Method of Recording Facsimile Signals

Facsimile signals of graphic material such as weather maps are now being printed with a thermal printing head having a linear array of, for example, 1024 minute thermal printing elements extending across the heat sensitive paper on which it will mark a line of picture elements. Such elements are called pixels, a term also identifying the elements of the original graphic matter and the corresponding electrical elements of the facsimile signal. As the paper is fed past the head it marks the paper line by line, each line comprising a series of dots at positions along the line, or the absence thereof. A printing element which receives marking current will mark a black pixel; other elements receiving no current will make no mark and leave the paper white.

While a recording having only black and white pixels is useful for many kinds of graphic material, it is often highly desirable to print pixels of gray tones or densities between black and white. It is, however, very difficult if not imposssible to supply different values of marking current to different printing elements at the same time, and thermal heads have not been used successfully hitherto for making recordings with a scale of gray tones from white to black.

Accordingly it is the object of the present to provide a method of recording facsimile recordings with a thermal print head and having a nearly continuous tone shading from black to white.

According to the invention a method of recording facsimile signals of different shades with a printing head marking individual pixels along a line comprises sorting out the signal pixels of a line according to shade and position along each line, storing the pixels of the line of the same shade according to their positions on the line and in separate series, successively and separately reading out of storage the series of pixels of different shades, and separately marking all pixels of each series of one shade with the same density on a record, each line being recorded by a plurality of markings of different densities, so as to produce a recording with the different shades of the facsimile signal.

FIG. 1 is a flow diagram of a method of recording facsimile signals according to the invention; and

FIG. 2 is a diagram illustrating the storage of lines of pixels in the method of FIG. 1.

As shown in FIG. 1, facsimile signals are demodulated from their AM OR FM carrier (1 DEMOD) converting them into series of analog or digital signals representing successive transverse lines of the original graphic material such as a weather map line by line and pixel by pixel. Where the signal pixels represent neither black nor white they may be some gray shade or density on the scale between black and white. One conventional gray scale has 16 different density values including black, white and increasingly lighter gray densities between. With prior methods of recording the signals on a thermal printing head, the gray shades of the scale are lost and only black is marked on the recording paper.

The first step of the present recording method is to analyse each line of signals pixel by pixel and sort out all the pixels of the same gray shade into a series of pixel signals having their original position in that signal line. (FIG. 1, 2 SORT SHADE BY SHADE) This sorting is repeated as many times, e.g., 16, as there are gray shades in the demodulated signal so that a plurality of series of pixels will be sorted out, each series containing all the pixels in a signal line of the same shade, each pixel being in the same position in its series as it was in the original signal.

FIG. 2 illustrates how the pixels of an individual line are sorted out to make up 16 separate lines or series of pixels, each series containing the same number of pixel positions, e.g., 1024, as the thermal print head has marking elements. In FIG. 2 a "O" represents a white or zero value pixel, and an "X" represents a black or gray pixel value. The actual shade of black or gray is designated by the line into which the pixel is sorted. The pixels of Line 1 all represent white. In Line 2, the X pixel represents a very light shade, while the pixels in lines 2 to 15 represent increasingly darker gray shades, Line 16 representing black.

The respective series of sorted pixels are stored separately shade by shade in a conventional memory by microprocessing. That is, each series containing pixels of the same gray shade are stored discretely in the memory (3 STORE 16 SERIES BY 1024 SHADE BY SHADE).

The stored pixel series representing different shades are then successively and separately read out of storage (4 READ SERIES BY SERIES), and applied to a moving, heat sensitive record such as thermal recording paper. All the pixel series, e.g., 16, of one signal line are marked on the same line of the recording paper transversely of its movement in superposition, that is, with the series of each shade linearly interspersed with the other shade series.

During marking, however, the printing density is varied between readout of successive shade lines of pixel series so that each separate series of pixels is recorded with a different marking density. Variation of the density is effected by increasing the energy of the marking current, preferably by increasing the time (by microseconds) that the marking current is applied to the thermal print during marking of each different shade series. Variation of marking energy occurs at the rate of readout of the 16 series, not at the pixel rate. The total time for printing one 1024 pixel line of 16 series is approximately 15 miliseconds so that there is no significant delay attributable to the 16 successive markings for one elemental picture line which produce a graphic recording with almost continuous shades of gray.

It should be understood that the foregoing description is for the purpose of illustration only, and that the invention includes all modifications and equivalents falling within the accompanying claims.