VIDIOC_G_MODULATOR·VIDIOC_S_MODULATOR ioctl

목적, 호출 형식과 인자

`VIDIOC_G_MODULATOR`와 `VIDIOC_S_MODULATOR`는 modulator 속성을 조회하거나 설정합니다. 두 명령은 `struct v4l2_modulator`를 사용하며 조회는 수정 가능한 포인터, 설정은 const 포인터를 받습니다.

`fd`는 `open()`이 반환한 파일 디스크립터이고 `argp`는 대상 modulator의 index와 속성을 담은 구조체를 가리킵니다.

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _VIDIOC_G_MODULATOR:

********************************************
ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR
********************************************

Name
====

VIDIOC_G_MODULATOR - VIDIOC_S_MODULATOR - Get or set modulator attributes

Synopsis
========

.. c:macro:: VIDIOC_G_MODULATOR

``int ioctl(int fd, VIDIOC_G_MODULATOR, struct v4l2_modulator *argp)``

.. c:macro:: VIDIOC_S_MODULATOR

``int ioctl(int fd, VIDIOC_S_MODULATOR, const struct v4l2_modulator *argp)``

Arguments
=========

``fd``
    File descriptor returned by :c:func:`open()`.

``argp``
    Pointer to struct :c:type:`v4l2_modulator`.

열거, audio 설정과 SDR

속성을 조회하려면 `index`를 초기화하고 `reserved`를 0으로 만든 뒤 G_MODULATOR를 호출합니다. 드라이버가 나머지를 채우며 index가 범위를 벗어나면 `EINVAL`입니다. 모든 modulator는 index 0부터 하나씩 늘려 `EINVAL`이 나올 때까지 열거합니다.

modulator의 쓰기 가능한 속성은 audio modulation 집합과 radio frequency입니다. audio subprogram은 `index`, `txsubchans`, `reserved`를 채워 S_MODULATOR로 설정합니다.

요청을 만족할 수 없으면 드라이버가 다른 audio modulation을 선택할 수 있습니다. 하지만 S_MODULATOR는 write-only여서 실제 선택된 modulation을 반환하지 않으므로 G_MODULATOR로 다시 조회해야 합니다.

SDR 전용 modulator type은 `V4L2_TUNER_SDR`과 `V4L2_TUNER_RF`입니다. SDR 장치에서는 `txsubchans = 0`이어야 하며 여기서 modulator는 SDR transmitter를 뜻합니다.

radio frequency를 바꾸는 작업은 이 ioctl이 아니라 `VIDIOC_S_FREQUENCY`를 사용합니다.

Description
===========

To query the attributes of a modulator applications initialize the
``index`` field and zero out the ``reserved`` array of a struct
:c:type:`v4l2_modulator` and call the
:ref:`VIDIOC_G_MODULATOR <VIDIOC_G_MODULATOR>` ioctl with a pointer to this structure. Drivers
fill the rest of the structure or return an ``EINVAL`` error code when the
index is out of bounds. To enumerate all modulators applications shall
begin at index zero, incrementing by one until the driver returns
EINVAL.

Modulators have two writable properties, an audio modulation set and the
radio frequency. To change the modulated audio subprograms, applications
initialize the ``index`` and ``txsubchans`` fields and the ``reserved``
array and call the :ref:`VIDIOC_S_MODULATOR <VIDIOC_G_MODULATOR>` ioctl. Drivers may choose a
different audio modulation if the request cannot be satisfied. However
this is a write-only ioctl, it does not return the actual audio
modulation selected.

:ref:`SDR <sdr>` specific modulator types are ``V4L2_TUNER_SDR`` and
``V4L2_TUNER_RF``. For SDR devices ``txsubchans`` field must be
initialized to zero. The term 'modulator' means SDR transmitter in this
context.

To change the radio frequency the
:ref:`VIDIOC_S_FREQUENCY <VIDIOC_G_FREQUENCY>` ioctl is available.

struct v4l2_modulator

audio capability flag는 현재 video standard가 바뀌어도 달라지지 않습니다. `txsubchans`는 tuner의 `rxsubchans` bit를 재사용하지만 의미는 다릅니다.

video output 장치는 1~3 channel의 analog 또는 PCM audio 입력을 가진다고 가정합니다. `txsubchans`는 변조할 channel과 stereo pilot tone 같은 audio subprogram indicator를 함께 선택합니다.

.. tabularcolumns:: |p{2.9cm}|p{2.9cm}|p{5.8cm}|p{2.9cm}|p{2.4cm}|

.. c:type:: v4l2_modulator

.. flat-table:: struct v4l2_modulator
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2 1 1

    * - __u32
      - ``index``
      - Identifies the modulator, set by the application.
    * - __u8
      - ``name``\ [32]
      - Name of the modulator, a NUL-terminated ASCII string.

	This information is intended for the user.
    * - __u32
      - ``capability``
      - Modulator capability flags. No flags are defined for this field,
	the tuner flags in struct :c:type:`v4l2_tuner` are
	used accordingly. The audio flags indicate the ability to encode
	audio subprograms. They will *not* change for example with the
	current video standard.
    * - __u32
      - ``rangelow``
      - The lowest tunable frequency in units of 62.5 KHz, or if the
	``capability`` flag ``V4L2_TUNER_CAP_LOW`` is set, in units of
	62.5 Hz, or if the ``capability`` flag ``V4L2_TUNER_CAP_1HZ`` is
	set, in units of 1 Hz.
    * - __u32
      - ``rangehigh``
      - The highest tunable frequency in units of 62.5 KHz, or if the
	``capability`` flag ``V4L2_TUNER_CAP_LOW`` is set, in units of
	62.5 Hz, or if the ``capability`` flag ``V4L2_TUNER_CAP_1HZ`` is
	set, in units of 1 Hz.
    * - __u32
      - ``txsubchans``
      - With this field applications can determine how audio sub-carriers
	shall be modulated. It contains a set of flags as defined in
	:ref:`modulator-txsubchans`.

