Sold for about $40, very high quality modulation to whatever analog channel.
Well, at least every TV sold now can support both NTSC and digital signal.
I can't seems to locate one model, but the cheap ones only modulate on Chan 3 and 4.
http://www.apogeekits.com/tv_transmitter.htm this is random google.
http://www.amazon.com/modulator-amplifier-video-sender-transmitter/dp/B003YPDWJE
