Appendix B Video Waveforms
This appendix illustrates the NTSC video waveform and is taken from EIA/TIA 250C, an earlier version of which formed the basis for the video waveform as specified in the FCC rules for broadcast television. See Chapter 2 for a more detailed description of the NTSC video waveform. The figures as shown in this appendix are drawn to conform as closely as possible to the defined amplitude and times of the NTSC signal. Other references frequently distort the time relationships.
The basic frequency in an NTSC waveform is the color subcarrier. All other video frequencies are defined in terms of the color subcarrier and for broadcast purposes are required to be locked to the subcarrier. The color subcarrier frequency is defined in the FCC rules as being
The digits … 45 … repeat continuously. This is often called the 3.58 signal.
This frequency is sometimes referred to as 15.75 kHz, which was the old horizontal frequency prior to color transmission. At that time the horizontal frequency was exactly 15.75 kHz but was changed with the addition of color to reduce the effect of a beat between the color subcarrier and the sound carrier. The time of one horizontal line is given by the reciprocal,
Since there are 525 horizontal lines in one frame and two fields in a frame, the vertical frequency is given by
The resultant period of one field is then
From the above relationships, you can show that there are 227.5 cycles of the color subcarrier in one horizontal line, so when you trigger an oscilloscope to the horizontal sync, you will see an apparent phase reversal in the color burst from one line to the next. The color subcarrier is not really changing phase, but the extra half cycle per line results in the apparent phase reversal.
Together the four fields make up two frames, and it takes the two frames to return to the initial relation between sync and color subcarrier. Shown to the left of each line are the last active video lines at the bottom of the picture for each of four consecutive fields. The vertical interval begins with sync pulses spaced every half horizontal line rather than every one line. Each pulse is one half the normal width of 4.7 µs. These half-width, twice-occurring pulses are called equalizing pulses. All sync pulses are defined as starting at the negative-going edge of the pulse, which is the zero time reference for the blanking interval. If you start counting horizontal time intervals of 63.5556 µs from the negative-going edge of line 1, then all interval edges will fall on negative-going edges of horizontal sync pulses.
Figure B.1 shows the vertical interval of four consecutive fields. The start of odd fields is identified by the beginning of the equalizing pulses following a full width horizontal line. Even fields start with the first equalizing pulse following a half line. This is the way interlace is achieved. Notice that line 1 in even fields doesn’t start immediately after the half active video line that ends the odd field.
Vertical sync is defined by three lines of video (lines 4-6), in which the voltage waveform is at the lowest level for all except 4.7µs in the center of the line and at the end of the line. Even in these three lines, the line begins at the negative-going transition of the waveform. Following common practice, the four vertical intervals are shown with the vertical sync pulses lined up. Vertical sync is followed by three more lines of equalizing pulses. During the nine line vertical interval, the color burst is not present. It resumes on line 10, occurring immediately after the sync pulse. Lines 10-20 are normal horizontal lines with sync and burst but no video. This is the retrace interval, which allows time for the electron beam to move from the bottom of the screen to the top. Video is never transmitted during these lines, but digital data and test signals may optionally be transmitted. Lines 17 and 18 are frequently used for vertical interval test signals (VITS), which may consist of various signals used to test the quality of the transmission path. Line 19 was at one time reserved for a vertical interval reference signal (VIRS), used to overcome some problems with processing signals in a network distribution system. Today, it may be used for a ghost-canceling reference signal.
Active video may start again on line 21, but most commonly today, line 21 is used for slow speed data transmission using a standard popularly known as the closed captioning standard. This is used for transmission of captioning data for viewers who are hard of hearing and also for transmission of a number of different types of information data for viewers.
Due to the way the color subcarrier is related to sync, the phase of the subcarrier relative to the start of sync changes between odd and even fields and from one odd (or even) field to the next odd (or even) field. This is indicated by the up and down arrows under the start of the sync pulse on line 10 in each field. This relationship is important when switching between two video sources. Fields I and III, the odd fields, are identical except that the phase of the color subcarrier is opposite in the two. Similarly, the even fields are identical except for the color phase.
Figure B.2 illustrates a typical horizontal blanking interval. All times are measured with respect to the 50% (−20 IRE) point of the negative-going edge of the horizontal sync pulse. The video at the right side of the picture tube ends 1.5 µs before the start of sync. The period from the end of active video to start of sync is known as the front porch. It is during the front porch that most sync suppression scrambling systems switch to suppressing the sync pulse, creating an artifact as shown in Chapter 21.
The actual sync pulse is 4.7 µs wide measured at the 50% (—20 IRE) points. The transitions from one level to another follow a sine squared shape with a duration of about 140 ns, limited by the 4.18-MHz baseband bandwidth of the NTSC signal. The sync is followed by a short breezeway between the end of sync and beginning of color burst. The color burst consists of 9 cycles of the 3.579-MHz color subcarrier. As explained above, due to the manner in which the sync relates to the color subcarrier, the subcarrier phase shifts with respect to the sync from line to line and field to field. It is shown here in both phases, because this is typically the way you will see the burst when observing it on a waveform monitor or oscilloscope. The color channel is limited in bandwidth, which is why the envelope of the color burst increases over about one cycle at the beginning of the burst and decreases over one cycle at the end. If the burst started and ended abruptly, the bandwidth of the signal would exceed the color channel bandwidth. The color burst is needed by the TV in order to demodulate the color subcarrier properly.
Following the color burst is another short interval before active video begins, called the color back porch. (In the black-and-white days, the color burst didn’t exist, and the entire interval from sync tip end to active video begin was called the back porch. When the color burst was added, the name breezeway was used to define the interval between the sync and burst.) The color back porch is the region in which most sync suppression systems stop suppressing and expanding the sync, leaving another artifact. Normally the front and back porches are offscreen in the TV receiver, so the artifacts are not seen.
NTSC video levels are measured in IRE units, named for one of the predecessor organizations to today’s IEEE, the Institute of Radio Engineers, which sponsored much of the early standardization of the NTSC system. The scale is referenced to the blanking level, occupied by the video waveform during the front and back porches. Sync extends from 0 IRE to — 40 IRE. Active video occupies the positive portion of the IRE scale. It rides on a 7.5 IRE setup level, which in the early days was thought to improve the operation of low-cost tube sync separator circuits. Black corresponds to 7.5 IRE and white to 100 IRE, the maximum level of the video signal (sometimes the color subcarrier will extend somewhat above 100 IRE but not much above when the signal is transmitted). Video can begin at the same time setup begins, so the 7.5 IRE setup level may not be visible in the waveform.
The voltage amplitude of the composite (video and sync) signal is 1 volt peak to peak when the signal is interfaced from one piece of equipment to another. Sync is always negative-going. This means that the active video (including setup) occupies 0.714 volts and the sync tip 0.286 volts.
Traditionally video interfaces between two pieces of equipment are ac coupled. This means that the sync tip level will move up and down as the signal’s average value changes with video content. 0 volts will be at the average value. Some modern systems clamp their output video so that 0 volts occur at the blanking level. The video is said to be dc coupled. Because each piece of equipment is capable of clamping the video as it requires, equipment with ac or dc coupling can usually be intermixed without regard for the coupling.