	.. note::

	   The tuner ``rxsubchans`` flags  are reused, but the
	   semantics are different. Video output devices
	   are assumed to have an analog or PCM audio input with 1-3
	   channels. The ``txsubchans`` flags select one or more channels
	   for modulation, together with some audio subprogram indicator,
	   for example, a stereo pilot tone.
    * - __u32
      - ``type``
      - :cspan:`2` Type of the modulator, see :c:type:`v4l2_tuner_type`.
    * - __u32
      - ``reserved``\ [3]
      - Reserved for future extensions.

	Drivers and applications must set the array to zero.

Modulator Audio Transmission Flags

STEREO에서 입력이 한 channel이거나 두 channel이면서 SAP도 설정되면 channel 1을 좌·우 모두에 사용합니다. 드라이버가 stereo를 지원하지 않으면 mono로 fallback해야 합니다.

LANG1은 channel 1·2를 primary·secondary language로 사용하고 한 channel만 있으면 양쪽 language에 같은 channel을 씁니다. 한쪽 language만 인코딩할 수는 없습니다. audio matrix 또는 현재 standard가 bilingual audio를 지원하지 않으면 `EINVAL`을 반환하고 mono 또는 stereo로 fallback합니다.

MONO+SAP에서는 첫 channel이 mono, 마지막 channel이 SAP입니다. 1 channel이면 두 track에 같이 쓰고 3 channel이면 mono track은 channel 1·2의 down-mix입니다.

STEREO+SAP에서는 channel 1·2가 좌·우이고 channel 3이 SAP입니다. 2 channel이면 첫 channel을 좌·우에, 둘째를 SAP에 쓰며 1 channel이면 모든 track에 같은 channel을 씁니다.

SAP는 MONO 또는 STEREO와 반드시 결합해야 합니다. audio matrix나 video standard가 SAP를 허용하지 않으면 S_MODULATOR가 `EINVAL`을 반환하고 드라이버는 mono 또는 stereo로 fallback합니다.

.. tabularcolumns:: |p{6.0cm}|p{2.0cm}|p{9.3cm}|

.. cssclass:: longtable

.. _modulator-txsubchans:

.. flat-table:: Modulator Audio Transmission Flags
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 1 4

    * - ``V4L2_TUNER_SUB_MONO``
      - 0x0001
      - Modulate channel 1 as mono audio, when the input has more
	channels, a down-mix of channel 1 and 2. This flag does not
	combine with ``V4L2_TUNER_SUB_STEREO`` or
	``V4L2_TUNER_SUB_LANG1``.
    * - ``V4L2_TUNER_SUB_STEREO``
      - 0x0002
      - Modulate channel 1 and 2 as left and right channel of a stereo
	audio signal. When the input has only one channel or two channels
	and ``V4L2_TUNER_SUB_SAP`` is also set, channel 1 is encoded as
	left and right channel. This flag does not combine with
	``V4L2_TUNER_SUB_MONO`` or ``V4L2_TUNER_SUB_LANG1``. When the
	driver does not support stereo audio it shall fall back to mono.
    * - ``V4L2_TUNER_SUB_LANG1``
      - 0x0008
      - Modulate channel 1 and 2 as primary and secondary language of a
	bilingual audio signal. When the input has only one channel it is
	used for both languages. It is not possible to encode the primary
	or secondary language only. This flag does not combine with
	``V4L2_TUNER_SUB_MONO``, ``V4L2_TUNER_SUB_STEREO`` or
	``V4L2_TUNER_SUB_SAP``. If the hardware does not support the
	respective audio matrix, or the current video standard does not
	permit bilingual audio the :ref:`VIDIOC_S_MODULATOR <VIDIOC_G_MODULATOR>` ioctl shall
	return an ``EINVAL`` error code and the driver shall fall back to mono
	or stereo mode.
    * - ``V4L2_TUNER_SUB_LANG2``
      - 0x0004
      - Same effect as ``V4L2_TUNER_SUB_SAP``.
    * - ``V4L2_TUNER_SUB_SAP``
      - 0x0004
      - When combined with ``V4L2_TUNER_SUB_MONO`` the first channel is
	encoded as mono audio, the last channel as Second Audio Program.
	When the input has only one channel it is used for both audio
	tracks. When the input has three channels the mono track is a
	down-mix of channel 1 and 2. When combined with
	``V4L2_TUNER_SUB_STEREO`` channel 1 and 2 are encoded as left and
	right stereo audio, channel 3 as Second Audio Program. When the
	input has only two channels, the first is encoded as left and
	right channel and the second as SAP. When the input has only one
	channel it is used for all audio tracks. It is not possible to
	encode a Second Audio Program only. This flag must combine with
	``V4L2_TUNER_SUB_MONO`` or ``V4L2_TUNER_SUB_STEREO``. If the
	hardware does not support the respective audio matrix, or the
	current video standard does not permit SAP the
	:ref:`VIDIOC_S_MODULATOR <VIDIOC_G_MODULATOR>` ioctl shall return an ``EINVAL`` error code and
	driver shall fall back to mono or stereo mode.
    * - ``V4L2_TUNER_SUB_RDS``
      - 0x0010
      - Enable the RDS encoder for a radio FM transmitter.

반환값과 오류

성공하면 0을 반환합니다. 오류가 발생하면 -1을 반환하고 `errno`를 설정하며 Generic Error Codes 장의 공통 오류도 적용됩니다.

Return Value
============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.

EINVAL
    The struct :c:type:`v4l2_modulator` ``index`` is
    out of bounds.