Method and apparatus for controlling a thermal color printer

A method of controlling a thermal printer of the type having plural thermal print heads for printing indicia of different colors by applying heat for a selectable number of unit heat times to a dye medium to transfer a color dye therefrom to a record medium, the relative intensity of a color indicium printed by a head being determined by the number of unit heat times for which heat is generated by said head, said method comprising the steps of:

• storing respective image signals for said print heads in addressable locations, each image signal having a value representing the intensity of the indicium to be printed by a corresponding head;
• cyclically addressing said locations in predetermined sequence to cyclically read out said image signals;
• generating an intensity level representation and incrementing said intensity level representation by a predetermined incremental amount after each image signal read-out cycle;
• comparing the read out image signals to said intensity level representation to produce respective heat signals, each heat signal being produced for a number of unit heat times that is determined by the value of a corresponding read out image signal with respect to said intensity level representation;
• supplying said heat signals to respective print heads to cause the latter to generate heat;
• and
• varying the rate at which said addressable locations are addressed and said intensity level representation is incremented as a function of the printing characteristics of said dye medium, thereby varying, in a predetermined manner, the duration of said unit heat time to compensate for non-linearities in the relationship between the intensity of a color indicium printed from said dye medium and the heating time during which heat is applied to that dye medium.

Apparatus for controlling a printer of the type having plural energizable print heads for printing indicia of different colors by transferring a color dye from a dye medium to a record medium, each print head being supplied with an energizing signal for a time duration that determines the relative intensity of the indicium printed thereby, said apparatus comprising:

• a source of image signals having respective values representing the intensities of indicia printed by said print heads;
• reference signal generating means for generating a reference signal having a value which changes with time;
• comparing means for comparing said image signals, in a predetermined sequence, with said reference signal to produce respective energizing signals of durations that are determined by the respective values of the image signals with respect to the value of said reference signal;
• supply means for supplying said energizing signals to said print heads;
• and
• duration varying means for varying the durations of said energizing signals to compensate for different print characteristics of the dye medium from which said print heads print said indicia.

Apparatus for controlling a thermal printer of the type having plural thermal print heads and a dye medium of different color sections adjacent thereto, each print head being selectively energizable for a number of unit heat times to generate heat and thereby transfer dye from said dye medium to a record medium to print an indicium whose relative intensity is determined by the number of unit heat times that heat is generated by said print head, each color dye section exhibiting a different non-linear relationship between the relative intensity of the indicia printered therefrom and the number of unit heat times during which heat is applied thereto;

• said apparatus comprising:
• storage means for storing image signals representing the intensities of the indicia to be printed by said print heads, respectively, said storage means having addressable locations therein;
• cyclical address generating means for cyclically addressing said storage means to read out the stored image signals in a predetermined sequence and for generating a unit heat time pulse after completing a read-out cycle;
• reference counting means for counting said unit heat time pulses to produce a reference count of changing value;
• comparator means for comparing the image signals sequentially read out of said storage means to each reference count so as to produce respective energizing signals which commence when said reference count is incremented to a value at least equal to the value of the respectively read out image signals and which terminate at a predetermined time;
• supply means for supplying said respective energizing signals to print heads to energize the latter for numbers of unit heat times determined by the durations of said respective energizing signals;
• and
• varying means for varying at least the period of said unit heat time pulse as a function of said reference count and the color dye section from which indicia then are printed.