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Old 24th Feb 2005, 4:20 pm   #10
David_Robinson's Avatar
Join Date: Feb 2004
Location: Nottingham, UK
Posts: 94
Default Re: VHF Modulator wanted

Originally Posted by oldeurope
I must look at the data sheet for the IC maybe there is a way to use the audio section too.
Trust me, there isn't! The sound-vision spacing can only have certain values and 3.5 isn't one of them! You could put an external audio subcarrier into the MC44BS373CA in the 20-pin version. Except the 20-pin version is unobtainable.

In the audio stage you don't need a tandem pot.
The problem with your circuit with the stereo mix to mono directly on the input, is if the source device (in my case a set top box) also feeds other places (my stereo amp). The mixer may reduce stereo separation on all destinations due to the output impedance of the source. I put the pot right on the input so that the system can't be overloaded.
...the bias adjust via the emitter follower. It must be temperature compensatet.
Good point!
...the IC makes not 100% video modulation.
True, but think about what the diode detector in the average vintage TV will do to the signal if it reaches 100% mod!
Hope the audio carrier can be switched off.
Not at the moment, but I can add that to the software.
Look that up in your Funk and Wagnall's

Last edited by Dave Moll; 27th Jun 2007 at 9:21 pm. Reason: correct for username change in quote
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