cable tech facts issue 101

In This Issue

• Understanding CATV Basedband Video
• HDTV Field Tests Released

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Understanding CATV Baseband Video

Understanding the NTSC Composite Video Signal can be very valuable in making video baseband performance measurements. In this issue of the Cable Tech Facts, we will cover the basics of composite video signal.

The RF television signal that is carried by all CATV systems begins and ends with a composite video signal. In theory, the composite video signal that is delivered to your customer's TV should be an exact duplicate of the signal you are receiving at your headend. Any changes in the composite signal at the headend or to the RF in the distribution plant will and do affect the signal your customer receives.

What Is The Composite Video Signal?

The composite video signal contains all the information that is needed by a television receiver to produce a complete video picture. This information includes three main components: 1) black and white luminance information, 2) sync pulse, and 3) color information (hue and saturation). A complete picture consists of 525 horizontal scan lines. Each horizontal scan line contains luminance and chrominance information as well as sync pulses.

Luminance Information

As the electron beam of the television is moved across the CRT, it must be turned on and off to produce a visible picture. When the beam is on, it produces a white color. When it is in the "off" state, the screen remains black. The portion of the composite video signal that turns the electron beam "on" and "off" ( and that turns all three electron beams in a color CRT on and off equally) is the luminance information.

The luminance (Y) information is represented by the amplitude variations in the composite video signal, as illustrated in Figure 1. These amplitude variations extend over a range of 0 to 4.2 MHz. The low video frequencies form the large picture areas, while the higher frequencies represent the small picture details. The NTSC standards dictate that a 1 VPP composite video signal has a maximum white level that is 714 mV above "blanking" level. Also notice that the black level is 54 mV above the blanking level. The difference between black and blanking is called the "Black Setup". Voltage levels between 54 and 714 mV correspond to the levels of gray in the picture. The video information is a constantly changing voltage levels, unless the picture it represents is constant image, such as a test pattern.

Sync information is at a voltage level that is "blacker than black". The sync tips are 286 mV lower than the blanking level. Unlike the constantly changing video information, the sync pulse is precisely repeated, and has a constant amplitude. The full voltage range from sync tip (-286 mV) to peak white (714 mV) equals 1 VPP.

The video signal illustrated in Figure 1 is called "negative sync" because the sync pulses are lower in voltage than the white level. This is the most commonly used composite video waveshape. However, you may also see "positive sync" video signals, as shown in Figure 2.

IRE/IEEE Levels

A more convenient video measurement scale is called the IRE or IEEE scale. It is often used in place of the millivolt scale to specify video signal levels. In the IRE scale, blanking is 0 IRE units, peak white is 100 IRE units: sync tip is -40 IRE units, and black is 7.5 IRE units. Since 140 IRE units corresponds to 1 VPP, 1 IRE corresponds to 7.14 mV. (An IRE unit is named after the Institute of Radio Engineers, which later became the Institute of Electrical and Electronic Engineers. Thus, an IRE and IEEE unit are the same). The units of IRE, IEEE or mV measurements can be used to specify video levels.

Composite Video Amplitude vs. Percentage of Modulation

An RF television signal uses negative modulation. This means that the composite video signal's sync tip level produces maximum peak-to-peak modulation of the RF envelope (100% P-P), and the white level produces minimum (12.5% P-P) modulation. The relationship of the composite video signal and the RF envelope is shown in Figure 3.

Sync Pulses And Blanking Intervals

Vertical and horizontal sync pulses are needed to synchronize the scanning of the television's electron beam with the incoming composite video signal. Without this synchronization the picture would "roll" vertically and/or horizontally. The sync pulses occur during the portion of the video signal called "blanking" when no "active" or viewable picture information is present.

Horizontal Blanking Intervals

When the electron beam reaches the far right side of the CRT (plus a small amount of overscan), the horizontal blanking interval, or blanking pedestal begins. During the entire horizontal blanking interval, the luminance levels are kept at or below the black level to prevent visible retrace lines as the electron beam returns to the left side of the CRT. The horizontal blanking interval consists of three parts: 1) the front porch, 2) sync pulse, and 3) back porch, as illustrated in Figure 4.

Horizontal blanking begins when the signal level drops to the blanking level. This is the front porch. Next, the signal level drops to -40 IRE units forming the horizontal sync pulse. The television receiver recognizes the negative peak sync pulse and initiates horizontal retrace.

Following the sync pulse is another period of time when the signal level is at the blanking level. This is the "back porch". When the composite video signal contains color (and usually even if it doesn't), 8 to 10 cycles of 3.579545 MHz is present during the back porch time. This is the color burst information.

