8.2.7 Switching between speech and data

26.2263GPPCellular text telephone modemGeneral descriptionRelease 17TS

The Cellular Text Telephone Modem includes an adaptive switching between CTM signal generation and the transparent transmission, if there are no text characters for transmission. This automatic switching is provided in both directions by means of the switches S1 and S2 in Figure 2.

Switch S1 is controlled by the functional blocks of the CTM transmitter with the intention to avoid forwarding the signal from the speech input of the CTM transmitter while the CTM modulator is active. The default position of switch S1 is its "closed" position so that the signal at the "speech" input of the CTM transmitter can bypass without modification as long as the CTM modulator is in idle mode. Switch S1 is set to its "open" position if the CTM modulator is active, i.e. the output signal of the modulator is not zero.

Switch S2 is controlled by the functional blocks of the CTM receiver with the intention to block any CTM signal. The default position of switch S1 is its "closed" position so that the CTM receiver’s input signal can bypass to the "speech" output. Switch S2 is set to its "open" position as soon as the CTM receiver has detected a CTM sequence. Switch S2 remains in its "open" position as long as the receiver has not detected the end of the CTM burst.