Itt Industries, Inc.

Ulrich Langenkamp

Reiner Backes

Thomas Fischer

Liubomir Micic

A data slicer circuit for separating and recovering digital teletext signals from a demodulated composite color signal derives the slicing level by using a framing code detector and a gating pulse having an active phase during horizontal and color sync pulses. By an adder or a multiplier, a start value is formed at the beginning of each picture line. A square wave reference signal is formed which is subtracted from the start value containing composite color signal. From the difference signal, a digital automatic control system forms the slicing level.

Data slicer circuit for separating and recovering digital teletext

With this correcting method, errors in bytes are corrected successively. To this end, the signal resulting from the parity check is stored into a page memory, along with the character bits of each byte. The parity check signal replaces the parity bit of each byte. Two successively received identical pages are stored together with their parity check bits into first and second segments of the page memory. The two pages are compared for equality of the character bits and parity check bits. The result of the comparison is evaluated as follows. In case of equality and correct parity, the byte stored in the first segment will remain unchanged; in case of inequality of the two bytes and correct parity of one of the two bytes, the byte having correct parity will be stored into or remain stored in the first segment; in case of inequality and incorrect parity of the two bytes, no change will be made in the first segment; in case inequality and correct parity of the two bytes, the byte in the first segment will be replaced by a blank byte. The next received identical page but one is stored into the second segment by overwriting, and the correction is repeated with this byte. By this method, single errors are detected and corrected and double errors are detected and replaced by a blank character. In a preferred embodiment with a third segment of the page memory, storage of the third received identical page, and corresponding comparisons of the bytes in the three segments, the method is extended so that double errors are corrected as well.

Method of correcting errors in bytes of teletext signals

A delay circuit provides adjustable delay in constant increments. In order to achieve adjustable but constant delay times of a chain of inverter pairs, each pair is completed by a capacitor, a third inverter, and a transfer transistor the gate of which is fed by a voltage controlling the pair delay time. This voltage is generated by a control circuit measuring the actual delay time of the chain with respect to the period of a constant clock signal.

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Freiburg, Germany

Ulrich Langenkamp