On Tue, 3 Nov 1998 22:59:30 +0000 Dan Larson writes: >On Tue, 3 Nov 1998 19:23:13 -0500, Sean Breheny wrote: >>, could I just amplitude modulate a >video >>carrier freq. with the output of the camera and then send that? >TV carriers use a vestigial side band. Only the center + the uppper >side band >are transmitted. Regulations may have changed, but it used to be legal for a low-power (1000 W or less) TV transmitter to send simple AM with both sidebands. However it does have to suppress the lower color subcarrier at Fv - 3.58 MHz, which can be done with a notch filter between the transmitter and antenna. Obviously that's not an issue using a black and white camera. VSB generation used to be done with a giant band-reject filter between an AM transmitter and the antenna. With today's prices of electricity, copper, and real estate, a modern transmitter generates a low level VSB signal at an IF frequency, then converts it up to the TV channel frequency and amplifies it with a linear amplifier. Such a transmitter's VSB filter can then be used for any channel without retuning it. The required suppression of the unwanted lower sideband is only about 20 dB if I remember correctly. >There is a small lower side band where the FM audio >hangs out. FM audio with +- 25 KHz deviation is transmitted 4.5 MHz *above* the video carrier. The audio carrier power is about 10% of the peak video carrier power. Full-power stations use an entirely seperate transmitter, maybe even a seperate antenna. A less desirable setup uses the same power amplifier for picture and sound, because distortions in the amplifier can intermodulate the two signals together, causing a degraded picture or illegal out of band signals. But, small hacker transmitters and RF modulators found in VCRs and video games simply modulate a 4.5 MHz carrier with the audio signal, then add this to the video going into the AM transmitter. The AM transmitter produces two sound carriers: one in the proper place and one below the video carrier which could be a source of considerable interference if allowed to get out over the air. An RF notch filter similar to the one used to remove the lower color subcarrier could be used. Better performance will result though from splitting the sound off to a seperate RF generator or transmitter. >Steal a cheap modulator from an old video game and amplify the output >of that, >perhaps? Not too bad of an idea. Some of these even have a VSB filter before the output. ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]