STB

PCR: 

To enable a decoder to present synchronized content, such as audio tracks matching the associated video, at least once each 100 ms a Program Clock Reference, or PCR is transmitted in the adaptation field of an MPEG-2 transport stream packet. The PID with the PCR for an MPEG-2 program is identified by the pcr_pid value in the associated Program Map Table. The value of the PCR, when properly used, is employed to generate a system_timing_clock in the decoder. The STC or System Time Clock decoder, when properly implemented, provides a highly accurate time base that is used to synchronize audio and video elementary streams. Timing in MPEG2 references this clock. For example, the presentation time stamp (PTS) is intended to be relative to the PCR. The first 33 bits are based on a 90 kHz clock. The last 9 are based on a 27 MHz clock. The maximum jitter permitted for the PCR is +/- 500 ns.

The STB no longer merely decodes video and audio. It has now become the primary Home entertainment device, with increased complexity and functionality, which must:

  • deal with multiple digital streams, simultaneously
  • manage the viewing, recording, storage, and sharing of rich media content
  • access the Internet, supporting multiple browsers
  • function as the Home gateway for Broadband and pay TV services
  • have a 3D graphical user interface
  • support for TV widgets
  • operate with much less dynamic and standby power

 

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