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Subject[RFC, WIP, v7 4/4] media: Documentation: vidtv: Add ReST documentation for vidtv
From: "Daniel W. S. Almeida" <>

Add documentation for the Virtual Digital TV driver (vidtv) in the
Restructured Text (ReST) format.

This discusses:
- What is vidtv
- Why vidtv is needed
- How to build and run vidtv
- How vidtv is structured
- How to test vidtv
- How to improve vidtv

Signed-off-by: Daniel W. S. Almeida <>
.../driver-api/media/drivers/index.rst | 1 +
.../driver-api/media/drivers/vidtv.rst | 415 ++++++++++++++++++
2 files changed, 416 insertions(+)
create mode 100644 Documentation/driver-api/media/drivers/vidtv.rst

diff --git a/Documentation/driver-api/media/drivers/index.rst b/Documentation/driver-api/media/drivers/index.rst
index 0df85fc966056..5f340cfdb4cc2 100644
--- a/Documentation/driver-api/media/drivers/index.rst
+++ b/Documentation/driver-api/media/drivers/index.rst
@@ -35,4 +35,5 @@ Digital TV drivers

+ vidtv
diff --git a/Documentation/driver-api/media/drivers/vidtv.rst b/Documentation/driver-api/media/drivers/vidtv.rst
new file mode 100644
index 0000000000000..698be9350e56b
--- /dev/null
+++ b/Documentation/driver-api/media/drivers/vidtv.rst
@@ -0,0 +1,415 @@
+.. SPDX-License-Identifier: GPL-2.0
+vidtv: Virtual Digital TV driver
+Author: Daniel W. S. Almeida <>, June 2020.
+Vidtv is a virtual DVB driver that aims to serve as a reference for driver
+writers by serving as a template. It also validates the existing media DVB
+APIs, thus helping userspace application writers.
+Currently, it consists of:
+- A fake tuner driver, which will report a bad signal quality if the chosen
+ frequency is too far away from a table of valid frequencies for a
+ particular delivery system.
+- A fake demod driver, which will constantly poll the fake signal quality
+ returned by the tuner, simulating a device that can lose/reacquire a lock
+ on the signal depending on the CNR levels.
+- A fake bridge driver, which is the module responsible for modprobing the
+ fake tuner and demod modules and implementing the demux logic. This module
+ takes parameters at initialization that will dictate how the simulation
+ behaves.
+- Code reponsible for encoding a valid MPEG Transport Stream, which is then
+ passed to the bridge driver. This fake stream contains some hardcoded content.
+ For now, we have a single, audio-only channel containing a single MPEG
+ Elementary Stream, which in turn contains a SMPTE 302m encoded sine-wave.
+ Note that this particular encoder was chosen because it is the easiest
+ way to encode PCM audio data in a MPEG Transport Stream.
+Building vidtv
+vidtv is a test driver and thus is **not** enabled by default when
+compiling the kernel.
+In order to enable compilation of vidtv:
+- Enable **DVB_TEST_DRIVERS**, then
+- Enable **DVB_VIDTV**
+When compiled as a module, expect the following .ko files:
+- dvb_vidtv_tuner.ko
+- dvb_vidtv_demod.ko
+- dvb_vidtv_bridge.ko
+Running vidtv
+When compiled as a module, run::
+ modprobe dvb_vidtv_bridge
+That's it! The bridge driver will initialize the tuner and demod drivers as
+part of its own initialization.
+You can optionally define some command-line arguments to vidtv.
+Command-line arguments to vidtv
+Below is a list of all arguments that can be supplied to vidtv:
+ Probability of losing the TS lock if the signal quality is bad.
+ This probability be used by the fake demodulator driver to
+ eventually return a status of 0 when the signal quality is not
+ good.
+ Probability recovering the TS lock when the signal improves. This
+ probability be used by the fake demodulator driver to eventually
+ return a status of 0x1f when/if the signal quality improves.
+ Simulate a power up delay. Default: 0.
+ Simulate a tune delay. Default 0.
+ Valid DVB-T frequencies to simulate.
+ Valid DVB-C frequencies to simulate.
+ Valid DVB-C frequencies to simulate.
+ Maximum shift in HZ allowed when tuning in a channel.
+ How often to send SI packets. Default: 40ms.
+ How often to send PCR packets. Default: 40ms.
+ Attempt to maintain this bit rate by inserting TS null packets, if
+ necessary. Default: 4096.
+ PCR PID for all channels. Default: 0x200.
+ Size for the TS buffer in multiples of 188 bytes. Default: 20
