Understanding & Measuring Video TV-RF Signals(I)

Understanding & Measuring Video TV-RF Signals

By Glen Kropuenske, CET, Sencore Application Engineer

Editor’s Note: This is the first part of a three-part article.

TV-RF signals deliver your favorite TV shows. They are transported by TV broadcast stations, CATV (cable television systems), MATV (master antenna television) systems, and whole-house TV distribution systems. Ideally the picture on your TV should be an exact duplicate of the original scanned by the camera. However, the delivery system affects the signal level and ultimately the picture you see on the TV screen.

This article (Part I) covers the makeup of an NTSC video signal and modulated TV-RF signal. Part II will explain the various designated TV channel frequency plans and Part III covers how TV-RF signals are measured for signal strength and how signal strength relates to picture quality.

NTSC Video

The pictures and sound you experience as you watch TV are scenes and sound converted to electrical signals at the point of origin, transported to you by TV-RF signals and converted back to scenes and sound by the television receiver.

The scene starts out as a light image and is converted to an electrical signal called a composite video signal by a television camera. It is called composite video as it includes several signals assembled together. The three main components of composite video include: 1) black & white picture information or luminance, 2) color information or chroma, and 3) picture sync information. The TV sound is changed to an electrical signal by the microphone and transported to the television independent of the video.

Video Scanning

The sound to the microphone is continuously converted to electrical signals for transport. However, to transport a scene requires capturing a picture in time, dividing the picture into a matrix of many tiny picture elements. Each picture element must be sampled one at a time producing a voltage output that represents a level between black and white.

The picture elements are sampled in an orderly sequence as your would read the page of a book. The scanning process starts in the upper left, scans straight across to the right side, and quickly resets back to the left just below the initial starting point. The process repeats scanning many lines horizontally until reaching the bottom right of the picture. The process then resets moving vertically to the top right to begin scanning a new picture.

During the scanning process each picture element outputs an electrical voltage relative to the level of light ranging form black to white. The voltage output is blanked and sync pulses are added when resetting from the right to left and bottom to top.

The scanning process in the television receiver follows the same order to recreate the image on the television screen. The cathode ray tube (CRT) produces an electron beam that scans the face of the picture tube as the camera scanned the scene. The electrical voltage representing the black and white levels of the original scene is applied

to the CRT. The voltage varies its beam conduction changing the light output from the picture elements on face of the CRT. This process recreates the original scanned picture image.

 

Fig. 1 - A composite video signal includes: 1) luminance information with blanking added, 2) color information, and 3) sync pulses.

 

The picture scanning process is fast. Each horizontal scan line (including retrace) occurs in just 63.5 microseconds (15,734 Hz). One complete vertical scan field from top to bottom, including 262.5 horizontal scan lines, is completed in 1/60^th of a second. While one scan is enough to reproduce a scene, higher resolution, or picture detail, is produced by combining two scanned fields in an interlaced fashion to complete a picture or frame.

Video mV/IRE Levels

The black-to-white information of the composite video signal is retained by the relationship of the voltage levels defining the video and blanking/sync. Levels are defined relative to a standard 1VPP signal with negative sync polarity terminated with 75 ohms. The picture voltage ranges approximately 714 mV and the sync pulse range approximately -286 mV. These levels may also be defined with an IRE unit of measurement created by the Institute of Radio Engineers. Zero to one hundred IRE represents the video voltages while 0 to –40 IRE represents sync level voltages

Fig. 2 - A 1VPP negative sync composite video signal is measured in mV or IRE units.

 

Video Frequencies

The black-to-white voltages of the composite video vary in both amplitude and frequency. While audio frequencies range from 20-20,000 Hz or cycles-per-second, video ranges in frequency from DC (0Hz) to 4.2 MHz. To understand video frequencies consider a single horizontal scan line from left to right. For a dark, white or constant light level the voltage remains at a constant DC level. Now consider the video frequency of a picture with the left side black and the right half white, as illustrated in Figure 3. The video signal forms a square wave with a video frequency of approximately 19 kHz. A total of 106 black to white voltage variations across the screen relate to approximately 2 MHz while 159 interruptions relate to approximately 3 MHz. The higher the frequency the more it relates to the detail or resolution of the scene.

Fig. 3 - The changing video voltages from black-to-white create video frequencies from 0-4.2MHz

 

Chroma Encoding

Color in the composite video signal begins when the camera filters out the electrical voltages of the individual red, green and blue light of the scene. These individual voltages is what a television receiver recovers and applies to the red, green and blue electron guns of the color picture tube or CRT to recreate the color image. Since it is not practical or possible in the bandwidth provided to transmit separately three-color signals, an encoding scheme was devised. The encoding scheme also permits black and white television receivers to function normally.

The encoding process mixes the red, green and blue video signal voltages to form three signals: the luminance or Y signal, plus two color mixture or difference signals called R-Y or I, and B-Y or Q. The TV receiver uses the Y or luminance mixture to recover the black and white picture information to produce the image brightness. The color mixtures are used to recover the separate red, green and blue electrical voltage to produce proper color saturation and color tint.

