Comb filter delays
Mixing two adjacent scan lines can be achieved by the introducing time delay. One kind of time delay is the use of tiny loudspeaker that plays the video signal into a tiny glass chamber where a tiny microphone picks it up at the other end. Another is similar except that a thin glass fibre transmits the signal from the speaker to the microphone like a toy tin can and string telephone. Still others (CCDs, or charged coupled devices) use silicon chips with a chain of ‘charged cells’ so as to make the signal travel slower as is passes through. The circuits are arranged so that the video signal goes through both the apparatus described above and through a straight wire.
At the other end where the signals are mixed, when the current line is coming down the wire, the previous line is coming out of the delay appa- ratus. This happens for each line in the field. For the 3L comb filter, there are two delay apparatus, so we have available for commingling one line delayed twice, the next line delayed once and the third line not delayed.