A Set Top Box system would include:
DSP - performs MPEG encoder/decoder, and voice/AC3/MPEG audio processing.
RF Demodulation - performs COFDM/QAM/QPSK demodulation, Forward Error Correction (FEC) and video mux.
MCU - controls system electronic, network, and user interface.
Memory - stores executing code and data/parameters.
Video Interface - selects video source to be decoded/encoded by ADC/DAC and DSP. Both the video front end and output stages require high-performance op-amps to amplify the video signal.
Audio Interface - allows audio to be digitized by the audio codec and processed by DSP to provide high-quality audio for MPEG/AC3 requirements.

User Interface - allows the user to communicate with system network, control video/audio I/O sources, and perform voice/data communication.
Broadband & Wireless Connectivity - allows interactive TV, gaming, streaming audio and video, voice over IP (VoIP), and video telephony to transmit and receive through ADSL, cable modem, and 802.11 interface.
Power Conversion - converts the input power from the AC adaptor to run various functional blocks.