A method and apparatus are described for modifying an analogue video signal to impair its conversion to a digital signal by an analogue to digital converter. The housing defines a first sidewalls and a second sidewall and two other sidewalls opposite to the first and second sidewalls respectively. A minimum output power process produces an error signal that drives the variable delay and adaptive filter to minimize the power in the signal of interest to maximize cancellation of the interfering signal. Concurrent graphical representations of the plurality of cannulas are superimposed over the images and displayed. Thus, different combinations of speeds of the first and second motors can be selectively chosen to meet cooling requirements.