----- Original Message -----
From: "John Rollins" <kd7bcy_at_teleport.com>
To: <cctalk_at_classiccmp.org>
Sent: Monday, June 30, 2003 8:17 PM
Subject: Re: Modifying a B&W TV for composite video input
> I would think the easiest and cheapest way is to use a plain old RF
> modulator box that will put composite video on to channel 3 or 4.
> I've taken apart one or two and they seem to have little more than a
> few coils and small parts, a far cry from the rack mounted cable TV
> modulators I just picked up(ooohh... so many trimmer caps and
> coils...). I don't know a whole lot about it, but I'm slowly learning
> as my interest in fast scan ATV increases... The converter box I have
> goes with my TI-99, I probably have one in the box with my C-64, too.
> I think you can still get them at Radio Shack. Otherwise, all you
> need to do is dump the composite signal into an AM modulator and then
> through a transmitter that the TV can pick up. I would guess these
> small boxes having only coils just use a tuned circuit for channel 3
> and the voltage from the composite signal to provide the signal. But
> then I'm just guessing, I've never poked and prodded those little
> boxes very much, I think I'll go do that now(when I find it,
> anyhow...).
>
> <SNIP> --
>
> /------------------------------------\
> | http://jrollins.tripod.com/ |
> | KD7BCY kd7bcy_at_teleport.com |
> \------------------------------------/
>
The SWTPC TV-Typewriter outputs baseband video, which is also a RF (radio
frequency) signal. What it is NOT is modulated. That is why it is called
baseband. Baseband is also sometimes called DIRECT video.
The Commodore 64 puts out a RF signal on TV channel 3 or 4 (switchable),
which is usually connected to a TV set using a RF SWITCH (required by the
FCC in the US). That switch is NOT a RF modulator, it is an isolation
device, to prevent your connection of your computer to an outside antenna
and the broadcasting your computer signal to your neighborhood; that
broadcast would be considered as unwanted interference by the FCC, thus the
isolation switch.
Please not that in the rest of this post that I am NOT talking about a RF
"switch" as described above, but about an actual RF modulator vs. baseband
video.
In my opinion, based on personal experience, it is cheaper, easier (if you
know what to do), and certainly provides a better picture if you use the
direct video approach when connecting a baseband device like the
TV-Typewriter to a TV set. When you use a RF modulator you cause the signal
to be modulated and then demodulated (by the TV); every time this is done,
some of the information is lost in the process. When you use the direct
video, little of the signal is lost or attenuated and the full bandwidth of
the TV's electronics are available. The lossy signal from a (cheap) RF
modulator causes the picture to be fuzzy or wavy and may not even be stable.
I used to have one of these TV-Typewriters and even modified it for 24x80
display by doubling the ram and making mods to the timing of the board.
There was an article published in one of the electronics / computer
magazines that showed how to do this, and it worked exactly as shown. I used
my TV Typewriter with a small black & white TV modified to have an RCA jack
for baseband RF input. It worked well for many years. My computer at that
time was a KIM-1, expanded with S-100 peripherals and memory, using a KIMSAI
motherboard / S-100 interface.
How one connects baseband to a TV depends on the TV itself; many small sets
have a "hot" chassis, that is to say that the chassis is connected to one
side of the 110 VAC mains, and insulated from the user by the plastic case.
This kind of TV can get you KILLED if you mess with it opened up or can
possibly blow your computer up if you connect to it improperly. This
procedure is therefore NOT recommended to the non-technically skilled. The
actual circuit can be as simple as a capacitor and a few resistors (for DC
restoration). What is done is to disconnect the TV's tuner and inject the
signal where the tuner did. Many people used a toggle switch so that the TV
or Video function could be selected as needed.
For what it is worth, TV doesn't use AM (Amplitude Modulation), it uses a
modulation technique called Vestigial Sideband, from before the days when
Single Sideband modulation was understood.
Also, RF modulators are ACTIVE devices requiring a power supply and ground
to operate; they do not derive their power from the signal; at least none
that I've ever seen do. With today's ultra-low powered devices, it may be
possible, but I doubt that anyone has gone to the trouble to design an
ultra-low power RF modulator considering that many TV's already have
composite (baseband having chrominance & luminance combined) video inputs.
Some specialty monitors, like those sold for use with the Commodore 64,
accepted separate chrominance (color) and luminance (brightness) inputs in
an attempt to give a better picture. The Commodore 64 has a special jack on
its back to connect to these monitors and uses a special cable to make that
connection.
Stuart Johnson
former electronics technician and present computer nut
Received on Mon Jun 30 2003 - 21:51:01 BST