The semiconductor memory device includes a refresh timer for determining a refresh cycle of self-refresh operation. The refresh timer includes a voltage regulator, a ring oscillator and a counter. The voltage regulator generates a bias voltage having positive temperature characteristics. The ring oscillator varies an oscillation cycle of a pulse signal according to the bias voltage. The counter counts a prescribed number of pulse signals and generates a refresh signal for executing refresh operation. The semiconductor memory device thus varies the refresh cycle according to a temperature change, and executes refresh operation with an appropriate refresh cycle.

A first location and a last location of the required part of a first piece of picture information displayed at a display part are specified by cursors. Cleared data is transferred to a picture element memory where said picture information is stored for removing unnecessary picture information. The extracted necessary picture information is recorded in a magnetic tape device. Only the necessary part of a second piece of picture information is extracted in a similar manner and is recorded in the magnetic tape device. A logic sum of a title input from the keyboard and said first piece of picture information is obtained for each scanning line to provide a first synthesized picture. A logic sum of the first synthesized picture and said second piece of picture information is obtained for each line to provide a second synthesized picture.

A raster scan display system for overlapping images. A non-interfering erasure and relocation capability is included by providing a multi-level digital signal specifying the status of each pixel of the display and the number of display images which overlap at each pixel. For each image to be displayed the associated stored digital word is incremented one count. For each image to be erased the associated stored digital word is decremented one count. An image or a portion of an image is relocated by decrementing the data word associated with the current location and incrementing the data word associated with the new location. Expanding the data storage permits the same technique to be applied to multi-color display systems.

Coder-decoder of television pictures for transmission and/or reception via digital low-speed channel. In transmission, it first carries out the bidimensional transform of digital sample blocks of the television signal and then a dichotomic quantization thus obtaining quantized coefficients with variable length as a function of the coefficient index; in reception, it first carries out a quantization for quantized coefficients re-assignment of suitable quantization levels, and then the bidimensional antitransform for obtaining again the digital television signal samples. An optimized configuration of digital sample memory in transmission and in reception is shown in FIG. 1.

A visually effective image switching apparatus includes a frame memory unit for storing image data for one picture and a decoder for expanding a compressed image data and for outputting the expanded image data. Also included is a graphic memory controller for writing the expanded image data outputted from the decoder directly in the frame memory unit. An I/O access unit is also provided for selecting whether the expanded image data is to be written or not written at the present address of the frame memory. A local bus unit is also provided for transferring the expanded image data to the graphic memory controller. The timing of the decoder, the graphic memory controller and the I/O access unit are controlled by a CPU unit.