The control voltage UST is coupled to {HEAD ~ a half wave rectifier D}, which produces {TAIL ~ a half wave voltage signal} 
produces
The control voltage UST is coupled to { HEAD ~ a half wave rectifier D } , which produces { TAIL ~ a half wave voltage signal }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL