127 lines
4.2 KiB
ReStructuredText
127 lines
4.2 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
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.. _frontend-properties:
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**************
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Property types
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**************
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Tuning into a Digital TV physical channel and starting decoding it
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requires changing a set of parameters, in order to control the tuner,
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the demodulator, the Linear Low-noise Amplifier (LNA) and to set the
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antenna subsystem via Satellite Equipment Control - SEC (on satellite
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systems). The actual parameters are specific to each particular digital
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TV standards, and may change as the digital TV specs evolves.
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In the past (up to DVB API version 3 - DVBv3), the strategy used was to have a
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union with the parameters needed to tune for DVB-S, DVB-C, DVB-T and
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ATSC delivery systems grouped there. The problem is that, as the second
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generation standards appeared, the size of such union was not big
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enough to group the structs that would be required for those new
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standards. Also, extending it would break userspace.
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So, the legacy union/struct based approach was deprecated, in favor
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of a properties set approach. On such approach,
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:ref:`FE_GET_PROPERTY and FE_SET_PROPERTY <FE_GET_PROPERTY>` are used
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to setup the frontend and read its status.
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The actual action is determined by a set of dtv_property cmd/data pairs.
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With one single ioctl, is possible to get/set up to 64 properties.
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This section describes the new and recommended way to set the frontend,
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with supports all digital TV delivery systems.
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.. note::
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1. On Linux DVB API version 3, setting a frontend was done via
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struct :c:type:`dvb_frontend_parameters`.
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2. Don't use DVB API version 3 calls on hardware with supports
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newer standards. Such API provides no suport or a very limited
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support to new standards and/or new hardware.
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3. Nowadays, most frontends support multiple delivery systems.
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Only with DVB API version 5 calls it is possible to switch between
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the multiple delivery systems supported by a frontend.
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4. DVB API version 5 is also called *S2API*, as the first
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new standard added to it was DVB-S2.
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**Example**: in order to set the hardware to tune into a DVB-C channel
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at 651 kHz, modulated with 256-QAM, FEC 3/4 and symbol rate of 5.217
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Mbauds, those properties should be sent to
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:ref:`FE_SET_PROPERTY <FE_GET_PROPERTY>` ioctl:
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:ref:`DTV_DELIVERY_SYSTEM <DTV-DELIVERY-SYSTEM>` = SYS_DVBC_ANNEX_A
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:ref:`DTV_FREQUENCY <DTV-FREQUENCY>` = 651000000
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:ref:`DTV_MODULATION <DTV-MODULATION>` = QAM_256
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:ref:`DTV_INVERSION <DTV-INVERSION>` = INVERSION_AUTO
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:ref:`DTV_SYMBOL_RATE <DTV-SYMBOL-RATE>` = 5217000
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:ref:`DTV_INNER_FEC <DTV-INNER-FEC>` = FEC_3_4
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:ref:`DTV_TUNE <DTV-TUNE>`
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The code that would that would do the above is show in
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:ref:`dtv-prop-example`.
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.. code-block:: c
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:caption: Example: Setting digital TV frontend properties
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:name: dtv-prop-example
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#include <stdio.h>
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#include <fcntl.h>
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#include <sys/ioctl.h>
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#include <linux/dvb/frontend.h>
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static struct dtv_property props[] = {
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{ .cmd = DTV_DELIVERY_SYSTEM, .u.data = SYS_DVBC_ANNEX_A },
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{ .cmd = DTV_FREQUENCY, .u.data = 651000000 },
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{ .cmd = DTV_MODULATION, .u.data = QAM_256 },
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{ .cmd = DTV_INVERSION, .u.data = INVERSION_AUTO },
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{ .cmd = DTV_SYMBOL_RATE, .u.data = 5217000 },
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{ .cmd = DTV_INNER_FEC, .u.data = FEC_3_4 },
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{ .cmd = DTV_TUNE }
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};
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static struct dtv_properties dtv_prop = {
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.num = 6, .props = props
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};
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int main(void)
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{
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int fd = open("/dev/dvb/adapter0/frontend0", O_RDWR);
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if (!fd) {
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perror ("open");
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return -1;
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}
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if (ioctl(fd, FE_SET_PROPERTY, &dtv_prop) == -1) {
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perror("ioctl");
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return -1;
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}
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printf("Frontend set\\n");
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return 0;
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}
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.. attention:: While it is possible to directly call the Kernel code like the
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above example, it is strongly recommended to use
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`libdvbv5 <https://linuxtv.org/docs/libdvbv5/index.html>`__, as it
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provides abstraction to work with the supported digital TV standards and
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provides methods for usual operations like program scanning and to
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read/write channel descriptor files.
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.. toctree::
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:maxdepth: 1
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fe_property_parameters
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frontend-stat-properties
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frontend-property-terrestrial-systems
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frontend-property-cable-systems
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frontend-property-satellite-systems
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frontend-header
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