Standards for Analog Video -Part I: Television (Display Interfaces) Part 2

PAL Color Encoding

Due to the effects of World War II, color television development in Europe lagged somewhat behind efforts in North America. By the time the European nations were ready to determine a color broadcast standard (the mid-1960s), the RCA/NTSC encoding system had already been adopted and implemented in the US and Canada. Still, it was clear that this system could not be simply transferred in a completely compatible form; if nothing else, the differences in the standard scanning formats and rates, coupled with the differing European channelization schemes, would require that different color frequency standards be set.

The system adopted by most of Western Europe is very close in its basic concepts to the NTSC standard. It differs in three major aspects. First, no change was made to the original line and frame rates, as had been done in the US. Next, while the basic idea of carrying the color information via quadrature modulation of two additional signals onto a subcarrier was retained, the definition of those signals was simplified. Rather than using the I and Q definitions of NTSC, the new European standards used simple color-difference signals, U and V, defined as

(It was later realized that these were within the adjustment range of NTSC-standard receivers, and the FCC permitted either these or the original IQ definitions to be used. At this time, the simpler color-difference definitions have essentially displaced the original versions completely.) The final change gave the new standard its common name. In order to minimize one source of color error in the NTSC system, the new European standard reversed the phase of one of the chrominance components (the V, or R-Y, signal) every line. This results in color errors in any given line being more-or-less compensated for by an “inverse” error in the following line, such that the observed result (when the two lines adjacent lines are seen by the viewer) is greater color accuracy. Thus, the new standard was referred to as “Phase-Alternating-Line”, or PAL.

Other than these changes, the PAL system is virtually identical to NTSC, although the European 625/50 scanning formats did result in different frequency and timing definitions.

One other minor change resulted from the phase alternation described above; this results in the spectral components of the two chrominance signals being offset by half the line rate, relative to one another. With this spacing of components, setting the color subcarrier in the same manner as was done for NTSC (at an odd multiple of half the line rate) would have resulted in interference between the luminance signal and one of the chrominance signals. To avoid this, PAL systems had to select a color subcarrier frequency which placed the chroma components at one-quarter the line rate from the luminance components, with an additional offset equal to the frame rate to further minimize interference. The final color subcarrier frequency chosen was

Figure 8-10 Block diagram of PAL encoder.

Note that the PAL standards did not involve a change to either the original monochrome line or frame rates of the 625/50 format, or a change to the audio subcarrier frequency. The wider channels used in Europe had permitted a greater spacing between the video and audio carrier frequencies in the first place, so no interference concern arose here with the addition of the chrominance signals. The PAL encoding process is shown in block diagram form in Figure 8-10; note the similarities and differences between this and the NTSC encoder of Figure 8-6.

SECAM

The observant reader will have noted that the PAL system was adopted by most of Western Europe. For a number of reasons, including what must be recognized as some significant political factors, a third – and completely incompatible – system was adopted by France, what was then the Soviet Union, and the former colonies and allies of those two nations.

The SECAM (for “SEquential Colour Avec Memoire”) system also utilized two color-difference signals added to the original luminance-only information, but provided these sequentially rather than simultaneously (as had been done in the NTSC and PAL systems). While the luminance information remains continuous, as in the monochrome, the B-Y and R-Y components are transmitted on successive lines. Properly decoding the signal requires the storage of a full line of information in the receiver (and hence the “avec memoire” part of the name), and also results in a reduction of resolution (by a factor of two) of the color information in the vertical direction. This loss of resolution is visually acceptable, however, by the same reasoning which permitted the bandwidth limitations of the chrominance components of the NTSC and PAL systems.

Other incompatibilities of most SECAM systems, relative to NTSC and PAL, include the use of two separate color subcarriers and the use of frequency modulation of these carriers by the color-difference signals. Neither of these carriers is at the same frequency as used in the 625/50 PAL systems. And, as mentioned earlier, transmission of SECAM video generally employs positive modulation of the video carrier by the combined luminance/sync signal, as opposed to the negative modulation standard in PAL and NTSC countries.

Due to the complexities of dealing with SECAM encoding in the production environment, the usage of this system is today almost completely for the actual television transmission. Studio equipment in SECAM countries most often uses PAL encoding or, more recently, is operating on digital composite video.

Relative Performance of the Three Color Systems

It is very tempting to try to claim performance advantages for one of the three color encoding systems relative to the others, or for either of the basic 625/50 or 525/60 format and timing standards. At the present state of television development, the actual performance differences in terms of as-delivered image quality between any of these is very slight. The phase error problem, which led to the development of PAL from the original NTSC method, has essentially been eliminated in modern NTSC systems; the increased line count of the 625/50 format might deliver somewhat higher potential resolution, but this is often lost to receiver and/or display limitations in the final image. The 60 Hz field rate is claimed to have improved flicker performance over the 50 Hz systems, but many modern “50 Hz” receivers actually deinterlace the transmission and display at 100 Hz. Still, each system still has some very vocal proponents, and the debate continues, albeit on points of ever-decreasing significance. A much more significant change is underway now, with the transition from analog systems of any variety to full-digital broadcasting.

Table 8-2 CCIR television channelization standards.

CCIR designation	Channel width (MHz)	Video carrier^a (MHz)	Audio subcarrier offset (MHz)	Chroma subcarrier offset (MHz)	Color encoding
A	obsolete UK 405-line, 50 Hz system
B	7	1.25	5.5	4.43	PAL
D	8	1.25	6.5	4.43 (PAL)	PAL,SECAM
E,F	obsolete French 819-line, 50 Hz system
G	8	1.25	5.5	4.43	PAL
I	8	1.25^c	6.0	4.43	PAL
K,L	8	1.25^c	6.5	4.25/4.4^b	SECAM
M	6	1.25	4.5	3.58	NTSC^d
N	6	1.25	4.5	3.58^e	PAL,SECAM

^a Video carrier frequencies are given from the lower channel edge.

