Modifying a B&W TV for composite video input

----- Original Message ----- From: "John Rollins" <kd7bcy(a)teleport.com> To: <cctalk(a)classiccmp.org> Sent: Monday, June 30, 2003 8:17 PM Subject: Re: Modifying a B&W TV for composite video input 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