Analog equalizer systems and methods for baseband video signals
An equalizer and methods for equalizing a cable-borne signal are described. The cable-borne signal includes a digital signal and a baseband analog signal separated by frequency. Distortions are removed from the digital signal and an analog equalizer compensates for attenuations of the analog signal caused by the cable based on equalization settings of a digital equalizer. A baseband analog filter can be selected based on an estimate calculated by the digital equalizer of difference in attenuation at different frequencies.
1. A system for use in a receiver that receives a plurality of signals separated by frequency and carried by a cable, wherein the system comprises:
a digital equalizer that removes distortions from a digital signal which has been separated from an analog signal in the plurality of signals received at the receiver and that calculates differences in attenuation of the digital signal at a plurality of different frequencies of the digital signal; and
an analog equalizer that compensates for attenuations of the analog signal caused by the cable, wherein the analog equalizer uses the calculated differences in attenuation of the digital signal to select one or more baseband analog filters to compensate for the attenuations in the analog signal after the analog signal has been separated from the digital signal.
2. The system of claim 1 , wherein the analog signal and the digital signal are received from a transmitter embodied in a camera and wherein the receiver provides an equalized analog signal representative of the analog signal to a monitor.
3. The system of claim 2 , wherein the cable comprises a coax cable.
4. The system of claim 3 , wherein the distortions increase with the length of the cable.
5. The system of claim 1 , wherein the distortions include multipath distortions.
6. The system of claim 1 , further comprising a diplexer that separates the plurality of signals to provide the digital signal and the analog signal.
7. The system of claim 1 , further comprising a diplexer that separates a baseband portion of the plurality of signals from a passband portion of the plurality of signals, wherein the baseband portion comprises the analog signal and the passband portion comprises the digital signal.
8. A system for use in a receiver that receives a plurality of signals separated by frequency and carried by a cable, wherein the system comprises:
a digital equalizer that removes distortions from a digital signal received at the receiver and that calculates differences in attenuation of the digital signal at a plurality of different frequencies of the digital signal; and
an analog equalizer that compensates for attenuations of an analog signal caused by the cable, wherein the analog equalizer uses the calculated differences in attenuation of the digital signal to select one or more baseband analog filters to compensate for the attenuations,
wherein the distortions include multipath distortions, and
wherein the calculated differences in attenuation includes an estimate of attenuation calculated from a frequency band having a power spectral density in which tilt is approximately linear.
9. The system of claim 8 , wherein the tilt is calculated using a fast Fourier transform for a plurality of filter taps.
10. The system of claim 8 , wherein frequency bins within the frequency band are selected to permit calculation of the frequency response of a filter of the digital equalizer using the summations:
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wherein G[k] is the discrete Fourier transform (DFT) of time-domain converged equalizer filter taps and k 1 corresponds to a specific frequency bin of the DFT.
11. The system of claim 1 , wherein the digital signal comprises a high-definition representation of video images captured by a camera, and wherein the analog signal comprises a standard-definition representation of the video images.
12. A method for equalizing an analog signal in a cable that also carries a digital signal separated from the analog signal by frequency, the method being performed by a modem that receives the analog and digital signals and outputs a baseband video signal comprising:
separating the analog signal from the digital signal; and
subsequent to the separating:
calculating tilt in the digital signal, wherein the tilt characterizes attenuation attributable to the cable as a function of frequency;
equalizing the digital signal based on the calculated tilt;
configuring an analog equalizer by using the calculated tilt to select one of a set of baseband analog filters; and
equalizing the analog signal using the selected baseband analog filter.
13. The method of claim 12 , wherein the analog signal comprises a baseband video signal and the digital signal comprises a high definition version of the baseband video signal.
14. The method of claim 13 , wherein the cable comprises a coax cable and wherein the tilt varies with length of the cable.
15. The method of claim 14 , wherein the tilt derives from multi-path distortions.
16. A method for equalizing an analog signal in a cable that also carries a digital signal separated from the analog signal by frequency, the method being performed by a modem that receives the analog and digital signals and outputs a baseband video signal comprising:
calculating tilt in the digital signal, wherein the tilt characterizes attenuation attributable to the cable as a function of frequency;
equalizing the digital signal based on the calculated tilt;
configuring an analog equalizer by using the calculated tilt to select one of a set of baseband analog filters; and
equalizing the analog signal using the selected baseband analog filter,
wherein calculating tilt includes estimating attenuations within a frequency band having a power spectral density in which tilt is approximately linear.
17. The method of claim 16 , wherein estimating attenuation includes using a fast Fourier transform for a plurality of filter taps.
18. The method of claim 17 , wherein estimating attenuation includes selecting frequency bins within the frequency band, wherein the selected frequency bins optimize the efficiency of the step of calculating the tilt.
19. A method for equalizing an analog signal in a cable that also carries a digital signal separated from the analog signal by frequency, the method being performed by a modem that comprises one or more processors configured to execute one or more program instructions, the method comprising:
executing, on the one or more processors, one or more program instructions that cause the modem to calculate tilt in the digital signal, wherein the tilt characterizes attenuation attributable to the cable as a function of frequency;
executing, on the one or more processors, one or more program instructions that cause the modem to equalize the digital signal based on the calculated tilt;
executing, on the one or more processors, one or more program instructions that cause the modem to configure an analog equalizer by using the calculated tilt to select one of a set of baseband analog filters;
executing, on the one or more processors, one or more program instructions that cause the modem to equalize the analog signal using the selected baseband analog filter after the analog signal has been separated from the digital signal; and
executing, on the one or more processors, one or more program instructions that cause the modem to output the equalized analog signal.