These audio outputs are each mixed in a linear balanced mixer with a carrier. The carrier drive for one of these mixers is also shifted by 90°. The outputs of these mixers are added in a linear circuit to give the SSB signal by phase cancellation of one of the sidebands. Connecting the 90° delayed signal from either the audio or the carrier (but not both) to the other mixer will reverse the sideband, so either USB or LSB is available with a simple DPDT switch.
