The short answer is 'It will probably not get A&E and SPIKETV'. Here's why:
 Your TV tuner will pick out signals (if available) from what ever is directed to it.
There are two formats of signal to consider: analog (the NTSC standard) and digital (the ATSC standard). If your TV tuner is analog only, it cannot interpret the digital signals, but it will tune in to any analog channel signals that are still used on the cable. (Apparently Comcast is showing you the 'advertisement' by transmitting it over the cable in analog format on some analog channel frequencies.)
 Broadcast analog (NTSC) channel frequencies and cablecast analog (NTSC) channel frequencies are not the same--cable channels are closer together. For example, with Cox (analog) cable, I can tune to around channel 30 and see cable channel 80. For some channels, the fine tuning ring on the knob on my 1985 TV allows slight adjustment to pick out the next cable channel up or down without clicking the knob to the next stop position. (To connect some old TVs, requires a cheap connector called a balun: it has an RF coax connector on one side and on the other side, two 'Y'-shaped ends on the wires to screw to the TV's antenna terminals.)
 I connected the cable to my (Insignia) DTV converter box and scanned for channels: 0 channels found. We do have a digital cable box on one TV in the house, so there are digital signals coming in on the coaxial cable. Our digital cable box (a Scientific Atlanta mode) is actually a computer, and it has an IP address. I believe the cable company transmits the digital signal to the digital cable box on a different frequency than over-the-air broadcast digital channel frequencies; otherwise, the DTV coverter would probably tune in to something.
 I asked Cox about getting a non-Cox digital receiver (rather than renting another one) for the services to which we already subscribe for use on another TV or even on a PC. The answer was that there are compatible devices available and there are no incompatibility issues known (keyword 'known').
The next question is: what will a DTV box do for an analog NTSC TV?
It will receive *digital* format signals for certain over-the-air digital frequencies and convert them to the analog (NTSC) format, and it will put that signal out as analog on the broadcast analog channel 3 frequency (or channel 4).
Things to know about for good DTV reception are:  signal strength and  the signal-to-noise ratio ('SNR'). Start with SNR: For analog TV high signal and low noise means a high SNR, and that gives a good clear picture. Low signal and/or high noise (like interference) gives a low SNR and the picture will be snowy or show the interference. Now the signal strength is also important: if you send a signal through an amplifier to make it stronger, the amplifier will also make any noise at the same frequency stronger too--it does not change the SNR much. Also, if the signal is too strong, there can be problems. As an analogy consider that it is difficult to pick out details in an overexposed film picture: likewise, it can be difficult for a tuner to pick out finer variations in an analog signal.
So, for good DTV you want a good SNR and signal strength in the right range. This means that the receiver antenna needed depends on the distance to the broadcast antenna.
If the broadcast antenna is very close, an attenuator might be needed to tone down the signal strength; If the broadcast antenna is very far, an amplifier may be needed.
The farther away the broadcast antenna is located, the better then receiver antenna must be to catch the 'airwaves'. The receiver antenna will catch the signal and whatever noise is present at the same frequencies.
A couple good website for more research are http://tvfool.com/ and http://www.antennaweb.org
answered 5 years, 9 months ago