Image transmission

The modulation techniques which are used for image transmission are similar to those which are employed for sending voices. Nonmoving pictures are sent within the same bandwidth as the voice. For high-resolution, fast-scan moving images, the required bandwidth is greater.
A thorough discussion of image transmission is beyond the scope here. The basics of the 3 most common video communications modes are discussed here. For the further detail, a text on video communications is suggested.

Facsimile
Still images are transmitted by the facsimile (fax). If the data is sent slowly sufficient, any amount of detail can be transmitted within the voice band. In this manner telephone fax works.The high resolution commercial fax image has upwards of 1000 lines of data. The image is scanned from top to bottom and from left to right like reading a book. The complete image takes some minutes to send. Many black-and-white photographs in daily newspaper are sent via fax. Practically all the weather satellite images are faxed.


A fax signal sounds like AFSK. But modulation occurs over a continuously variable range of the audio tones, rather than at only 2 frequencies.To send the image by fax, a document or photo is wrapped around the drum. The drum is rotated at the slow, controlled rate. A spot of light scans from left to right; drum moves document or photo so that a slice, or line, can be scanned with each pass of light spot. This continues, line by line, until complete frame, has been scanned. The reflected light is picked up by the photodetector. Darker parts of image reflect less light than whiter parts, so current through photodetector varies. This current modulates the carrier in one of the modes which is described earlier, such as AM, FM, or SSB. Characteristically, black is sent as a 1.5-kHz tone, and white as 2.3 kHz. Gray shades generates intermediate tones.At receiver, scanning rate and pattern are duplicated, with the cathode-ray tube or special printer used to reproduce image in black and white. Cathode-ray-tube the reception of fax is popular among the radio amateurs. Personal computers are programmed to behave as fax receivers.

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 the band of frequencies as narrow as that of human voice. And, just like fax, SSTV transmission is of still pictures, not the moving ones.The major difference between SSTV and fax is that SSTV images are sent in less time. The frame time is 8 seconds, instead of several minutes. This speed bonus comes with the tradeoff: lower resolution, means less fineness of detail. The resolution of the SSTV image is a little less than that of a normal television picture.

All the SSTV signals can be received on cathode-ray-tube (CRT) displays. A computer is programmed so that the monitor of it will act as SSTV receiver. Converters are available which allow SSTV signals to be viewed on the consumer type TV set.The SSTV frame has 120 lines. The black and white frequencies are same for fax transmission; the darkest parts of picture are sent at 1.5 kHz and brightest at 2.3 kHz.

Synchronization pulses, which keep the receiving apparatus in step with 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 around 30 milliseconds (ms). A horizontal sync pulse tells to the receiver that it is time to begin a new line in a frame; the duration of it is 5 ms.. These pulses prevent rolling or tearing of the image.Ham radio operators send SSTV with SSB transmitters. It is also possible to transmit SSTV by using AM, FM, or PM. But these modes take more spectrum space than SSB.An SSTV signal, such as a fax signal, can be sent over telephone. This puts video phone within the reach of current technology, and equipment is not too expensive. Although images do not convey movement, as the frame time is long, telephone SSTV lets you see people on other end of line, and lets them see you. The bugaboo is that other person—or you—might not want to be looked at. The camera can be switched off easily. And, as often as not, users of the video phones prefer it this way.
 
Fast-scan television
Conventional television is known as fast-scan TV (FSTV). This is TV which brings you sports events, newscasts, and all other programming with which you are familiar with.In FSTV, frames come at rate of 30 every second. The human eye/brain perceives bursts of motion down to the time resolution of around 1/20 second. Thus, in FSTV, the sequence of still images blends together to give appearance of continuous motion.
The FSTV image has 525 lines in each frame. In the recent years, technological advances have been made which promise to make high-resolution TV available widely and affordable. This mode has over525 lines per frame.

The quick frame time, and increased resolution, of FSTV make it necessary to use wider frequency band than is the case with fax or SSTV. A characteristic video FSTV signal takes up 6 MHz of spectrum space, or 2000 times bandwidth of a fax or SSTV signal.Fast scan TV is always sent by using conventional AM. Wideband FM can be used. With the AM, one of the sidebands are filtered out, leaving the carrier and the other sideband. This mode is called as vestigial sideband (VSB) transmission.  It cuts bandwidth of an FSTV signal down to around 3 MHz.

Due to the large amount of spectrum space required to send FSTV, this mode is not practical at frequencies below around 30 MHz (10 times bandwidth of a VSB signal). All the commercial FSTV transmission is done over 50 MHz, with great majority of channels having frequencies far higher than this. Channels 2 through 13 on your TV receiver are called as VHF (very-high-frequency) channels; the higher channels are called as UHF (ultra-high-frequency) channels.

An amplitude versus time graph of waveform of a TV signal is illustrated in the figure given below. This represents one line of one frame, or 1/525 of the complete picture. The highest instantaneous signal amplitude corresponds to blackest shade, and lowest amplitude to the lightest shade. Therefore, the FSTV signal is sent negatively.The reason that FSTV signals are sent upside down is that retracing should stay synchronized between transmitter and receiver. This is assured by a defined, strong blanking pulse. This pulse tells receiver when to retrace; it shuts off the beam while CRT is retracing. You have probably noticed that weak TV signals have bad contrast. Weakened blanking pulses result in the incomplete retrace blanking. The weak TV signals are received better when strongest signals correspond to black, instead of to white. This was discovered, as things so are often, by experimentation.

When you tune TV set to the vacant channel, you see snow, or white and gray, fast moving dots. If the TV signal comes on air without modulation, screen goes dark. Only when there is modulation do portions of screen get light again.Color FSTV works by sending 3 separate mono color signals, corresponding to primary colors blue, red, and green. The signals are literally black and red, black and blue, and black and green. These are recombined at receiver and displayed on screen as a fine, interwoven matrix of blue, red, and green dots. When viewed

                                                   Figure--  An FSTV signal. This shows one line of data.


from some distance, the dots are too small to be discernible individually. Various combinations of blue, red, and green intensities result in the reproduction of all the possible hues and saturations of color.