Extraction of data from a television signal using an adaptable slicing level
From a television signal data such as teletext or closed caption data is extracted by slicing. A binary data signal is formed dependent on whether the television signal level is above or below a slicing level (S). The slicing level (S) is selected by defining a reconstructed data signal (D) with signal values equal to plus and minus an amplitude value relative to a middle level and transitions between these signal values at time points where the television signal crosses the slicing level. An auxiliary signal (R) is formed that contains the television signal (Vi) from which the reconstructed data signal (D) has been subtracted. A feedback loop is provided that adapts the amplitude value (A) to regulate a residual data signal amplitude in the auxiliary signal (R) to zero.
1. A television signal receiver apparatus comprising:
a receiving circuit for receiving a television signal having a level;
a data signal slicing circuit for forming a binary data signal dependent on whether the television signal level is above or below a slicing level; and
a slicing level selection circuit for selecting the selected slicing level from a difference between television signal and a reconstructed data signal the slicing level selection circuit comprising: a data signal reconstruction circuit for defining the reconstructed data signal with signal values equal to plus and minus an amplitude value (A) relative to a middle level and transitions between these signal values at time points where the television signal crosses the slicing level a subtraction circuit for forming an auxiliary signal that contains the television signal from which the reconstructed data signal has been subtracted; a feedback loop arranged to adapt the amplitude value to regulate a residual data signal amplitude in the auxiliary signal to zero.
2. The television signal receiver apparatus according to claim 1 , wherein the feedback loop contains an integrator for integrating the residual data signal amplitude as a function of time during at least a part of the auxiliary signal that derives from a part of the television signal that contains a data signal.
3. The television signal receiver apparatus according to claim 1 , comprising a subsampling circuit in the feedback loop, arranged to adapt the amplitude value in the feedback loop once per horizontal television line that contains a data signal.
4. The television signal receiver apparatus according to claim 1 , wherein the subtraction circuit is arranged to form the auxiliary signal by subtracting both the slicing level and the reconstructed data signal from the television signal.
5. The television signal receiver apparatus according to claim 4 , comprising a further feedback loop arranged to adapt the slicing level to regulate at least a time average of the auxiliary signal to zero.
6. The television signal receiver apparatus according to claim 5 , wherein the data slicing circuit comprises the subtraction circuit the slicing level selection circuit using the adapted slicing level to slice the data signal.
7. The television signal receiver apparatus according to claim 5 , wherein the further feedback loop is arranged to initialize the slicing level at least for each particular horizontal line period that contains a part with a data signal during a clock run in part of the particular horizontal line period.
8. The television signal receiver apparatus according to claim 1 , wherein the feedback loop comprises a residual amplitude detector that is arranged form a detection signal that corresponds to a product of the auxiliary signal and a reference signal that changes sign when the television signal crosses the slicing level.
9. A method of recovering digital data from a television signal, the method comprising:
receiving the television signal;
forming a binary data signal dependent on whether the television signal level is above or below a slicing level the slicing level being determined by:
defining a reconstructed data signal with signal values equal to plus and minus an amplitude value relative to a middle level and transitions between these signal values at time points where the television signal crosses the slicing level;
forming an auxiliary signal that contains the television signal from which the reconstructed data signal has been subtracted;
providing a feedback loop that adapts the amplitude value to regulate a residual data signal amplitude in the auxiliary signal to zero;
determining the slicing level from a difference between television signal and the reconstructed data signal.
10. The method according to claim 9 , wherein the feedback loop contains an integrator for integrating the residual data signal amplitude as a function of time for at least a part of the auxiliary signal that derives from a part of the television signal that contains the data signal.
11. The method according to claim 9 , the method adapting the amplitude value once per horizontal television line that contains the data signal.