ones in each character received. The return channel will include the RQ signal if an error is detected. ltrs is the symbol to activate the letters shift. figs is the symbol to activate figures shift. Space is equivalent to the space bar cr is carriage return lf is line feed Blank entries have undefined meaning in international communications, but may be defined within a country. The code was invented by Hendrick van Duuren during World War II, and so has been called van Duuren code. Marking Seven bit characters have the alternative to be sent erect, meaning that 0 stays as 0 and 1 stays as 1, or inverted where each 0 changes to 1 and 1 changes to 0. The standard specifies a marking pattern which says which characters are erect and which are inverted in a particular pattern, that enables the system phase, channel and subchannel to be determined. Since the system is synchronous there is no start bit, and a receiver has to examine the bit stream to tell where the start of a marked cycle is. When system phase is established the correct received bit can be assigned to the correct place in each character in each channel. The receiver can determine that system phase is established as each character will contain three one bits and four zero bits. In recommendation 342-2 here are two different marking lengths schemes. For the four character repetition cycle, Channel A is encoded ↓↑↑↑ (4333) and channel B is marked ↑↓↓↓ (3444). These are combined in a time division multiplex method with characters in the order A1 B4 A2 B1 A3 B2 A4 B3 resulting in a marking pattern ↓↓↑↑↑↓↑↓ (44333434 one bits) to match the first character from channel A at the start. For the eight character repetition cycle, Channel A is encoded ↓↑↑↑↑↑↑↑ (43333333) and channel B is marked ↑↓↓↓↓↓↓↓ (34444444). The combination marking pattern ↓↓↑↑↑↓↑↓↑↓↑↓↑↓↑↓ (4433343434343434 one bits). In practice the eight character repetition cycle has not been observed in use on the airwaves. In a four channel system, channel C is encoded the same way as channel B, and channel D the same as for channel A. Bits for channel C are interleaved with channel A and those for D interleaved with channel B. Elements of A come before C, and B come before D. In CCIR recommendation 242 the marking is much simpler with the