Gain control for a high definition television demodulator
View Patent ↗A demodulator for a high definition television receiver includes a first circuit element which receives a signal, and is operative in a first arrangement to produce an output signal having a first dynamic range, and operative in a second arrangement to produce an output signal having a second dynamic range different from the first dynamic range. A second circuit element is arranged for optimally processing a signal having a substantially predetermined fixed dynamic range. A gain control circuit is coupled between the first and second circuit elements, and produces a signal having the substantially predetermined fixed dynamic range.
1. A demodulator for a high definition television receiver, comprising:
a first circuit element, for receiving a signal, and operative in a first arrangement to produce an output signal having a first dynamic range, and operative in a second arrangement to produce an output signal having a second, dynamic range different from the first dynamic range;
a second circuit element arranged for optimally processing a signal having a substantially predetermined fixed dynamic range; and
a gain control circuit, coupled between the first and second circuit elements, for producing a signal having the substantially predetermined fixed dynamic range.
2. The demodulator of claim 1 wherein the first and second dynamic ranges are wider than the predetermined fixed dynamic range.
3. The demodulator of claim 1 wherein:
the first circuit element comprises a carrier tracking loop for processing the received signal to generate a baseband signal;
the first arrangement further comprises a matched filter coupled to the carrier tracking loop and the second arrangement does not include a matched filter preceding the carrier tracking loop; and
the second circuit element comprises a decision-directed circuit element, wherein decisions are made based on decision points having predetermined fixed signal levels.
4. The demodulator of claim 1 wherein the gain control circuit is responsive to a gain control signal exhibiting a switchable fixed gain.
5. The demodulator of claim 4 wherein the gain control circuit operates according to the table:
Gain control signal
Gain
0
1.0
1
1.5
2
1.75
3
2.0.
6. The demodulator of claim 4 wherein the gain control signal is supplied from outside the demodulator.
7. The demodulator of claim 1 wherein the gain control circuit comprises an adaptive gain control circuit, responsive to a reference signal from the second circuit element to adjust the gain.
8. The demodulator of claim 7 wherein the adaptive gain control circuit comprises a detector for generating gain adjustment signals responsive to the reference signal and adjusting the gain in response to the gain adjustment signals.
9. The demodulator of claim 8 wherein the adaptive gain control circuit further comprises an accumulator for accumulating gain adjustment signals and adjusting the gain in response to the accumulation.
10. The demodulator of claim 7 wherein:
the second circuit element comprises a symbol timing loop, for producing a sequence of symbol representative signals; and
the adaptive gain control circuit is responsive to the sequence of symbol representative signals as the reference signal to adjust the gain.
11. The demodulator of claim 10 wherein:
the sequence of symbol representative signals is arranged in successive segments each containing a segment synchronization portion containing a fixed number of symbols; and
the adaptive gain control circuit is responsive to all or part of the segment synchronization portion to adjust the gain.
12. The demodulator of claim 11 wherein:
the second circuit element further comprises a synchronization detector generating a segment synchronization signal having a first state during the segment synchronization portion and a second state otherwise; and
the adaptive gain control circuit is responsive to the sequence of symbol representative signals and the segment synchronization signal, to adjust the gain in response to the segment synchronization portion.
13. The demodulator of claim 10 wherein:
the sequence of symbol representative signals is arranged in successive segments, and further arranged into fields, each containing a field synchronization portion containing a fixed number of segments; and
the adaptive gain control circuit is responsive to all or part of the field synchronization portion to adjust the gain.
14. The demodulator of claim 13 wherein:
the second circuit element further comprises a synchronization detector generating a field synchronization signal having a first state during the field synchronization portion and a second state otherwise; and
the adaptive gain control circuit is responsive to the sequence of symbol representative signals and the field synchronization signal to adjust the gain in response to the field synchronization portion.
15. The demodulator of claim 10 wherein:
the adaptive gain control circuit further comprises a slicer, responsive to the sequence of symbol representative signals for generating corresponding sequence of ideal symbol representative signals; and
the adaptive gain control circuit is responsive to the sequence of symbol representative signals and the corresponding sequence of ideal symbol representative signals to adjust the gain.
16. The demodulator of claim 15 wherein the gain detector in the adaptive gain control circuit operates according to the equation
y=x *( xs−x )
where y is a gain adjustment signal, x is a symbol representative signal and xs is an ideal symbol representative signal corresponding to x.
17. The demodulator of claim 15 wherein the gain detector in the adaptive gain control circuit operates according to the equation
y= sign( x )*( xs−x )
where y is a gain adjustment signal, x is a symbol representative signal, xs is an ideal symbol representative signal corresponding to x and sign (.) is the sign function.