+ packets.
+vidtv internal structure
+The kernel modules are split in the following way:
+ Implements a fake tuner DVB driver.
+ Implements a fake demodulator DVB driver.
+ Implements a bridge driver.
+The MPEG related code is split in the following way:
+ Code to work with MPEG TS packets, such as TS headers, adaptation
+ fields, PCR packets and NULL packets.
+ This is the PSI generator. PSI packets contain general information
+ about a MPEG Transport Stream. A PSI generator is needed so
+ userspace apps can retrieve information about the Transport Stream
+ and eventually tune into a (dummy) channel.
+ Because the generator is implemented in a separate file, it can be
+ reused elsewhere in the media subsystem.
+ Currently vidtv supports working with 3 PSI tables: PAT, PMT and
+ SDT.
+ The specification for PAT and PMT can be found in *ISO 13818-1:
+ Systems*, while the specification for the SDT can be found in *ETSI
+ EN 300 468: Specification for Service Information (SI) in DVB
+ systems*.
+ It isn't strictly necessary, but using a real TS file helps when
+ debugging PSI tables. Vidtv currently tries to replicate the PSI
+ structure found in this file: `TS1Globo.ts
+ <>`_.
+ A good way to visualize the structure of streams is by using
+ `DVBInspector <>`_.
+ Implements the PES logic to convert encoder data into MPEG TS
+ packets. These can then be fed into a TS multiplexer and eventually
+ into userspace.
+ An interface for vidtv encoders. New encoders can be added to this
+ driver by implementing the calls in this file.
+ Implements a S302M encoder to make it possible to insert PCM audio
+ data in the generated MPEG Transport Stream. The relevant
+ specification is available online as *SMPTE 302M-2007: Television -
+ Mapping of AES3 Data into MPEG-2 Transport Stream*.
+ The resulting MPEG Elementary Stream is conveyed in a private
+ stream with a S302M registration descriptor attached.
+ This shall enable passing an audio signal into userspace so it can
+ be decoded and played by media software. The corresponding decoder
+ in ffmpeg is located in 'libavcodec/s302m.c' and is experimental.
+ Implements a 'channel' abstraction.
+ When vidtv boots, it will create some hardcoded channels:
+ #. Their services will be concatenated to populate the SDT.
+ #. Their programs will be concatenated to populate the PAT
+ #. For each program in the PAT, a PMT section will be created
+ #. The PMT section for a channel will be assigned its streams.
+ #. Every stream will have its corresponding encoder polled in a
+ loop to produce TS packets.
+ These packets may be interleaved by the muxer and then delivered
+ to the bridge.
+ Implements a MPEG TS mux, loosely based on the ffmpeg
+ implementation in "libavcodec/mpegtsenc.c"
+ The muxer runs a loop which is responsible for:
+ #. Keeping track of the amount of time elapsed since the last
+ iteration.
+ #. Polling encoders in order to fetch 'elapsed_time' worth of data.
+ #. Inserting PSI and/or PCR packets, if needed.
+ #. Padding the resulting stream with NULL packets if
+ necessary in order to maintain the chosen bit rate.
+ #. Delivering the resulting TS packets to the bridge
+ driver so it can pass them to the demux.
+Testing vidtv with v4l-utils
+Using the tools in v4l-utils is a great way to test and inspect the output of
+vidtv. It is hosted here: `v4l-utils Documentation
+From its webpage::
+ The v4l-utils are a series of packages for handling media devices.
+ It is hosted at, and packaged
+ on most distributions.
+ It provides a series of libraries and utilities to be used to
+ control several aspect of the media boards.
+Start by installing v4l-utils and then modprobing vidtv::
+ modprobe dvb_vidtv_bridge
+If the driver is OK, it should load and its probing code will run. This will
+pull in the tuner and demod drivers.
+Using dvb-fe-tool
+The first step to check whether the demod loaded successfully is to run::
+ $ dvb-fe-tool
+This should return what is currently set up at the demod struct, i.e.::
+ static const struct dvb_frontend_ops vidtv_demod_ops = {
+ .delsys = {
+ },
+ .info = {
+ .name = "Dummy demod for DVB-T/T2/C/S/S2",
+ .frequency_min_hz = 51 * MHz,
+ .frequency_max_hz = 2150 * MHz,
+ .frequency_stepsize_hz = 62500,
+ .frequency_tolerance_hz = 29500 * kHz,
