1. A demodulator, comprising:
- (a) an input for receiving a signal having a particular type of modulation and including in-phase and quadrature modulated signal components;
- (b) a circuit configured to variably control a carrier frequency depending on the particular type of modulation of the received signal;
- (c) an in-phase channel including a downshifter responsive to the carrier frequency and configured to downshift the in-phase modulated component to form an in-phase baseband signal and a baseband filter for filtering the in-phase baseband signal;
- (d) a quadrature channel including a downshifter responsive to the carrier frequency and configured to downshift the quadrature modulated signal component to form a quadrature baseband signal and a baseband filter for filtering the quadrature baseband signal;
- (e) a summer for summing the filtered in-phase and quadrature baseband signals; and
- (f) said in-phase and quadrature baseband filters having transfer functions such that said demodulator outputs a baseband signal originally used to form said modulated signal.
2. The demodulator of claim 1, wherein said in-phase baseband filter is a linear phase FIR filter and said quadrature baseband filter is a linear phase FIR filter.
3. The demodulator of claim 2, wherein said in-phase baseband filter has a purely real even symmetric transfer function.
4. The demodulator of claim 2, wherein said quadrature baseband filter has a purely imaginary odd symmetric transfer function.
5. The demodulator of claim 1, wherein said VSB modulated signal is a VSB-PAM modulated signal.
6. The demodulator of claim 1, wherein said modulated signal is a VSB-AM signal.
7. A modulator for performing VSB-PAM or QAM modulation, comprising:
- (a) an in-phase channel configured to receive an in-phase baseband signal and including an in-phase baseband filter and a frequency upshifter configured to modulate the in-phase baseband signal onto an in-phase carrier;
- (b) a quadrature channel configured to receive a quadrature baseband signal and including a quadrature baseband filter and a frequency upshifter configured to upshift the quadrature baseband signal onto a quadrature carrier; and
- (c) a summer configured to sum the modulated in-phase and quadrature signals to generate a modulated output signal which is a QAM signal or a VSB-PAM signal;
- (d) said in-phase and quadrature baseband filters being FIR filters, said modulator including a circuit for controlling tap weights of the FIR filters depending on whether the modulated output signal is a QAM signal or a VSB-PAM signal.
8. The modulator of claim 7, wherein:
- (a) said in-phase and quadrature baseband signals are independent signals;
- (b) said in-phase and quadrature baseband filters have the same transfer function which is purely real and even symmetric; and
- (c) said modulated output signal is a QAM signal.
9. The modulator of claim 7, wherein:
- (a) said in-phase and quadrature baseband signals are generated by distributing a single input baseband signal to said in-phase and quadrature channels;
- (b) said in-phase filter has a transfer function which is purely real and even symmetric;
- (c) said quadrature filter has a transfer function which is purely imaginary and which is odd symmetric; and
- (d) said modulated output signal is a VSB-PAM signal.
10. The modulator of claim 7, wherein said modulator includes a circuit configured to control the frequency of said in-phase and quadrature carriers.
11. A demodulator which can demodulate a QAM signal, a VSB-PAM signal, and a VSB-AM signal, comprising:
- (a) an input for receiving an input modulated signal which is a QAM signal, a VSB-PAM signal, or a VSB-AM signal;
- (b) an in-phase channel for receiving said modulated signal from said input, and including a frequency downshifter configured to downshift said modulated signal to form an in-phase baseband signal and a baseband filter for filtering the in-phase baseband signal;
- (c) a quadrature channel for receiving said modulated signal from said input and including a frequency downshifter configured to downshift said modulated signal to form a quadrature baseband signal and a baseband filter for filtering the quadrature baseband signal;
- (d) an output for outputting a demodulated baseband signal; and
- (e) said in-phase baseband filter and said quadrature baseband filter having transfer a circuit configured to control transfer functions of said in-phase and quadrature baseband filters depending on whether said input modulated signal is a QAM signal, a VSB-PAM signal, or a VSB-AM signal.
12. The demodulator of claim 11, wherein said transfer functions of said filters are the same for a VSB-AM or a VSB-PAM signal.
13. The demodulator of claim 11, wherein said demodulator includes a local oscillator and a controller being configured to control a frequency of the local oscillator depending on whether the input modulated signal is a QAM signal, a VSB-PAM signal, or a VSB-AM signal.
14. The demodulator of claim 11, wherein said baseband filters are FIR filters and said circuit controls tap weights of said FIR filters.
15. The demodulator of claim 11, wherein for a VSB-AM or a VSB-PAM signal said in-phase baseband filter has a purely real even symmetric transfer function and said quadrature baseband filter has a purely imaginary odd symmetric transfer function.
16. The demodulator of claim 11, wherein when said input modulated signal is a QAM signal, said output outputs independent in-phase and quadrature baseband signals.
17. The demodulator of claim 11, wherein when said input modulated signal is a VSB-PAM signal or VSB-AM said output is formed by summing the in-phase and quadrature baseband signals.
18. The demodulator of claim 11, wherein said input modulated signal is a VSB-AM signal and said output is an analog baseband signal.
19. The demodulator of claim 11, wherein said QAM signal is a digital video signal, said VSB-AM signal is an NTSC video signal, and said VSB-PAM signal is an HDTV video signal.
20. The demodulator of claim 11, further comprising:
- (a) a first switch responsive to the circuit and configured to selectively connect an output of the in-phase channel baseband filter between an in-phase output and the summer; and
- (b) a second switch responsive to the circuit configured to selectively connect the output of the quadrature channel baseband filter between a quadrature output and the summer.
21. A demodulator, comprising:
- (a) an input for receiving an input modulated signal;
- (b) downshifting circuitry for generating an in-phase baseband signal and a quadrature phase baseband signal;
- (c) an in-phase baseband filter for filtering said in-phase baseband signal;
- (d) a quadrature baseband filter for filtering said quadrature baseband signal;
- (e) a circuit for varying the transfer functions of said in-phase baseband filter and quadrature baseband filter depending on a type of modulation used to generate said input modulated signal; and
- (f) a local oscillator connected to the downshifting circuitry and responsive to the circuit to output a carrier frequency depending on the type of modulation used to generate said input modulated signals.
22. The demodulator of claim 21, wherein said input modulated signal is a QAM signal and said filter circuitry enables said in-phase and quadrature baseband filters to have even symmetric transfer functions.
23. The demodulator of claim 21, wherein said input modulated signal is a VSB signal and said filter circuitry enables said in-phase baseband filter to have a real even symmetric transfer function and said quadrature baseband filter to have an imaginary odd symmetric transfer function.
24. The demodulator of claim 23, wherein said filter circuitry varies said transfer functions of said filters by varying filter tap coefficients.
25. The demodulator of claim 21, further comprising:
- (a) a first switch responsive to the circuit and configured to selectively connect an output of the in-phase channel baseband filter between an in-phase output and the summer; and
- (b) a second switch responsive to the circuit and configured to selectively connect the output of the quadrature channel baseband filter between a quadrature output and the summer.
