Systems and methods for decoding teletext messages
View Patent ↗A system and method for efficiently detecting and decoding valid teletext message sequences is provided. A system is provided that includes a correlator, a sine wave generator, two time window generators, a phase detector, a framing code search engine, and a match filter. The method includes determining the phase of the run-in burst of a teletext message sequence, identifying the framing code location, deciding whether the teletext sequence is valid, and decoding the teletext data for display.
1. A method of processing data within a teletext sequence, comprising:
(a) determining a phase of a run-in burst within the teletext sequence;
(b) identifying a location of a framing code within the teletext sequence; and
(c) validating the teletext sequence.
2. The method of claim 1 , further comprising:
(d) decoding the teletext sequence.
3. The method of claim 1 , wherein step (a) comprises correlating the teletext signal with a sine wave of an expected teletext clock frequency, wherein zero crossing points of a correlation output signal are used to determine the phase of the run-in burst.
4. The method of claim 3 , wherein the correlating comprises using a correlator function,
Corr (nT)=Σ(In((n+i)*T)*Cos(nT))),
wherein i=±Δ,
wherein Corr (nT) is an output sample of a correlator,
wherein In (nT) is an actual sample of the teletext sequence,
wherein Cos (nT) is a sample of the sine wave, and
wherein Δ is a range of the correlation.
5. The method of claim 1 , wherein step (b) further comprises:
(i) using a sliding window mask to search teletext data for a match with a predefined framing code value and to identify a framing code detection location;
(ii) calculating a delay from a H-SYNC signal to the framing code detection location; and
(iii) comparing the delay determined in step (ii) with delays calculated during previous framing code acquisition cycles.
6. The method of claim 5 , further comprising declaring the framing code valid when the delay determined in step (ii) is within a predefined time from an averaged position of previous framing codes.
7. The method of claim 5 , further comprising declaring the framing code invalid when the delay determined in step (ii) is not within a predefined time from an averaged position of previous framing codes.
8. The method of claim 1 , further comprising outputting the decoded teletext sequence to a set-top box.
9. The method of claim 1 , further comprising outputting the decoded teletext sequence to a television.
10. A system for processing teletext message sequences, comprising:
a correlator coupled to an input;
a sine wave generator coupled to said correlator;
a first time window generator coupled to an H-SYNC input;
a phase detector coupled to outputs of said correlator and said first time window generator;
a second time window generator coupled to said H-SYNC input; and
a framing code search engine coupled to outputs of said phase detector and said second time window generator.
11. The system of claim 10 , further comprising:
a matched filter coupled to an output of said framing code search engine.