+ .symbol_rate_min = 1000000,
+ .symbol_rate_max = 45000000,
+ .caps = FE_CAN_FEC_1_2 |
+ FE_CAN_FEC_2_3 |
+ FE_CAN_FEC_3_4 |
+ FE_CAN_FEC_4_5 |
+ FE_CAN_FEC_5_6 |
+ FE_CAN_FEC_6_7 |
+ FE_CAN_FEC_7_8 |
+ FE_CAN_FEC_8_9 |
+ FE_CAN_QAM_16 |
+ FE_CAN_QAM_64 |
+ FE_CAN_QAM_32 |
+ FE_CAN_QAM_128 |
+ FE_CAN_QAM_256 |
+ }
+ ....
+For more information on dvb-fe-tools check its online documentation here:
+`dvb-fe-tool Documentation
+Using dvb-scan
+In order to tune into a channel and read the PSI tables, we can use dvb-scan.
+For this, one should provide a configuration file known as a 'scan file',
+here's an example::
+ [Channel]
+ FREQUENCY = 330000000
+ SYMBOL_RATE = 6940000
+.. note::
+ The parameters depend on the video standard you're testing.
+.. note::
+ Vidtv is a fake driver and does not validate much of the information
+ in the scan file. Just specifying 'FREQUENCY' and 'DELIVERY_SYSTEM'
+ should be enough for DVB-T/DVB-T2. For DVB-S/DVB-C however, you
+ should also provide 'SYMBOL_RATE'.
+You can browse scan tables online here: `dvb-scan-tables
+Assuming this channel is named 'channel.conf', you can then run::
+ $ dvbv5-scan dresden_dvbc_channel.conf
+For more information on dvb-scan, check its documentation online here:
+`dvb-scan Documentation <>`_.
+Using dvb-zap
+dvbv5-zap is a command line tool that can be used to record MPEG-TS to disk. The
+typical use is to tune into a channel and put it into record mode. The example
+below - which is taken from the documentation - illustrates that::
+ $ dvbv5-zap -c dvb_channel.conf "trilhas sonoras" -r
+ using demux '/dev/dvb/adapter0/demux0'
+ reading channels from file 'dvb_channel.conf'
+ service has pid type 05: 204
+ tuning to 573000000 Hz
+ audio pid 104
+ dvb_set_pesfilter 104
+ Lock (0x1f) Quality= Good Signal= 100.00% C/N= -13.80dB UCB= 70 postBER= 3.14x10^-3 PER= 0
+ DVR interface '/dev/dvb/adapter0/dvr0' can now be opened
+The channel can be watched by playing the contents of the DVR interface, with
+some player that recognizes the MPEG-TS format, such as *mplayer* or *vlc*.
+By playing the contents of the stream one can visually inspect the workings of
+vidtv, e.g.::
+ $ mplayer /dev/dvb/adapter0/dvr0
+For more information on dvb-zap check its online documentation here:
+`dvb-zap Documentation
+See also: `zap <>`_.
+What can still be improved in vidtv
+Add *debugfs* integration
+Although frontend drivers provide DVBv5 statistics via the .read_status
+call, a nice addition would be to make additional statistics available to
+userspace via debugfs, which is a simple-to-use, RAM-based filesystem
+specifically designed for debug purposes.
+The logic for this would be implemented on a separate file so as not to
+pollute the frontend driver. These statistics are driver-specific and can
+be useful during tests.
+The Siano driver is one example of a driver using
+debugfs to convey driver-specific statistics to userspace and it can be
+used as a reference.
+This should be further enabled and disabled via a Kconfig
+option for convenience.
+Add a way to test video
+Currently, vidtv can only encode PCM audio. It would be great to implement
+a barebones version of MPEG-2 video encoding so we can also test video. The
+first place to look into is *ISO 13818-2: Information technology — Generic
+coding of moving pictures and associated audio information — Part 2: Video*,
+which covers the encoding of compressed video in MPEG Transport Streams.
+This might optionally use the Video4Linux2 Test Pattern Generator, v4l2-tpg,
+which resides at::
+ drivers/media/common/v4l2-tpg/
+Add white noise simulation
+The vidtv tuner already has code to identify whether the chosen frequency
+is too far away from a table of valid frequencies. For now, this means that
+the demodulator can eventually lose the lock on the signal, since the tuner will
+report a bad signal quality.
+A nice addition is to simulate some noise when the signal quality is bad by:
+- Randomly dropping some TS packets. This will trigger a continuity error if the
+ continuity counter is updated but the packet is not passed on to the demux.
+- Updating the error statistics accordingly (e.g. BER, etc).
+- Simulating some noise in the encoded data.
 \ /
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