Just a doubt about HDTV. The set top box which I have connects to the Tv using RCA cables. Now is the Rca cable able to carry video at full hdtv resolution or we are losing the quality there as well?
thanks

Not enough information.
Do you have a HD monitor? If so, why are you not using the DVI/HDMI input? What input are you using? Composite(one line), component(3 lines), or what?

To answer your question, unless you are using DVI/HDMI input, you are not watching HD.

No just a normal TV. Connection is component(3 lines) I think coz it has video and two for audio. I have the option to connect s video from this set top box to TV. What resolution does the component(3 lines) output and what does the Svideo output.
thanks

Component is actually three video signals, which means you would have three phono (RCA) leads just for the video alone. With component, the first line 'Y' carries the luminance (brightness signal), the second line carries the blue level (BY) and the third carries the red level (RY). The receiver (TV, Projector, etc.) gets the green level when it combines the luminance level with the red & blue components.

Composite video on the other hand is the combination of the video chrominance (colour) and luminance (brightness) all on a single cable. Composite is not used for HDTV and due to everything being combined on a single cable, the colour rendering is generally not even as good as SVHS. As you mention two of your cables are for audio, then the remaining cable must be composite video.

As you say your TV is a regular TV, SVHS is the best choice assuming your TV has an SVHS input. SVHS carries the chrominance on one wire and the luminance on another wire, thus giving improved colour rendering and sharpness on a colour image. Just note that SVHS does not carry HDTV either.

HDTV can be carried over DVI/HDMI and component video. The four common resolutions are 480i, 480p (NTSC), 576i, 576p (PAL), 720i, 720p (HDTV), 1080i and 1080p (HDTV), where ‘i’ means interlaced (odd lines in one draw followed by even lines in next draw) and p means progressive (all lines in one draw). The two most common HDTV resolutions in use are 720p (intended for digital displays such as projectors) and 1080i (broadcast services such as HDTV over satellite and better suited for HDTV CRT displays). Some DVD players provide component output at 480i or 480p to provide better picture quality on TVs that support component video, but just note that resolutions below 720i/p are generally standard definition TV.