Documentation/admin-guide/media/dvb_intro.rst

Source file repositories/reference/linux-study-clean/Documentation/admin-guide/media/dvb_intro.rst

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Documentation/admin-guide/media/dvb_intro.rst
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Support Tooling And Documentation
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.. SPDX-License-Identifier: GPL-2.0

==============================
Using the Digital TV Framework
==============================

Introduction
~~~~~~~~~~~~

One significant difference between Digital TV and Analogue TV that the
unwary (like myself) should consider is that, although the component
structure of DVB-T cards are substantially similar to Analogue TV cards,
they function in substantially different ways.

The purpose of an Analogue TV is to receive and display an Analogue
Television signal. An Analogue TV signal (otherwise known as composite
video) is an analogue encoding of a sequence of image frames (25 frames
per second in Europe) rasterised using an interlacing technique.
Interlacing takes two fields to represent one frame. Therefore, an
Analogue TV card for a PC has the following purpose:

* Tune the receiver to receive a broadcast signal
* demodulate the broadcast signal
* demultiplex the analogue video signal and analogue audio
  signal.

  .. note::

     some countries employ a digital audio signal
     embedded within the modulated composite analogue signal -
     using NICAM signaling.)

* digitize the analogue video signal and make the resulting datastream
  available to the data bus.

The digital datastream from an Analogue TV card is generated by
circuitry on the card and is often presented uncompressed. For a PAL TV
signal encoded at a resolution of 768x576 24-bit color pixels over 25
frames per second - a fair amount of data is generated and must be
processed by the PC before it can be displayed on the video monitor
screen. Some Analogue TV cards for PCs have onboard MPEG2 encoders which
permit the raw digital data stream to be presented to the PC in an
encoded and compressed form - similar to the form that is used in
Digital TV.

The purpose of a simple budget digital TV card (DVB-T,C or S) is to
simply:

* Tune the received to receive a broadcast signal. * Extract the encoded
  digital datastream from the broadcast signal.
* Make the encoded digital datastream (MPEG2) available to the data bus.

The significant difference between the two is that the tuner on the
analogue TV card spits out an Analogue signal, whereas the tuner on the
digital TV card spits out a compressed encoded digital datastream. As
the signal is already digitised, it is trivial to pass this datastream
to the PC databus with minimal additional processing and then extract
the digital video and audio datastreams passing them to the appropriate
software or hardware for decoding and viewing.

Getting the card going
~~~~~~~~~~~~~~~~~~~~~~

The Device Driver API for DVB under Linux will the following
device nodes via the devfs filesystem:

* /dev/dvb/adapter0/demux0
* /dev/dvb/adapter0/dvr0
* /dev/dvb/adapter0/frontend0

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