Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!uunet!ogicse!milton!whit From: whit@milton.u.washington.edu (John Whitmore) Newsgroups: sci.electronics Subject: Re: Pulse detecting filter wanted Keywords: filter pulse video Message-ID: <13911@milton.u.washington.edu> Date: 7 Jan 91 23:09:12 GMT References: <22397@well.sf.ca.us> Distribution: sci Organization: University of Washington, Seattle Lines: 38 In article <22397@well.sf.ca.us> nagle@well.sf.ca.us (John Nagle) writes: > > I need to build a filter that will pass only 200ns or shorter >pulses found in a video signal. It's really very simple; you connect a shorted delay line to your signal input (after preamplification, if you prefer). For full 200 ns, you'll need enough coax cable for 100 ns one-way delay; about 60 feet of RG-59 should do it. After 200 ns, the cable is a short-circuit; until then, the pulse passes with only the characteristic impedance of the cable loading it. The cable is equivalent to some horrendous infinite array of inductors/capacitors (any time-selective filter to match its performance would take a LOT of components). A positive-going pulse becomes a positive pulse followed by a negative echo; the large slew from positive pulse to negative echo is easily discriminated from a lot of noise-like artifacts, IF you know that 200 ns pulse width accurately enough to tune for it. Example: --- --- short pulse ----- -------- => ------- -- ---------- --- ----- ----- medium pulse ----- ----- => ------- -------- ----- ------- ----- long pulse ----- ---- => ------- -- -------- ----- As shown above, only the pulse of the correct length (like, 200 ns) has a twice-amplitude fast slew in the as-processed signal. John Whitmore whit@milton.u.washington.edu