NTSC video consists of (approx.) sixty fields each with 240 scan lines worth of picture information every second. After doubling, we have the same 60 fields per second but they each have 480 scan lines of not necessarily unique picture information. After quadrupling, we have 960 scan lines per field. Using a process known as de-interlacing, we can call the 480 scan line fields full frames because each has information from both even and odd original fields, while plain doubling can be taken to mean just outputting each scan line twice. De-interlacing methods vary and nowadays it is hard to draw the line between doubling and de-interlacing. Intrinsically the result of plain doubling and the result of good de-interlacing are both the same video format, which for NTSC is referred to as 480p or progressive scan.
Doubling (or de-interlacing) is of no use to you if your TV set does not accept the resulting signal. If your TV set instructions does not say the TV accepts 480p, the TV almost certainly does not.
The better units preserve as much picture detail as possible and can enhance the picture compared with display on a standard TV set without getting involved with doublers. Extremely good de-interlacers are available nowadays that can display ordinary video on a computer monitor. They can also be used to display ordinary video on an HDTV set but almost all HDTV sets have a built in doubler so buying one separately is not mandatory.
The so-so units just convert the incoming video signal to be compatible with what a TV set (or monitor or projector) expects for example a "960p" projector (rarely encountered outside professional environments) requires that incoming NTSC video be "quadrupled". (The projector might have the quadrupler built in.) It so happens that good de-interlacing followed by redoubling, is better than straight quadrupling.
By definition, interpolation is guessing. Good de-interlacing includes some interpolation of what should go in the intervening added scan lines, based on the existing neighboring scan lines. I have not heard of any video devices called "interpolators". Using interpolation exclusively is slightly better than plain doubling but is far from the best.
Video hints:
http://members.aol.com/ajaynejr/viddoubl.htm