^b The CCIR-K, SECAM system uses two chroma subcarriers and FM modulation of the chroma information, as noted in the text.

^c The vestigial lower sideband is permitted to extend below the lower channel limit in the I and L standards.

^d A variant usually referred to as “PAL-M,” using the PAL encoding system but in the CCIR-M 6 MHz channel, using the common 3.58 MHz chroma subcarrier frequency and a 525/50 timing, is in use in Brazil.

^e The chroma subcarrier frequency of PAL-N is close to that of NTSC-M, but not identical.

Worldwide Channel Standards

As mentioned earlier, television channel utilization systems are identified using a letter-based system established by the CCIR. With the specifics of the three color encoding systems in common use now understood, Table 8-2 gives the details of the more popular CCIR channelization standards, and the countries or regions in which each is used.

Usage of these standards by country is shown in Table 8-3.

Table 8-3 Usage of channelization standards by country.

CCIR code	Country
B	Australia, Austria, Azores, Bahrain, Belgium, Cyprus, Denmark, Egypt, Finland, Germany, Greece, Hungary, Iceland, India, Indonesia, Israel, Italy, Jordan, Kenya, Luxembourg, Malaysia, Morocco, Netherlands, New Zealand, Norway, Pakistan, Portugal, Saudi Arabia, Singapore, Spain, Sweden, Switzerland, Thailand, Turkey
D	Bulgaria, Czech Republic, Hungary, People’s Republic of China, Poland, Russia, Slovakia
G	Australia, Austria, Belgium, Finland, Germany, Greece, Hungary, Israel, Italy, Luxembourg, Netherlands, New Zealand, Norway, Portugal, Romania, Spain, Sweden, Switzerland
I	Hong Kong, Ireland, South Africa, United Kingdom
K	Czech Rep., Hungary, N. Korea?, Poland, Russia
L	France
M	Canada, Japan, Mexico, Peru, Philippines, S. Korea, Taiwan, United States; Brazil (PAL-M)
N	Argentina, Jamaica, Paraguay, Uruguay

Physical Interface Standards for “Television” Video

With the timing, color-encoding, and signal-level standards reasonably well defined, at least for a given market or region, there are still several possible options for the physical connector standard to be used with these. In the case of analog television interconnects, there is also a separation of applications into the consumer market and the professional/production environment.

Component vs. composite video interfaces

One major distinguishing feature of wired video interfaces is whether they are considered as carrying component or composite video. While technically a difference in the form of electrical interface, this distinction also has a great impact on the physical connector choice. Simply put, a composite video interface is one which carries the signal in the same form as an over-the-air transmission; the color information is encoded per the appropriate standard and composited into a single electrical signal along with the luminance and sync information.

(Audio may or may not be included per the relevant broadcast standard.) Most often, a “composite video” connection refers to a baseband signal, one which has not been placed on a higher-frequency carrier through modulation. However, many consumer products, especially television receivers, lack a separate input for such signals, and must accept all signals through the RF tuner/demodulator. Therefore, it is common for consumer-class video sources such as video-cassette recorders (VCRs) or DVD players to provide both a baseband composite video output, and the same signal modulated onto an RF carrier on a locally unused broadcast channel. In the US, VHF channels 3 and 4 (60-66 MHz and 66-72 MHz, respectively) are typically provided, and can be selected by the user.

Component video interfaces place the various components of the television signal on physically separate connections and cables. The primary advantage of this, at least in the consumer environment, is to ensure that these components do not interfere with one another. (This is, of course, of benefit only if these components have not previously been composited.) Also, since the component signals do not have to be carried within a limited bandwidth channel or comply with the other requirements of the “broadcast-style” composite signal, the bandwidth of these signals can be increased. The transmission channel therefore need not be the limiting factor in the quality of the displayed image.

One of the more common consumer video interfaces, provided by many different types of equipment, simply separates the luminance (Y, with syncs) and the combined chrominance or color-difference signals (C), placing them on physically separate channels. The chrominance signals are otherwise encoded and combined per the appropriate system specifications. This connection is generally referred to as a “Y/C” interface, although it is often mistakenly referred to in the generic sense as an “S-Video” connection. As will be discussed shortly, “S-Video” properly refers only to this form of interface using a specific physical connector. While not a purely composite interface, the Y/C form of connection generally is not referred to as “component” video either, as the chrominance signals are not separated into their most basic form.

The “RCA Phono” connector

A very common connector used for consumer-market baseband, RF, and component video connections is the “RCA” or “phono” connector, shown in Figure 8-11. This is a simple, inexpensive connector system which works reasonably well with small-diameter coaxial cabling. It is also, however, in common use in other consumer applications, especially for audio connections. Physically, the plug is characterized by a rounded-tip center pin, into which the center conductor of the cable may be inserted and soldered. This is surrounded by an insulating cylinder, over which the outer contact is provided in the form of a cylindrical shell with four lengthwise slots. The plug is held onto the jack solely by friction between this shell and the outer cylindrical surface of the jack, over which it fits. While this type provides an acceptable coaxial connection, it is not truly an impedance-matched connector system, and is therefore not the best choice for RF connections; it also can suffer from loosening of the physical connection with repeated insertions. Also, due to the other common uses of this type, especially as an audio connector, one must be careful not to use cable assemblies intended for less-critical application as video interconnects. (Audio cabling which uses this connector, for example, is almost certainly not constructed from coaxial cable at all, let alone being of the proper impedance.)

Figure 8-11 “RCA” or “phono” plug and jack.