HF-Fax

b
The standard use over HF is as follow


First at all you need a Single Side  Band (SSB) Receiver, tune in to a Fax
signal, in the Upper Side Band (USB),  at a frequency 1.9 KHz lower as given in theFrequencies lists.
Example  to tune in Northwood on 11.086.5 KHz set your Receiver to 11.084,6 KHz.

The most Fax transmissions are send with a LPM (RPM) of 120  and a IOC (sometimes called Module) of 576.
Only stations with Russian equipment sometimes use RPM 60 or 90 and sometimes a IOC of 288.
Photofax transmissions such as from north Korea use RPM 60 and a IOC  352 with gray tones, and satellite rebroadcast use also RPM 120 IOC 576, with gray tones (4 or more bit Depth)

For software decoding best way is to decode with Black and White (2 bit Depth).

The Start-tone is 95 % black and 5 % white with tones with a duration of 15 to 20 seconds (called phasing) (300 Hz), the stopsignal is 450 Hz, and  this is called Automatic Picture Transmission ( APT).

Now you should heard the typical "Scratch" sound from a Fax signal, with white-tones are send with  2300 Hz, and black-tones are send with 1500 Hz.

If you use Software for  decoding Fax transmissions, use B&W with 2 bit Depth for the  B&W weather charts


By Donald G. Fink
Facsimile, the transmission of photographs, drawings, maps, and written or printed words by electric signals. Light waves reflected from an image are converted into electric signals, transmitted by wire or radio to a distant receiver, and  reconstituted on paper or film into a copy of the original.

 Facsimile is used by news services to send news and photos to  newspapers and television stations, by banks, airlines, and railroads to transmit the content of documents, and by many other businesses as an aid in data handling and record keeping.

 Facsimile systems involve optical scanning, signal encoding,  modulation, signal transmission, demodulation, decoding, and copy making. 

Scanning

Scanning is done in a manner similar to that used in television. An original, a photo for example, is illuminated and systematically examined in small adjacent areas called pixels (picture elements). Light reflected from each pixel is converted into electric current by an electronic device, a photocell,  photodiode, or charge-coupled device (CCD).

A single such device may be used to cover one pixel after another  in a row, row after row from top to bottom until the entire image has been  translated into electric impulses. This is rectilinear scanning. Scanning may also be done a row at a time by a battery of devices; this is array scanning.

In multispot scanning, a vertical array of photodevices moves  across the image, examining the pixels column by column. As the array passes  down the copy, it produces a set of current pulses from each photodevice. The separate currents, however produced, are then transmitted successively over a  single circuit to the distant receiver.

To secure fine detail in the reproduced image it is necessary to use very small pixels. In one standard, Group 3 of the International Telegraph  and Telephone Consultative Committee ( CCITT ), each pixel is a rectangle 0.12  by 0.13 mm ( 1 inch=25.4 mm ). On this standard, subjec copy measuring 8 by 11 inches ( 20 x 28 cm ) is divided into 3.6 million pixels.

This compares with about 200,000 pixels for televised images. The  pixels used in high-resolution facsimile systems have dimensions one-fifth those  of the CCITT standard mentioned above, whereas in low-definition systems the  dimensions may be twice as great.

The image may be illuminated as in rectilinear scanning, or a  relatively large area of the image may be illuminated, the photodevice viewing  the image through a lens aperture that restricts its field to a single pixel at a time.

In a commonly used facsimile scanning system ( invented by Frederick Bakewell in 1848 and based on Alexander Bain's work of 1842 ) the  subject copy is wrapped around a drum. A finely focused spot of light falls on  the copy and the light reflected from that pixel is picked up by the  photodevice. The drum is rotated so that the light spot traces a line across the copy, examining each pixel in turn.

As the drum rotates, the light source is moved slowly on a carriage parallel to the drum axis, tracing out a spiral of adjacent lines until the entire area of the copy has been scanned. At least once in each rotation of  the drum a signal transmitted to the recorder keeps the scanner and the recorder in step.

In drum scanning, the copy may also be illuminated broadly and examined by a photodevice fitted with a lens aperture.

Copy cannot always be conveniently wrapped around a drum. In such  cases, flat copy may be scanned by a spot of light directed across its surface  by a moving mirror. Mirror scanning may also be used when the copy is wrapped on  a drum, or while it is being pulled from a roller. Laser light produces a very fine beam that travels across the copy, row by row, as the copy moves  vertically.

In one arrangement the mirror is rocked back and forth, moving the beam across the copy. In another, a rotating polygonal mirror is used. This mirror typically has 18 flat mirror surfaces on its periphery, each capable of scanning a row of pixels.

Very fast scanning can be achieved by rapid rotation of the mirror and corresponding vertical motion of the copy. The beam is reflected from each  pixel into a photodevice that converts successive light values into corresponding currents. Electronic scanning of flat copy may also be done by arrays of photodiodes or charge-coupled devices.

For scanning rates higher than about 6 rows per second laser beams with polygonal mirrors and arrays of photodevices are favored.

Signal Encoding

In early facsimile systems the current impulses or analog signals  resulting from scanning were sent directly over telegraph or telephone wires Today, the signal current is transformed ( encoded ) before transmission.

Two forms of facsimile signal are produced, depending on the type  of copy and the recording medium. If the copy is black and white ( as in a  newspaper page ), a two-valued coded signal suffices, that is, a signal having  one current for black and another for white.

