Television with slow sweeping

The television with slow sweeping (or S low S edge T ele' v' ision SSTV ), in opposition to the abbreviation FSTV - television with fast sweeping - more known under the name of ATV: has mator T ele' v' ision, is mainly used by the Radioamateur S in Short waves, VHF, UHF and SHF. The basic principle of the SSTV is to make it possible the Radioamateur S to transmit still images using a Band-width reduced corresponding to that of the word. Of this emergent postulate the elementary disparities which differentiate the SSTV from ATV in the technical field and, consequently, in the field it hardware requirement to transmit in one or the other mode.

Hardware requirement

Although SSTV has existed for several decades, she always was somewhat been sulky by the Radioamateur S because of the high costs which she implies and of technical complexity making difficult a design “Makes House”. However, the availability of the microphone Informatique allowed the establishment of the individual Ordinateur S in a number unceasingly crescent of hearths and the latter replace on very good account complex and expensive equipment SSTV preceding.

The material breaks up into 2 parts: test

the Transceiver: That it is about a transceiver short waves, VHF or UHF, in mode SB or FM, that does not have any importance! SSTV using a reduced band-width of about 3 Khz, all the apparatuses usable in to transmit voice can also transmit SSTV. The use of transmitters old (for example deprived of filters) risk to pose some problems, in particular as for the quality of the images.
the interface: It is about the only element specific to the SSTV. The interface plays the part of interpreter between two units which were not conceived to be included/understood, namely the transceiver on the one hand and the computer on the other hand. This interface is declined in various models generally showing similar characteristics. Model HAMCOM is well established in HB9 and gives excellent results. The interface can take the shape of a material done everything, an element to build oneself or duet ordinateurs+cartes sound.

Technical considerations

A world of images

There roughly speaking exist two methods to transmit images by the means of the waves radios (without going into the details, one can reasonably connect the WEFAX (weather facsimile) with a form derived from the SSTV):

ATV (or FSTV), mode very similar on television traditional (animation of images color) and requiring a band-width of about 5 MHz, which confines the transmission in the band UHF
SSTV , mode allowing the transmission of fixed images color using transceivers traditional, thus requiring a reduced band-width of about 3 Khz. Obviously, the narrowness of the band-width lengthens considerably the duration of transmission (of a few seconds to several minutes for an image according to the protocol of transmission used) and a quality of irreproachable image

does not guarantee

Basic principles of the transmission of images

The mode SSTV initially seeks to break up the image selected to be able to transmit it by means of a transmission channel (radio waves) and to reconstitute it at the other end in its primitive form. Since such a channel makes it possible to transmit only one variable phenomenon in time, the space structure of the image must first of all be converted into a structure distributed in time and then reconverted. This operation is carried out by the line by line scan of the image, as if the image were cut out in a certain number of small narrow bands, then in points, whose variation of the luminosity is successively transmitted and reconstituted other side in complete lines. Not to lose the richness of the details of an image, it is necessary that the latter is broken up into a number of lines as large as possible and than each line the greatest possible number of points of image counts. But the larger this decomposition is large, will be the requirements to which will have to satisfy the transmission channel. In general, in the field of the SSTV computer-assisted, the pixel is used like unit of decomposition.

Image with the radio and vice versa

The following stage consists in coding the units of decomposition of the image so that they can be emitted by the transceiver the ones following the others. The system of coding used is of an astonishing simplicity: in the case of protocol SSTV 8 second white black/used at the time of the beginnings of the SSTV in 1958, one makes correspond to the black color a frequency of 1500 Hz, with the white color the frequency of 2300 Hz, all the levels of gray sharing the frequencies ranging between these two terminals. The system “then sweeps” the image pixel after pixel and, through the modem, sends to the transceiver the corresponding frequencies the ones after the others, from where strange sonorities of a transmission SSTV. With the reception, the transceiver séquenciellement collects the various frequencies and transmits them to the computer through the modem. Each frequency is reconverted in level of gray and is posted on the screen of the receiving station.

In addition to the Pixels, the protocol also codes significant events, namely the beginning of the transmission of the image, as well as the end of each line swept. In mode SSTV 8 second white black/, the beginning of transmission corresponds to a frequency of 1200 Hz transmitted during exactly 30 ms. With the reception of this signal (called signal of vertical synchronization), the computer of the receiving station prepares to receive the image itself. Then, at the end of each swept line, the transmitting system sends a signal of 1200 Hz during exactly 5 ms. With the reception of this signal (called signal of horizontal synchronization), the computer of the receiving station “understands” that it is time to pass to the following line. This principle avoids with the receiver receiving images completely of through.

White black/with the color

It goes without saying that current protocols (Robot, Wraase, Martin, Scottie,…) more readily the colors code than the levels of gray. Technically, the principle is hardly more complicated: the color is transmitted by three successive sweepings, the first for the red, the second for the green and the last for blue, according to the principle of composition RGB (Red Green Blue) of the colors. The Robot protocol is different somewhat from the others on this point, coding the colors according to the principles of brightness and chrominance, rather than according to system RGB.

Protocols

The exchange of information between several computers is not possible that if all the machines respect given regulations and conventions. Those cover a whole series of factors such as the code, the system of synchronization, transmission speed, the detection of error, etc These conventions or rules are called procedures of transmission, or protocols. The protocols of transmission SSTV can reasonably be grouped in five groups:

Robot: developed with the range of interfaces SSTV Robot (California)
Wraase: developed with the range of interfaces Wraase (Germany)
Martin: developed by the English Martin Emmerson G3OQD
Scottie: developed by the Scot Eddie Murphy GM3BSC
AVT: developed by Ben Blish-Williams AA7AS with the range of interfaces SSTV AVT (Montana)

The modes Wraase, Martin and Scottie present many similarities as for the Fréquences of coding and synchronization. They require different transmission speeds on the other hand. In a general way, the quality of the image is proportional to the time necessary for the transmission. The mode Scottie DX, for example, specialized in the transmissions long distances, request a very important time of transmission. In addition to a coding of the colors different, the Robot protocol uses a sequence of longer vertical synchronization, containing 7 bits of information and a bit of parity. This system allows an automatic identification of the format of the transmitted image, which, for the systems which recognize this principle of coding, avoids a manual selection of the protocol. Protocols AVT (Video Amiga Transceiver) are, for their part, radically different. They do not use a frequency of horizontal synchronization but are based on a digital system “of heading” to prevent that the image is not received with a shift. (The principle of coding of protocols AVT exceeds the framework of this document) the Radioamateur S of North America appreciate the protocol Scottie S1 enormously (80% of the images are sent in this mode). The 20% remainders being distributed between the protocols Scottie S2, Martin M1, Robot 36 and 72. The Radioamateur S of Japan prefer protocols Robot and AVT. In Europe, finally, the 95% of traffic SSTV are carried out using the protocol Martin M1.

Frequencies

FAX

144.700 M Hz
432.700 MHz
433.700 MHz
1296.700 MHz

SSTV

144.500 MHz
432.500 MHz
433.400 MHz
1296.500 MHz

SSTV/FAX

3.730 - 3.740 MHz
7.035 - 7.040 MHz
14.225 - 14.235 MHz

Area 1 (Europa and Africa)

21.335 - 21.345 MHz
28.675 - 28.685 MHz
Activity SSTV CB on 27 ' 700 MHz

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