Fig. 4 - To produce NSTC color requires red, green, blue matirxing and multiplexing to a 3.58MHz subcarrier.

The encoding process combines 30% red, 59% green and 11% blue to produce luminance (Y). It mixes 60% red, 28% of green’s complementary color magenta, and 32% of blue’s complementary color yellow to produce an I signal. Mixing 21% red, 52% magenta and, 31% blue forms the Q signal. The color mixtures are then converted to a single chroma signal and placed onto a 3.58 MHz sub-carrier by a multiplexing process.

The multiplexing stage converts the two color mixtures, either the I (R-Y) or Q (B-Y) to a chroma signal subcarrier at 3.579545 MHz (3.58MHz). In the multiplexing process two balanced AM modulators convert the I and Q signals to frequency sidebands of 3.58MHz. Phase shifting of 90 degrees puts the Q signals in quadrature with the I signal sidebands. The sideband frequencies are added to the luminance and output as the composite video signal.

To reproduce color requires chroma frequencies from zero to approximately 1.3 MHz for the I signal and zero to .5 MHz for the Q signal. These color signals are converted to signals ranging from approximately 2.28 MHz to 4.08 MHz in the composite video signal and co-exist with the luminance frequencies from 0-4.2MHz.

 

Chroma Frequencies - Interleaving

For the chroma and luminance frequencies to occupy the same frequencies and co-exist on the same wire is attributed to interleaving. For signals and harmonics of the signals to interleave, the color subcarrier was chosen to be an odd multiple of one-half the horizontal scan frequency. The luminance signals exist as signal energy that falls in the gaps of signal energy produced by the chroma sidebands.

 

Fig. 5 - Interleaving permits luminance and chroma signals to co-exit in the same frequency band.

 

NTSC Standards

The standards that define the NTSC color composite video signal were established over 40 years ago by a group of industry leaders who formed the National Television System Committee (NTSC). Table 1 summarizes the NTSC composite video signal standards. The frequencies have an intricate and purposeful relationship. The vertical field frequency is exactly 59.94 Hz and the horizontal line frequency is 15,734.26 Hz. These are exact sub-multiples of the chosen color subcarrier 3.5795454 MHz.

NTSC STANDARDS

Horizontal Scan Frequency15,734.26Hz

Vertical Scan Frequency59.94 Hz

Color subcarrier frequency3.579545 MHz

Lines per frame525

Lines per field262.5

Frames per second30

Fields per second60

Fields per frame2

Video bandwidth4.2 MHz

Aspect ratio4:3

Video signalAm modulation

Video modulationnegative

Audio signalFM modulation

RF channel bandwidth6 MHz

Table 1 - NTSC standards define the composite video signal specifications.

 

Video/Audio Modulation – The TV-RF Channel

TV broadcast or cable distribution requires the composite video and audio signals be contained in a 6 MHz bandwidth RF channel. Modulators are used to convert the composite video and audio signals to RF carriers. A RF video carrier is used to carry the composite video including luminance, color and sync information. A second RF carrier is used for the audio signal information. Both carriers are part of a 6 MHz RF channel.

The video carrier is amplitude modulated (AM) by the composite video signal using negative sync modulation. This means the sync tips produce maximum carrier amplitude (0% modulation) and the white peaks produce minimum carrier amplitude (87.5% modulation). The relationship of the video signal and resulting amplitude modulated RF carrier is shown in figure 6.

Fig. 6 - The composite video signal amplitude modulates the video RF carrier.

The audio carrier is frequency modulated (FM) by the composite audio signal, which may include multi-television sound signals (MTS). The RF carrier is deviated from its resting frequency by +/- 25 kHz producing sidebands ranging from approximately 0-200 kHz above and below the carrier.

The video modulation is AM vestigial sideband. Full sideband modulation produces sideband frequencies above and below the RF carrier frequency ranging from 0-4.2MHz. The upper and lower sidebands would equally contain the full 4.2MHz luminance and chroma signal information covering over 8 MHz bandwidth. Vestigial sideband operation limits the lower sideband to approximately .75MHz below the carrier frequency and permits the full 4.2MHz above the carrier frequency.

All RF-TV channels have a designated 6MHz bandwidth containing the separate video and audio RF carriers. Sidebands above and below the carrier frequencies contain the video and audio signal information. In the RF channel the video carrier is positioned 1.25MHz above the lower edge of the channel. The color subcarrier is positioned 3.58 MHz above the video carrier. The FM modulated audio carrier is positioned 4.5 MHz above the video carrier.

TV or cable channel 3 has a 6 MHz bandwidth from 60-66 MHz. The video carrier is positioned at 61.25 MHz with lower sidebands limited to –1.25MHz and upper sidebands extending +4.2 MHz above the carrier (65.45MHz). The color subcarrier is positioned +3.58MHz from the video carrier (64.83MHz). The I color sidebands extend 1.3 MHz below the color subcarrier frequency and the I and Q color sidebands extend +.5 MHz above this frequency. The audio carrier is positioned +4.5MHz above the video carrier or at 65.75MHz.

 

 

Fig. 7 - The spectrum of luminance, color, and audio signals within a RF – TV channel.