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sampling resolution, or simply the resolution. A resolution of 23 = 8 (as shown in Fig. 25-8D) is good enough for voice transmission, and is the standard for commercial digital voice circuits. A resolution of 24 = 16 is adequate for high-fidelity (hi-fi) music reproduction.
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Sampling Rate The efficiency with which a signal can be digitized depends on the frequency at which sampling is done. In general, the sampling rate must be at least twice the highest data frequency. For an audio signal with components as high as 3 kHz, the minimum sampling rate for effective digitization is 6 kHz; the commercial voice standard is 8 kHz. For hi-fi digital transmission, the standard sampling rate is 44.1 kHz, a little more than twice the frequency of the highest audible sound (approximately 20 kHz).
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Nonmoving images can be sent within the same bandwidth as voice signals. For high-resolution, moving images, the necessary bandwidth is greater.
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Facsimile 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). Slow-Scan Television 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.
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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).
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Fast-Scan Television 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. Figure 25-9 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. High-Definition Television 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
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