Psvbi

From M1Research

Revision as of 18:37, 7 January 2006 by Verem (Talk | contribs)
Jump to: navigation, search

psvbi - Pseudo VBI

Contents

Overview

psvbi software complex was developed for trasmitting control commands in TV video signal. Data block coded into some lines of video signal and could be used as marks for remote stations or analytic system. It much easy to implement and control (and diag) instead of real VBI due to high cost of equipment.

How does it works

Base Theory

Data to transmit coded by 14 bytes block. Each block transmitted in one video frame. Data encoded as luminance part of video signal (most stable to MPEG encoding/decoding) in some video lines (in our case we prefer to use top 2..4 lines - not visible on home equipment, only professional monitors with turned on underscan area could be used to SEE if data real present). During coding data used Hamming Error Correction Code and permutations alorithms to make more stable transmitting data, but some times it was noticed false data detection :-(((, use control sum algorithms in analizator to check if it's your real data packed :-))).

Embedding data

Embedding data performed by VirtualDub software (real greate program). Embedding and encoding data performed by our plug-in.

Make sure you have VirtualDub and copied pseudovbi.vdf to VirtualDub's plugin folder.

Start VirtualDub, open desired video file (in our case we use Matrox Digisuite for capturing and playback). Open Video -> Filters and select pseudovbi plugin. As result you can see plugin dialog window:

Psvbi Plugin Dialog.png

Encoding/Embeding parametes:

  • Start encoding from - define start frame number of video to insert data;
  • Frames to encode - number of frames (duration on frames) of encoded data block;
  • Data block - hexadiminal presentation of 14 bytes lenght data block;
  • Setup/"Luma" level - defines value is used to encode data (0...255);
  • Inserting Position - this blocks define where data will be inserted (position on screen). We prefer use top 2 rows (video lines) to insert;
    • BLOCK - enable/disable insert block;
    • COLUMN - start position (from left) where data begin to inserts
    • ROW - row/line number where data begin to inserts;

Result of plugin: (fragment of VirtualDub program interface)

Psvbi Plugin Result.png

As result you can see a dot sequence in first 2 lines of frame. Now data block embedded. You can print this video to tape or video server.

Demembedding of data

Dembedding of data performed by another part of software.

This software capture video using V4L2 base interface (we use bttv878 based capture card) perform alization and in a case of detection datablock it call plugins method that perform higher level data decoding (application level).

Application startup parameters:

[root@dev-2 decoder]# ./psvbid --help
Usage: psvbid <args>
args:
        --demonize=N    0-no daemon mode, 1-demonize (default 0)
        --white-level=N White level of signal (default 96)
        --pos-x-start=N left 'x' position of search block (default 10)
        --pos-x-stop=N  right 'x' position of search block (default 30)
        --pos-y-start=N top 'y' position of search block (default 0)
        --pos-y-stop=N  bottom 'y' position of search block (default 10)
        --video-dev=S   video device to grab video from (default '/dev/video0')
        --load-plugin=S load handler plugin (multiple allowed, max 10)
        --verbose-video-init    show more info during init v4l2 device (default 0)
        --help  Show program usage

Real application startup:

[root@dev-2 decoder]# ./psvbid --load-plugin=m1cmds --verbose-video-init --pos-y-stop=5

API interface for own analizator

Demebedder (psvbid) application perform all operation on "transport" level. To operate on higher level (application) software use plugins set. Each new task required new plugin developing (depends on application level data usage).

