NTSC Analog Television
As with radio, the FCC regulates all television broadcast transmissions in the United States. It stipulates what standards or systems must be used and licenses stations to operate on particular frequency allocations at specific power levels. As before, some international treaties also apply.
The arrangements for NTSC transmission were defined in the original NTSC specifications in 1941 and 1953, most of which are incorporated into the FCC rules. The FCC rules also specify various parameters, such as channels to be used, and the bandwidth of the transmitted signal. These rules enable U.S. stations to coexist with each other and allow receivers to operate in an optimum fashion. NTSC television signals use a combination of AM and FM modulation, with subcarriers for certain parts of the signal. In Chapter 12, we learned about these methods for radio transmission. If you understand the concepts discussed there, understanding analog TV transmission is quite straightforward.
Carriers and Channels for Analog TV
TV channels 2 to 13 are assigned in the frequency range 54 to 216 MHz in the VHF band. TV channels 14 to 69 are assigned in the range 470 to 806 MHz in the UHF band. However, not all frequencies in these ranges are assigned to television; there are some gaps for other services (e.g., the 88 to 108MHz FM radio band) and for other purposes such as radio astronomy. In addition, changes in the FCC rules, related to spectrum recovery following the transition to DTV, mean that channels 52 to 69 will no longer be available for either analog or digital television broadcasting.
Each television channel in the United States is 6 MHz wide. The visual carrier (also known as the video or picture carrier) frequency is 1.25 MHz above the lower boundary of the channel, and the aural carrier (also known as the audio or sound carrier) center frequency is 4.5 MHz higher than the visual carrier. The color subcarrier is placed at 3.58 MHz above the visual carrier—all as shown in Figure 14.1.
Figure 14.1. NTSC RF Channel
Sidebands and Bandwidth
In order to conserve radio spectrum, a form of AM modulation, known as vestigial sideband, is used for the visual signal. This means that most of the lower sideband of the TV visual signal is cut off, as shown in the figure. As explained in Chapter 4, the normal double sideband signal would extend from the video carrier minus the modulating frequency to the video carrier plus the modulating frequency.
Video Signal
Video Carrier
The video signal is amplitude modulated onto the video carrier. The process is similar to an AM radio station—except, of course, that the TV station is transmitting video information that is used by a TV receiver to paint a picture on the screen, whereas the AM radio station is transmitting audio information. In addition, the modulated AM picture signal occupies nearly 300 times as much of the radio frequency spectrum as a modulated AM radio signal. Although the lower video sideband is curtailed, the signal carries enough information to allow the receiver to recover the video picture; the demodulation process is slightly more complex than it would be with two full sidebands available.
Color Subcarrier
In Chapter 5, we explained how the NTSC chrominance information is carried on a subcarrier within the video signal. The way this works is similar to the subcarriers for stereo and ancillary audio services described in Chapter 12, with three main differences. The first is that this color coding is usually carried out early on in an analog system chain (e.g., at the studio camera), in order to allow single-wire video distribution. Also, to allow the two color difference signals to be carried, the color subcarrier is modulated using QAM (quadrature amplitude modulation). The third difference is that the subcarrier frequency is within the frequency range of the video luminance information. Although the receiver does its best to separate out the luminance and chrominance, there can be some interference of one with the other, called cross-color. This is why pictures with fine patterns, such as a check or striped jacket, can at times produce shimmering colored interference over the pattern. In that case, the particular brightness or luminance frequencies are being interpreted by the receiver as false color information.
Just as a mono radio ignores the stereo subcarrier and uses only the main mono program information, a black-and-white television ignores the color subcarrier and uses only the luminance information to produce the monochrome picture.
The reason that chrominance and luminance share some of the same frequencies is that, when color television was first introduced, the color subcarrier had to be placed within the existing video bandwidth so that the existing channel frequency allocations could be maintained. Otherwise, all stations would have needed new carrier frequencies, and existing receivers would have been made obsolete.
The audio signal is frequency modulated onto the audio carrier at the top end of the television channel, as shown in Figure 14.1. The audio baseband signal may be a single mono audio channel or may include subcarriers for stereo and additional services, as discussed below.