After the back porch, the signal returns to "active video." The time duration of the entire horizontal blanking interval is between 10.49 and 11.55 µS. Active video is present for approximately 53 µS, for a total horizontal line time of 63.5 µS.

To produce one complete picture, a television receiver scans the CRT twice. The complete picture is called a frame, and is made up of two fields, each containing 262 1/2 horizontal scan lines. The scan lines of each field are interlaced, meaning that the scan lines from one field fall inbetween the scan lines of the other field. This is done to reduce visible flicker. Referring to Figure 5, all of field 1 odd is scanned first. Next, field 2 even is scanned. Notice these scan lines fall inbetween the others. This completes one full picture or frame. The fields are scanned 60 times per second (actually 59.94 Hz), so approximately 30 complete pictures occur each second.

After each field of 262.5 horizontal lines are scanned, the beam must return to the top of the screen so that the next field can be scanned. This retrace time is called the vertical blanking interval or VBI. The VBI lasts for a time equal to 21 horizontal lines, or 1333.5 uS, and includes the vertical sync pulse. During the vertical blanking time, no picture information is included in the composite video signal. However, the 21 horizontal lines that occur during vertical blanking contain information such as VITS, VIRS, and closed caption information.

Vertical Blanking Interval

Figure 6 illustrates the vertical blanking interval. The VBI begins with 6 equalizing pulses. These equalizing pulses are essentially horizontal sync pulses at twice the normal rate. Their purpose is to synchronize the video information between field 1 and field 2. The first group of equalizing pulses lasts for 190.5 µS, a time equal to 3 horizontal lines.

The next portion of the composite video signal is the vertical sync pulse. The vertical sync pulse interval again lasts for a time equal to 3 horizontal lines, and contains pulses at twice the horizontal line rate. But notice that the duty cycle of the pulses (called serrations) are inverted from the equalizing pulses. The serrations time the vertical sync for interlaced scan. The change in duty cycle between the equalizing pulses and the serrations are what allows an integrator inside the television to identify the vertical sync.

Following the vertical sync pulse is a second set of six equalizing pulses. This set of equalizing pulses insure field frequency regularity. The remaining 12 lines of vertical blanking are included to insure adequate vertical retrace time. They may contain information and test signals, but no picture information. Televisions and video displays should be adjusted to position these lines off the top of the visible picture area.

Test Signals In The Vertical Blanking Interval

Most RF television transmissions include special reference signals that are located on various lines of the vertical blanking interval. Several common signals include the VITS (Vertical Interval Test Signal), VIRS (Vertical Interval Reference Signal), and CC (Closed Caption).

VITS signals are transmitted by most broadcast networks as an aid in evaluating system performance. The test signals are transmitted during active operation to insure continuous quality and accuracy. The VITS signals are typically located on line 17 and 18 of the vertical blanking interval.

The VIRS signal is used by television receivers and other devices to establish correct chroma amplitude, phase values, proper luminance, as well as black setup levels. It is typically located on line 19 of the vertical blanking interval.

The CC signal was established for the hearing impaired and is transmitted by the broadcast networks. This information is typically located on line 16 of the vertical blanking interval.

Do not confuse the vertical blanking line number with the even and odd scan line numbering or with the 525 or 262 1/2 frame and field numbering. The vertical blanking line numbers refer only to the horizontal lines within the vertical blanking interval. Thus, both the VBI for field 1 and the VBI for field 2 have line numbers 1-21. Figure 7 shows how the lines occurring during the vertical blanking interval are numbered. Notice the first line in field two starts 1/2 line later than line 1 in field one, thus, offsetting the second field downward by 1/2 line and interlacing the lines from the two fields.

As you can see, understanding the composite video signal will play an important part of making video performance tests. Each component of the signal is critical to delivering a quality signal to the customer. If you would like to learn more, or have questions, give your Area Technical Representative a call at 1-800-SENCORE (736-2673) today.

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HDTV Field Tests Released

Results of a summer long field test of vestigial sideband (VSB) modulation technology for HDTV went well enough that some technologists are convinced that the long awaited HDTV standard could emerge very soon.

The test showed that the digital HDTV transmission technology proposed by the Grand Alliance performed very well under "real-world" conditions. The field tests were designed to compare the proposed system's coverage with that of the current NTSC television systems. At 8-VSB and in the UHF band, "acceptable reception" of the HDTV signal occurred 92 percent of the time compared to 76 percent for the NTSC signal reception. In the VHF band, transmissions were received satisfactorily in twice as many locations as NTSC.

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