a digital effects loop is inserted in the video and key signal buses which extend to a mixer of a switcher apparatus , and a digital effects unit is coupled to the loop . circuitry is included in the loop to interrupt the particular bus or buses and the signal or signals are re - routed to provide inputs to the digital effects unit . the latter supplies transformed video and / or key signals back to the continuation of the respective interrupted bus or buses , and then to the mixer to be used thereby in place of the original signals .