HDD sends control signals to the preamp and the preamp selects the head which HDD needs at the current moment.

In the transmit direction, the transceiver provides RF signals to the power amplifier, which amplifies the signals based on gain and ramp information for a given modulation type.

Furthermore, a gain value may be sent which may be used to control a variable gain amplifier (VGA) within the transceiver during 8-PSK operation.