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The efficiency with which a signal can be digitized depends on the frequency at which sampling is done. In general, the sampling rate must have a frequency that is 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, or one sample every 167 microseconds ( s). Ideally, the sampling rate should be somewhat higher; the commercial voice standard is 8 kHz, or one sample every 125 s. For music and hi-fi digital transmission, the standard sampling rate is 44.1 kHz, or one sample every 22.7 s. This is based on a maximum audio frequency of 20 kHz, the approximate upper limit of the human hearing range.
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The modulation techniques used for image transmission are similar to those employed for sending voices. Nonmoving pictures can be sent within the same bandwidth as a voice. For high-resolution, fast-scan moving images, the necessary bandwidth is greater. A thorough discussion of image transmission is beyond the scope of this book. The basics of the three most common video communications modes are discussed here. For further detail, a text on video communications is recommended.
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Still images are transmitted by facsimile ( fax). If data is sent slowly enough, any amount of detail can be transmitted within a voice band. This is how telephone fax works. A high-resolution commercial fax image has upwards of 1000 lines of data. The image is scanned from left to right and from top to bottom like reading a book. The complete image takes several minutes to send. Many of the black-and-white photographs in the daily newspaper are sent via fax. Practically all weather satellite images are faxed.
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488 Data transmission A fax signal sounds somewhat like AFSK. But the modulation occurs over a continuously variable range of audio tones, rather than at only two frequencies. To send an image by fax, a 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 or photo so that a slice, or line, is scanned with each pass of the light spot. This continues, line by line, until the complete frame, or picture, has been scanned. The reflected light is picked up by a photodetector. Darker parts of the image reflect less light than whiter 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 tone, and white as 2.3 kHz. Gray shades produce intermediate tones. At the receiver, the scanning rate and pattern can be duplicated, with a cathode-ray tube or special printer used to reproduce the image in black-and-white. Cathode-ray-tube reception of fax is popular among radio amateurs. Personal computers can be programmed to act as fax receivers.
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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 frame time is 8 seconds, rather than several minutes. This speed bonus comes with a tradeoff: lower resolution, meaning less fineness of detail. The resolution of an SSTV image is a bit less than that of an ordinary television picture. All SSTV signals are received on cathode-ray-tube (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 it s time to begin a new frame; it lasts for 30 milliseconds (ms). A horizontal sync pulse tells the receiver it s time to start a new line in a frame; its duration is 5 ms. These pulses prevent rolling or tearing of the image. Ham radio operators like to send SSTV with SSB transmitters. It s also possible to transmit SSTV using AM, FM, or PM. But these modes take up more spectrum space than SSB. An SSTV signal, like a fax signal, can be sent over the telephone. This puts the video phone within reach of current technology, and the equipment isn t too expensive. Although the images don t convey movement, because the frame time is long, telephone SSTV lets you see people on the other end of the line, and also lets them see you. The bugaboo is that the other person or you might not want to be looked at. The camera can be switched off easily enough. And, as often as not, users of video phones prefer it that way.
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