Nonmoving images can be sent within the same bandwidth as voice signals. For high-resolution, moving images, the necessary bandwidth is greater.
Nonmoving images (also called still images) are commonly transmitted by facsimile, also called fax. If data is sent slowly enough, any amount of detail can be transmitted within a 3-kHz-wide band, the standard for voice communications. This is why detailed fax images can be sent over a plain old telephone service (POTS) line. In an electromechanical fax machine, a paper document or photo is wrapped around a drum. The drum is rotated at a slow, controlled rate. A spot of light scans from left to right; the drum moves the document so a single line is scanned with each pass of the light spot. This continues, line by line, until the complete frame (image) has been scanned. The reflected light is picked up by a photodetector.
Dark parts of the image reflect less light than bright parts, so the current through the photodetector varies. This current modulates a carrier in one of the modes described earlier, such as AM, FM, or SSB. Typically, black is sent as a 1.5-kHz audio sine wave, and white as a 2.3-kHz audio sine wave. Gray shades produce audio sine waves having frequencies between these extremes. At the receiver, the scanning rate and pattern can be duplicated, and a cathode-ray tube (CRT), liquid crystal display (LCD), or printer can be used to reproduce the image in grayscale (shades of gray ranging from black to white, without color).
One way to think of slow-scan television (SSTV) is to imagine “fast fax.” An SSTV signal, like a fax signal, is sent within a band of frequencies as narrow as that of a human voice. And, like fax, SSTV transmission is of still pictures, not moving ones. The big difference between SSTV and fax is that SSTV images are sent in much less time. The time required to send a complete frame (image or scene) is 8 seconds, rather than several minutes. This speed bonus comes with a tradeoff: lower resolution, meaning less image detail.
Some SSTV signals are received on CRT displays. A computer can be programmed so that its monitor will act as an SSTV receiver. Converters are also available that allow SSTV signals to be viewed on a consumer-type TV set.
An SSTV frame has 120 lines. The black and white frequencies are the same as for fax transmission; the darkest parts of the picture are sent at 1.5 kHz and the brightest at 2.3 kHz. Synchronization (sync) pulses, that keep the receiving apparatus in step with the transmitter, are sent at 1.2 kHz. A vertical sync pulse tells the receiver that it’s time to begin a new frame; it lasts for 30 milliseconds (ms). A horizontal sync pulse tells the receiver that it’s time to start a new line in a frame; its duration is 5 ms. These pulses prevent rolling (haphazard vertical image motion) or tearing (lack of horizontal synchronization).
Conventional television is also known as fast-scan TV (FSTV). The frames are transmitted at the rate of 30 per second. There are 525 lines per frame. The quick frame time, and the increased resolution, of FSTV make it necessary to use a much wider frequency band than is the case with fax or SSTV. A typical video FSTV signal takes up 6 MHz of spectrum space, or 2000 times the bandwidth of a fax or SSTV signal.
Fast-scan TV is almost always sent using conventional AM. Wideband FM can also be used. With AM, one of the sidebands can be filtered out, leaving just the carrier and the other sideband. This mode is called vestigial sideband (VSB) transmission. It cuts the bandwidth of an FSTV signal down to about 3 MHz.
Because of the large amount of spectrum space needed to send FSTV, this mode isn’t practical at frequencies below about 30 MHz. All commercial FSTV transmission is done above 50 MHz, with the great majority of channels having frequencies far higher than this. Channels 2 through 13 on your TV receiver are sometimes called the very high frequency (VHF) channels; the higher channels are called the ultrahigh frequency (UHF) channels.
Time-domain graph of a single line in an FSTV video frame.
Above figure is a time-domain graph of the waveform of a single line in an FSTV video signal. This represents 1⁄ 525 of a complete frame. The highest instantaneous signal amplitude corresponds to the blackest shade, and the lowest amplitude to the lightest shade. Thus, the FSTV signal is sent negatively. The reason that FSTV signals are sent this way is that retracing (moving from the end of one line to the beginning of the next) must be synchronized between the transmitter and receiver. This is guaranteed by a defined, strong blanking pulse. This pulse tells the receiver when to retrace; it also shuts off the beam while the receiver display is retracing. Have you noticed that weak TV signals have poor contrast? (You have, if you’re old enough to remember “rabbit ears”!) Weakened blanking pulses result in incomplete retrace blanking. But this is better than having the TV receiver completely lose track of when it should retrace.
Color FSTV works by sending three separate monochromatic signals, corresponding to the primary colors red, blue, and green. The signals are literally black-and-red, black-and-blue, and blackand-green. These are recombined at the receiver and displayed on the screen as a fine, interwoven matrix of red, blue, and green dots. When viewed from a distance, the dots are too small to be individually discernible. Various combinations of red, blue, and green intensities result in reproduction of all possible hues and saturations of color.
The term high-definition television (HDTV) refers to any of several similar methods for getting more detail into a TV picture, and for obtaining better audio quality, compared with standard FSTV. A standard FSTV picture has 525 lines per frame, but HDTV systems have between 787 and 1125 lines per frame. The image is scanned about 60 times per second. High-definition TV is often
sent in a digital mode; this offers another advantage over conventional FSTV. Digital signals propagate better, are easier to deal with when they are weak, and can be processed in ways that analog signals cannot.
Some HDTV systems use interlacing in which two rasters are meshed together. This effectively doubles the image resolution without doubling the cost of the hardware. But it can cause annoying jitter in fast-moving or fast-changing images.
Digital Satellite TV
Until the early 1990s, a satellite television installation required a dish antenna several feet in diameter. A few such systems are still in use. The antennas are expensive, they attract attention (sometimes unwanted), and they are subject to damage from ice storms, heavy snows, and high winds. Digitization has changed this situation. In any communications system, digitization allows the use of smaller receiving antennas, smaller transmitting antennas, and/or lower transmitter power levels. Engineers have managed to get the diameter of the receiving dish down to about 2 ft.
A pioneer in digital TV was RCA (Radio Corporation of America), which developed the Digital Satellite System (DSS). The analog signal is changed into digital pulses at the transmitting station via A/D conversion. The digital signal is amplified and sent up to a geostationary satellite. The satellite has a transponder that receives the signal, converts it to a different frequency, and retransmits it back toward the earth. The return signal is picked up by a portable dish. A tuner selects the channel. Digital signal processing (DSP) can be used to improve the quality of reception under marginal conditions. The digital signal is changed back into analog form, suitable for viewing on a conventional FSTV set, by means of digital-to-analog (D/A) conversion.