Re: Multi-Channel VHF Modulator Design
HI all,
David's KB Majestic looks well-kept, but can we see it on 405 lines! (and with real telly, not this set-top box muck!).
PNP, NPN, why not have a NPN on Tuesdays and Thursdays, PNP all other times!.
No but really, if you must have one (filter that is! and you don’t need it really) put the filter before the base with a stopper resistor like you have, maybe a series L/C type rather that a T filter, it (the MC44..) may be happier with a PNP as a source as the internals of the device (P31 of data sheet) shows a 'NPN emitter clamp' but have a look they show 500 ohm as a series feed!.
Jeff, remember the modulator is working on the whole of the video waveform, it 'clamps' the sync tips to a known level to suit its modulation so back porches and colour bursts are way up there from the sync tip level.
This goes back to what I said about the bandwidth of the video you feed to the modulator, it shouldn’t have anything near 3.5MHz in it!, make the output of the converter fit the 405 standard, then all is well.
The 'Gold Standard' Dinosaur converter has a 3.0MHz filter in its video output stage with a fancy feedback restorer in the modulator and that produces very good results.
Malcolm
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