be done by inserting the extra signals into cable systems on nonstandard frequencies, typically either below VHF channel 7 (midband) or directly above VHF channel 13 (superband). These frequencies corresponded to non-television services (such as two-way radio) over-the-air and were therefore not on standard TV receivers. Before cable-ready TV sets became common in the late 1980s, an electronic tuning device called a cable converter box was needed to receive the additional analog cable TV channels and transpose or convert the selected channel to analog radio frequency (RF) for viewing on a regular TV set on a single channel, usually VHF channel 3 or 4. The box allowed an analog non-cable-ready television set to receive analog encrypted cable channels and was a prototype topology for later date digital encryption devices. Newer televisions were then converted to be analog cypher cable-ready, with the standard converter built-in for selling premium television (aka pay per view). Several years later and slowly marketed, the advent of digital cable continued and increased the need for various forms of these devices. Block conversion of the entire affected frequency band onto UHF, while less common, was used by some models to provide full VCR compatibility and the ability to drive multiple TV sets, albeit with a somewhat nonstandard channel numbering scheme. Newer television receivers greatly reduced the need for external set-top boxes, although cable converter boxes continue to be used to descramble premium cable channels according to carrier-controlled access restrictions, and to receive digital cable channels, along with using interactive services like video on demand, pay per view, and home shopping through television. Closed captioning box Set-top boxes were also made to enable closed captioning on older sets in North America, before this became a mandated inclusion in new TV sets. Some have also been produced to mute the audio (or replace it with noise) when profanity is detected in the captioning, where the offensive word is also blocked. Some also include a V-chip that allows only programs of some television content ratings. A function that limits children's time watching TV or playing video games may also be built in, though some of these work on main electricity rather than the video signal. Digital television adapter The transition to digital terrestrial television after the turn of the millennium left many existing television receivers unable to tune and display the new signal directly. In the United States, where analog