Technology of Q 2500 colour TV set

Technology of Q 2500 colour TV setThe operating commands for videotext arespecified as a sub-program in "Level 0".The transmitter IC produces a code word foreach command and adds two start bits and acontrol bit. The two start bits are always H,and the control bit changes each time the buttonis pressed. It indicates to the central controlunit, whether a new command is present,or the current command is still valid.This 14 bit codeword is modulated with a 36kHz carrier and applied serially to pin 21 of 84C 122.The digital Information is expressed by thestep direction within the bit (Biphase).If the button is pressed for a long time on thereport control the codeword is repeated after114 ms.This signal is supplied to T 121 via resistorR 124, amplified and radiated via infrared diodeD 121. The transmitter diode is now nolonger covered by a dispersion window. Thismeans the current can be reduced to f 900mAss. The current can be taken directly fromthe battery, which means that no additionalElkos are necessary on the operating voltage.14 BIT - CodewordStart bitsAdress bitsCommand bitsExample:Codeword for command searchCarrier = 36kHz1,78 1,78 1,78 1,78 1,78 1,78 1,78 1,78 1,7814 BIT = 24,92 mst (ms)HH H L L L L L L H H H L HOn pins 8 and 9 of 84 C 122, the ceramic oscillatorX 111 is the component that determinesthe frequency of the pulse oscillator forthe synchronisation of the system and for theproduction of the carrier frequency (fTr = 1/12fCycle). Using the ceramic oscillator X 111means that tuning of the cycle frequency isnot needed.The circuit is functional at an operating voltageof 2.5 – 3.5 V and draws a normally energisedcircuit current of = 1 uA and an operatingcurrent of 35 mA. The transmitter therebyachieves a range of >10 m.The voltage source is two micro-alkalimanganesebatteries.Document Q 2500 64 © Loewe ProCollege