A digital IF demodulator receives and demodulates an analog IF input
signal to produce a digital audio signal and a digital video signal. The
digital IF demodulator includes an A/D converter, a first digital complex
mixer, a second digital complex mixer, and various digital filters. The
first digital complex mixer receives the output of the A/D converter and
down-converts the output of the A/D converter to baseband. Additionally,
the picture carrier is recovered from the output of the first digital
complex mixer, and fed back to a direct digital synthesizer to control
the tuning accuracy of the first digital complex mixer. More
specifically, a feedback loop is formed to so that the picture carrier is
down-converted to DC so as to control the tuning accuracy of the first
digital complex mixer. The complex output of the first complex mixer is
further processed using Nyquist filtering and other filtering to recover
the digital video signal. The digital audio signal is recovered by
further processing the output of the first digital complex mixer. With
the picture carrier located at DC, the audio signal is shifted off DC by
approximately 4.5 Mhz. A second complex mixer down-converts the output of
the first digital complex mixer so that the audio signal at 4.5 MHz is
down-converted to baseband. After filtering and demodulation, the digital
audio signal is recovered.