Audio Subcarriers
As previously mentioned, the composite baseband signal of an analog television station’s audio channel looks very much like that of an FM station, but with the stereo pilot frequency at 15.734 kHz instead of 19 kHz. In addition to the stereo signal, two standardized subcarrier channels are defined that can be carried with the TV audio signal: the separate audio program (SAP) channel and the professional (PRO) channel. The SAP channel is a mono audio channel that television broadcasters can use to transmit any type of audio information (it is often used for a second language track). The PRO channel is a narrower channel than the SAP channel, and therefore only permits voice transmissions or data information. The PRO channel, like the SAP channel, is optional, and not used by all broadcasters, but it is often used for communications with station staff at remote locations (e.g., for news gathering). An illustration of the typical analog TV station’s aural baseband signal is provided in Figure 14.2.
Figure 14.2. TV Aural Baseband Signal
Vertical Blanking Interval Ancillary Information
Because there is no picture information on the VBI lines (see Chapter 5), some of them can be used to carry additional, nonvideo information. The FCC has specified what type of information can be transmitted on various VBI lines. A summary is as follows:
Lines 1 to 9 Vertical synchronization information only
(needed to ensure that the TV receiver knows this is the start of a new field)
Lines 10 to 18 Test, cue, and control signals telecommunications (or other applications with prior FCC approval)
(e.g., control signals could be sent from a network to local affiliates to alert the affiliates to the fact that a local commercial break is coming)
Line 19 Ghost canceling reference signal only
(planned to be used by receivers to reduce “ghosts” caused by signal reflections, but very rarely used)
Line 20 Test, cue, and control signals; telecommunications (or other applications with prior FCC approval)
(see above)
Line 21 Closed captioning and other program-related information only
(descriptive text used by hearing-impaired persons, “V-chip” content advisory ratings, and so forth)
Closed Captioning and Content Advisory Ratings
The FCC rules require broadcasters in the United States to carry closed captioning information for a certain percentage of most types of programming. This allows receivers to decode and display captions on screen, so hearing-impaired viewers can follow and understand the program dialogue and narration. Captioning text is coded into digital form at the studio and carried as a low-bit rate data signal inserted onto VBI line 21 in the video signal. This data is then modulated onto the visual carrier along with the rest of the video signal.
Line 21 can also carry other data associated with the program, in particular the content advisory information. This so-called V-chip rating can be displayed on screen and allows parents to control what types of television programs can be viewed by children. The system for closed captioning and other information carried in line 21 is specified in the Consumer Electronics Association (CEA) standard CEA-608.
Analog TV Data Broadcasting
VBI Data
It is possible to carry a limited amount of data in an analog television signal using spare capacity in the VBI, in a similar way that closed captioning is carried in line 21. About 12 kbps of data can be carried on each VBI line, and several lines can be combined to provide greater bandwidth. Several such services have been implemented at different times. In the United Kingdom and elsewhere, this includes a Teletext service that provides many pages of text and simple graphics pages, as well as program captions or subtitles. In the United States, analog datacasting has mainly been implemented in closed systems such as cable television, where the data decoder and user controls can be provided in the cable set-top box and remote control, although the Public Broadcasting Service (PBS) has also used the technique for various data services.
dNTSC In-Band Wideband Data
Recent developments have been made that allow an analog television station to transmit wideband data signals transmitted in the same channel as regular NTSC program material. Known as dNTSC (presumably for data NTSC), this allows data services to be carried that are similar to those proposed for DTV datacasting (see Chapter 15). The system, developed by Dotcast Inc., uses special techniques to reduce interference from the data signals into the television picture and sound to imperceptible levels. In this way, data services with bandwidth up to about 4.5Mbps can be carried. Special receivers are required for this system, which is incompatible with other data broadcast systems.
In 2004, the Walt Disney Company introduced its MovieBeam® subscription service that uses Dotcast technology, over PBS member station channels. MovieBeam provides downloading of feature films as data files, which are then stored on a hard drive in the receiver set-top box, and can be played back on a regular television.