From: "Michael A. Terrell"
Subject: Re: Homemade RF Modulator?
Date: Mon, 18 Nov 2002 23:59:15 -0500
Organization: Have you seen my bench? No, really! Where is it?
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Peter Bennett wrote:
> On 18 Nov 2002 15:02:05 -0800, email@example.com (Jeff Bowles)
> >Hello, All.
> >I would like to insert an NTSC composite video signal (videotext) into
> >my home cable TV setup, so that any TV in the house can tune to the
> >"special" channel and look at the videotext.
> >My budget is near zero, so commercially available RF modulators are
> >not an option for me.
> >I am capable of reading schematics and soldering, however I'm not very
> >good at RF theory or circuit design.
> >Is it feasible to modify an old "ch 3/ch 4" video game modulator to
> >use a cable TV channel much higher up, say anywhere from channel 70 or
> >99? These are empty channels in my community.
> I expect that a Channel 3/4 modulator could fairly easily be modified
> to cover up to Channel 6. High VHF (7 - 13) would be considerably
> more difficult.
> The UHF channels (14 and up), particularly the high end, would require
> a whole new design, and considerable experience with UHF circuit
> design and construction.
> Peter Bennett, VE7CEI
Some of the CH 3/4 modulators have a SAW filter in them to remove
most of the lower sideband on the visual carrier. It would be better to
make yourself a crystal controlled upconverter, or got to
http://www.skyvision.com and see what they have in open box or closeout
If you want to try to convert or build your own, CH 95 is the lowest
frequency in the range you listed. Below is an excerpt from one of my
online reference charts showing the frequency, depending on the system
your TV or cable box uses. You need a crystal on the video carrier
frequency. The audio is offset 4.5 MHz higher than the video by the
existing audio modulator. Using CH 96 will let you get by without
building and aligning a vestigial sideband filter to prevent
interference to CH 6. You will still need to tune the output circuit in
the modulator, but most use air wound coils, so you can stretch them to
reduce the inductance, and raise the frequency.
CATV CHANNELS, North America
EIA Standard Incremental Harmonic
Ch Video Audio Video Audio Video Audio
06 83.2500 87.7500 85.2625 89.7625 84.0042 88.5042
95 91.2500 95.7500 91.2625 95.7625 90.0045 94.5045
96 97.2500 101.7500 97.2625 101.7625 96.0048 100.5048
Take care, and God bless!
Michael A. Terrell