The present invention relates to a teletext converter which is external to a television receiver ("set-top" converter), and more particularly, to such converters having a "transparent mode".
Teletext is a service that broadcasts digital information within the vertical blanking interval (VBI) of a standard broadcast television signal and at the receiver presents the information as digitally generated text and/or pictures on a display screen. In the most commonly considered mode of operation, the standard TV picture is completely replaced by the digitally generated picture. "Transparent mode" refers to the situation in which some region of the screen, rather than being defined by the teletext signal, is occupied by the regular video signal. One example is a regular video picture having a teletext generated sub-title or caption. Ideally, the teletext converter, which converts the digital signals into video signals, is located within the receiver. However, almost all existing receivers do not have such an internal converter. Thus an external set-top converter is used if teletext signal reception is desired on most existing sets. A set-top converter receives the antenna signal, converts the digital teletext signal into a video signal, and then provides an output signal for connection to the receivers antenna input terminal.
One prior art method of producing a transparent mode set-top converter is to decode the regular video to baseband red, green and blue signals, combine these signals with R, G, and B signals derived from the decoded teletext, re-encode to composite NTSC, and then modulate with the composite NTSC signal an output carrier having a selected channel frequency. This approach suffers the deficiencies of high cost and significant signal quality degradation.
In particular, the decoding of composite video to baseband color signals, involving the separation of luminance and chrominance, band-limiting these signals and various other distortion producing processes degrades the fidelity of the standard TV picture produced after re-encoding and finally decoding again in the TV receiver.
It is therefore desirable to provide a set-top teletext decoder with transparent mode that does not cause signal degradation and has accurate color fidelity.