If the copy incorporates tones between black and white, that is, gray tones, a multiple-valued coded signal is required. In this case the signal  from the scanner may be encoded, in the form of binary digits, into computer  "words." Each word represents a different value in the scale of grays from black to white.

The facsimile code most often used is the Huffman code. A  digitally encoded facsimile signal is a series of two-valued currents, one for  the binary digit 0, the other for the digit 1; it thus resembles the two-valued signal used for black-and-white copy and is transmitted in the same  fashion.

Modulation

In early work, the analog signal from the scanner was used to vary the maximum value of a carrier wave, but this method, known as amplitude modulation, produced variations in shading in the record analogous to that caused by fading in radio transmission.

In modern systems the frequency of the carrier wave is continuously varied. The frequency range used must be suitable for transmission  over telephone lines. In the CCITT Group 1 standard, the frequency variation  corresponds to the analog signal from the scanner. A somewhat different  modulation scheme is used in the CCITT Group 2 standard, in which the phase of  the carrier is reversed between the black and white levels of the encoded facsimile signal.

In the CCITT Group 3 standard, the digital version of the scanner  signal is transmitted at 2,400 bits ( binary digits, 0 and 1 ) per second by a  device known as a modem. Modems operating at 4,800, 7,200, and 9,600 bits per second are also permitted under this standard.

When digital signals are used in transmitting classified or  sensitive information, the signal may be further encoded ( scrambled) prior to  transmission and unscrambled at the receiving end.

A CCITT Group 4 standard uses digital signals and special codes to secure error-free transmission. The time required for transmission is about half  that required under Group 3.

Signal Transmission

The modulated facsimile carrier signal is usually transmitted over telephone facilities. When facsimile signals in digital form ( Groups 3 and 4 )  are used the telephone circuit must be suitable for data transmission.

Long-distance telephone circuits usually involve microwave or  satellite links. Radio and wire transmission facilities outside the telephone  systems may also be used.

An important aspect of this use of the telephone is cost.

Group 1 facsimile systems require about 6 minutes to scan and  reproduce an 8 by 11-inch ( 20 x 28-cm ) page.

Group 2 operates faster, at about 3 minutes per page, or less than 2 minutes if the signal representing white space is compressed.

Group 3 is faster still, averaging about one minute per page. (  Group 4 is fastest, as noted above. )

Demodulation and Decoding

Following transmission the encoded and modulated signal is demodulated by a frequency detector. This circuit recovers the analog signal ( in Group 1 systems ) or the encoded version of the signal ( Groups 2 and 3).

The encoded signals are decoded and the analog version of the  scanning signal recovered. The analog signal is then applied to the facsimile recorder, which marks the recording medium ( the paper or film ) in the same sequence of rows and columns used in scanning the copy. 

Recording Medium

The method of reproducing the facsimile image depends on the recording medium, which may be plain paper, treated paper, or photographic film.

Many specialized forms of paper are used. The earliest, invented by Alexander Bain in 1842, is electrolytic paper, paper wetted with a conducting salt and which changes color when current passes through it. An electrosensitive paper is a dry bond paper coated with carbon and an insulating film of zinc  oxide.

When a varying voltage ( the facsimile analog signal ) is applied  to the paper the coating is removed, revealing the black surface beneath. In  electrosensitive recording, a stylus is passed over the paper in the same pattern of scanning used at the transmitter; the voltage applied to the  recording stylus causes the pixels to be reproduced.

Another facsimile recording medium responds to heat. This thermal  paper is bond paper impregnated with a colorless dye. Heat is applied by a  thermal print head, an array of resistance elements, one for each pixel in the  row.

The analog facsimile signal is applied successively to each  element, raising its temperature to above 200C. ( 392F. ) in a few thousandths of a second. The element cools rapidly when the signal passes to an adjacent element. The pixels are thus formed on the paper for the row being scanned.

Successive rows are reproduced as the paper moves past the print head. An electrostatic recording system uses bond paper treated to retain electric charge. An array of styli ( nibs ) applies a latent image of electric charge to the paper, one row of pixels at a time. A toner transforms the charge  image into black and white dots.

Still another method of marking the pixels on plain paper uses an  array of tiny ink jets. The nozzle of each jet deposits small globules of ink  onto the paper.

Marks can also be deposited on plain paper by pressure; an array of electrically actuated styli presses against the paper through typewriter ribbon or carbon paper. These systems make black-and-white markings only, the  halftone values being simulated by varying the size of the dots, as in  halftone printing. The principles of xerographic copying are also used in facsimile recording.

Some recorders recreate the light of each pixel by applying the  analog signal to a light source, such as a crater lamp or a solid state laser.  The process can be thought of as scanning in reverse.

The recording medium is photosensitive paper or film.  High-resolution lithographic film is used, for example, in transmitting facsimile copies of newspaper or magazine pages from composing room to press room.

History

Facsimile was invented in 1842 by the Scotsman Alexander Bain. Many improvements were made to Bain's contraption in the following half century, including synchronization of transmitter and receiver ( 1869 ) and the use of a  photoelectric cell for transmitting photos ( 1902 ).

In 1924 the first wire photo was sent from Cleveland to New York.  Facsimile began to spread rapidly; by the mid-1950's the International News  Service was using the system to transmit pictures and voice signals.

Facsimile transmission and computer graphics are related fields, using methods closely similar in principle and detail. As personal computers  allow information handling and processing to be carried out in the home it is to  be expected that facsimile systems will come to play a similar role.