The input level to the return optical transmitter, TX2, is critical. If it is set too low, the signal may get lost in noise. If it is set too high, the laser will clip, causing a number of problems. The acceptable signal level range typically is limited because the cost of the transmitter needs to be kept low. Early modem systems required the installer to set the output level of the return transmitter, TX4.
Modern systems use long loop automatic level control.
The headend receiver, RX4, measures the signal level it gets and a signal is sent to the home equipment, telling it to raise or lower the signal level until the correct signal level is received back at RX4 in the headend. When the home equipment is first powered, it doesn't know at what level it should transmit. It starts by transmitting at the lowest level and if it doesn't get a response from the HDT, it increases its level on the next try. Sooner or later, the home equipment receives a response, and from there the HDT will communicate the level at which the home equipment is to transmit. The good and the bad This system makes it unnecessary for the installer to set the level, and it allows the level to be corrected automatically for any changes in gains in the return path.