ATSC (Advanced Television Systems Committee) version of MPEG-2, which is used primarily for terrestrial broadcasting and cable TV. The objective of the ATSC standard (Refs. 16, 17) is to specify a system for the transmission