Abstract:
A set-top teletext converter decodes the chroma burst signal of a color television signal with the substantially same amplitude and phase as the burst in the television signal. The teletext signal along with its transparent mode signal are decoded. The decoded teletext signal is then re-encoded as a standard television signal using the decoded burst signal. When switching between the encoded teletext signal and the regular television signal under the control of the transparent mode signal, no color shift is present.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a teletext converter which is external to a television receiver (&#34;set-top&#34; converter), and more particularly, to such converters having a &#34;transparent mode&#34;. 
     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. &#34;Transparent mode&#34; 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. 
     SUMMARY OF THE INVENTION 
     Method and apparatus for use with a color television signal having teletext information, comprising decoding the color subcarrier signal as a continuous signal with a selected amplitude and phase, converting said teletext signal into a video signal, encoding said video signal into a standard signal using said continuous signal, and selecting between said color television signal and said standard signal in accordance with a control signal. 
    
    
     DESCRIPTION OF THE DRAWING 
     The sole FIGURE is a block diagram of the invention. 
    
    
     DETAILED DESCRIPTION 
     Input terminal 10 receives from, e.g. a transmission line (not shown) coupled to an antenna (not shown), an RF television signal which comprises a digital teletext signal transmitted during the VBI. The television signal is applied to a circuit 12 comprising a tuner, IF circuit, detectors, and sync and burst separators, all as known in the art. Circuit 12 also receives a user command, i.e. channel selection, and provides as output signals a burst gate to chroma processor 14 and S and H (sample-and-hold) circuit 36, horizontal and vertical sync to teletext decoder 16, composite NTSC video including the teletext signal to decoder 16, chroma separator 17, and contact 18 of switch 20, and sound to modulator 22. Chroma separator 17 can comprise a conventional bandpass filter or comb filter and supplies I and Q signals modulated on a 3.58 MHz subcarrier and chroma burst to processor 14. 
     Processor 14 is a conventional color decoding circuit. It receives the I, Q, and burst signal from separator 17, and burst gate signal from tuner 12 and provides baseband R-Y, G-Y, and B-Y signals to matrix 24. The magnitude of these color difference signals is controlled by the saturation control 28 associated with processor 14 and it thus comprises the teletext saturation control. Another signal provided by processor 14 is a constant amplitude 3.58 MHz color subcarrier signal having the same phase as the input color burst and which is applied to phase shifter 30. The subcarrier can be generated by a phase-locked-loop (not shown) within processor 14 or by having the burst ring a crystal 31. The phase shift of shifter 30 is controlled by teletext hue control potentiometer 32, and therefore the phase of the subcarrier signal from processor 14 need not exactly match that of the input color burst. The subcarrier signal from shifter 30 is applied to variable gain amplifier 34. 
     Matrix 24 matrixes the R-Y, G-Y, and B-Y signals together so that during the occurrence of burst, an output pulse is provided by matrix 24 which has an amplitude proportional to the color burst of the video signal. This pulse is sampled by S and H circuit 36 under the control of the burst gate since the output signal of matrix 24 at times other than the burst occurence is of no interest. The output level signal from S and H circuit 36 is applied as a gain control signal to amplifier 34 so that the amplitude of the output signal from amplifier 34 is proportional to that of the received burst. 
     It will thus be appreciated that the output signal of amplifier 34 is an essentially constant amplitude color subcarrier whose phase is essentially the same as that of the burst of the video signal. Fine control of this subcarrier&#39;s amplitude and phase is respectively provided by potentiometers 28 and 32. Since as explained below, this subcarrier is used to convey the decoded teletext signal, potentiometers 28 and 32 respectively control the saturation and hue of the teletext signal relative to the regular video signal. Potentiometer 32 also allows compensation for phase shifts in ensuing filters. This subcarrier signal is applied to a standard NTSC encoder 38, in particular, to ±45 degree phase shifters 40 and 42 therein. The outputs of phase shifters 40 and 42 are respectively applied to multiplier-type modulators 44 and 46. 
     As known in the art, teletext decoder and processor 16, using a user supplied page select signal, decodes the digital teletext signal into a transparent mode signal for control of switch 20. In addition, decoder 16 decodes the digital teletext signals into R, G, and B output signals and applies the output signals to matrix 48. Teletext decoders are shown in the publication &#34;Viditexas&#34; published by Texas Instruments-France. Matrix 48 matrixes the R, G, and B signals into Y, I, and Q signals. Obviously, if decoder 16 directly provides Y, I and Q signals, matrix 48 can be eliminated. The I and Q signals are respectively applied to modulators 44 and 46. The modulated color subcarrier output signals from modulators 44 and 46 are respectively applied to BPFs (band pass filters) 50 and 52. BPF 50 passes frequencies from about 2 MHz to 4 MHz, while BPF 52 passes frequencies from about 3 MHz to 4 MHz. The output signals from BPFs 50 and 52 are applied to adder 54 along with the Y signal from matrix 48. The output signal from adder 54, which is also the output signal from encoder 38, is thus an NTSC encoded teletext signal having a color subcarrier with the same amplitude and phase (saturation and hue respectively) as that of the NTSC video signal from circuit 12, except for the possible fine adjustments provided by potentiometers 28 and 32. This encoded signal is applied to contact 56 of switch 20. 
     The transparent signal from decoder 16 controls the position of switch 20 so that the composite video from circuit 12 is applied to LPF (low pass filter) 58 when the transparent mode is used. When the non-transparent mode is in use, the teletext composite video from encoder 38 is applied to LPF 58. LPF 58 has a cutoff frequency of about 4.2 MHz so that high-frequency transients in the teletext signal do not disturb the sound signal. The output signal from LPF 58 is applied to modulator 22 along with the sound signal from circuit 12. Modulator 22 has an R.F. signal (usually channels 3 or 4) that is modulated by the two input signals and provides the modulated signal to output terminal 60 which terminal is connected to the television set antenna terminals.