Basic plugin structure:

Include required headers:

#include <stdio.h>
#include "psvbi.h"
#include "psvbid-plugins.h"

Plugins is dynamic loadable library. You can use my_init and my_fini to initialize data structures:

void __attribute__ ((constructor)) my_init(void)
{
    printf("debug::my_init\n");
};

void __attribute__ ((destructor)) my_fini(void)
{
    printf("debug::my_fini\n");
};

After detecting of code block (or fail to detect) main program calls function detect_ok (or detect_ok, that you can make empty) of all plugins chain loaded:

static int detect_ok(struct psvbi_block* block)
{
    // some code to operate with block
    // calc checksum, check data is correct
    return 0;
};
static int detect_fail(struct psvbi_block* block)
{
    return 0;
};

Argument supplied to functions is struct psvbi_block:

struct psvbi_block
{
    // in/out datas
    unsigned char decoded[PSVBI_DATA_LEN];              // data block to coded/restored
    unsigned char encoded[PSVBI_PACKET_LEN_PACKED];     // coded data block
                                                                                                                                               
    // statistic
    unsigned char decode_errors[PSVBI_DATA_LEN];        // errors numbers for bytes
    unsigned char decode_corrections[PSVBI_DATA_LEN];   // number of corrected bit
    int errors;                                         // total errors count during decoding
    int corrections;                                    // total count of corrected bits
                                                                                                                                               
    // decoding/coding controls
    int _h;                                             // horizontal and vertical positions of
    int _v;                                             // found block
/*
    int state;                                          // state-machine controls
    void* data;                                         // input - output data
    int offset;                                         // offset from begining
*/
};

At last you should create a plugin description struct, that define methods and name (classic):

/* ---------------------------------------------------------------------------
                                                                                                                                               
    Define plugin attributest
                                                                                                                                               
----------------------------------------------------------------------------*/
                                                                                                                                               
struct psvbid_plugin PLUGIN =
{
    .name = "test1",
    .ok = detect_ok,
    .fail = NULL        // detect_fail
};
                                                                                                                                               

Software

Version 1.0

Real example

This is most interesting part of whole this story. I try to describe step by step system configuration and creation additional software plugins.

Task

There is playback videoserver that accept command throw RS-232 interface to start/stop playback. There are a lot of video materials that should be replaced by another one that supplied by videoserver.

Videoserver Control

Lets next commands set / config options will be default to control videoserver.

Interface RS-232:
    BaudRate = (UINT) CBR_38400
    ByteSize = (BYTE) 8
    Parity = (BYTE) ODDPARITY
    StopBits = (BYTE) ONESTOPBIT 

Control commands (fixed length, 4 byte command with STX and ETX)
     02    01    01    FF
    ----  ----  ----  ----
    STX   CMD1  CMD2  ETX

    where:
        STX - start head
        CMD1,CMD2 - 2 byte command code
        ETX - end of command

Reply (sent from videoserver, 2 byte block) :
    04    00   - ACK
    05    XX   - NACK (where XX is error code)

List of available commands:
    CMD1  CMD2
    ----  ----
    01    01   - start playback (and control MCS)
    02    01   - stop playback (and control MCS)

Command encoding

This is a little agreement about command encoding/embedding into video stream:

# REPLACE
02 01 01 FF 02 01 01 FF 02 01 01 FF 19 78
                                                                                                                                               
# RETURN
02 02 01 FF 02 02 01 FF 02 02 01 FF 19 78
                                                                                                                                               
# INCORRECT (not tripled data)
01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E
                                                                                                                                               
# INCORRECT (incorrect command)
02 02 01 FA 02 02 01 FA 02 02 01 FA 19 78
                                                                                                                                               
# INCORRECT (unknown command)
02 03 03 FF 02 03 03 FF 02 03 03 FF 19 78
                                                                                                                                               
***NB
command interval is 100 frames
command repeated in 2 frames
luma 128
blocks (24,0),(25,1)

This defines data blocks, that makes videos "marked". (Incorrect commands used with test video to check system for correct application level error detection)

Writing own plugin

Now wer are ready to write own plugin for translating commands from videostream into commands set accepted by videoserver:

  • 1. Create a skeleton application (using debug.c plugin);
  • 2. Add initialization section to setup tty device;
  • 3. Create a block code to check if command correct.
  • 4. Create a block code to operate with videoserver, detect incorrect reply and timeouts.

As result you can see the file: m1cmds.c

Licence

Personal tools