Digitizer for use in an overlay system with digital optical transmitter for digitized narrowcast signals
Methods and apparatuses are provided to digitize an analog multi-channel RF narrowcast signal in an overlay system by dividing the signal into a plurality of subband signals and digitizing each subband signal using practical A/D converters.
1. A digital transmitter for narrowcast signals in the forward path of an overlay transmission system comprising:
a plurality of digitizers wherein each digitizer is configured to receive a plurality of analog subband signals of an analog signal representing one or more digital bit streams that each have been QAM encoded and modulated and wherein each digitizer is configured to digitize the plurality of analog subband signals and combine the plurality of digitized subband signals to produce a digitized signal;
a plurality of digital optical transmitters configured to receive the plurality of digitized signals from the plurality of digitizers, respectively, and configured to convert the plurality of digitized signals to a plurality of optical signals, respectively; and
a multiplexer configured to multiplex the plurality of optical signals.
2. The overlay system of claim 1 wherein each digitizer is configured to downconvert each of the plurality of analog subband signals to an analog baseband subband signal, convert each of the plurality of analog baseband subband signals to a digital baseband subband signal, and combine the digital baseband subband signals to produce a digitize signal.
3. The overlay system of claim 1 wherein each digitizer is configured to undersample each of the plurality of analog subband signals to produce a digital baseband subband signal, filter each of the plurality of digital baseband subband signals, and combine the filtered digital baseband subband signals to produce a digitize signal.
4. A method of digitizing narrowcast signals in the forward path of an overlay system, the method comprising:
receiving a plurality of analog subband signals of a plurality of analog signals, respectively, wherein each analog signal represents one or more digital bit streams that have been QAM encoded and modulated;
for each analog signal of the plurality of analog signals, digitizing each of the plurality of analog subband signals and combining the plurality of digitized subband signals to produce a digitized signal;
converting the plurality of digitized signals to a plurality of optical signals; and
multiplexing the plurality of optical signals.
5. The method of claim 4 , wherein for each analog signal of the plurality of analog signals, digitizing each of the plurality of analog subband signals and combining the plurality of digitized subband signals to produce a digitized signal comprises downconverting each of the plurality of analog subband signals to an analog baseband subband signal, converting each of the plurality of analog baseband subband signals to a digital baseband subband signal, and combining the digital baseband subband signals to produce a digitize signal.
6. The method of claim 4 , wherein for each analog signal of the plurality of analog signals, digitizing each of the plurality of analog subband signals and combining the plurality of digitized subband signals to produce a digitized signal comprises undersampling each of the plurality of analog subband signals to produce a digital baseband subband signal, filtering each of the plurality of digital baseband subband signals, and combining the filtered digital baseband subband signals to produce a digitize signal.