NAME

SYNOPSIS

#include <endian.h>
#include <stddef.h>

Detailed Description

Linear (or Longitudinal) Timecode (LTC) is an encoding of timecode data as a Manchester-Biphase encoded audio signal. The audio signal is commonly recorded on a VTR track or other storage media.

libltc facilitates decoding and encoding of LTC from/to timecode, including SMPTE date support.

Author:

Copyright:

Copyright (C) 2006-2014 Robin Gareus robin@gareus.org

Copyright (C) 2008-2009 Jan Weiß jan@geheimwerk.de

Inspired by SMPTE Decoder - Maarten de Boer mdeboer@iua.upf.es

This program is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this library. If not, see http://www.gnu.org/licenses/.

Data Structure Documentation

struct LTCFrame

The datastream for each video frame of Longitudinal Timecode consists of eighty bit-periods.

At a frame-rate of 30 fps, the bit-rate corresponds to 30 [fps] * 80 [bits/f] = 2400 bits per second. The frequency for a stream of zeros would be 1.2 kHz and for a stream of ones it would be 2.4 kHz. With all commonly used video-frame-rates and audio-sample-rates, LTC timecode can be recorded easily into a audio-track.

In each frame, 26 of the eighty bits carry the SMPTE time in binary coded decimal (BCD).

These Bits are FRAME-UNITS, FRAME-TENS, SECS-UNITS, SECS-TENS, MINS-UNITS, MINS-TENS, HOURS-UNITS and HOURS-TENS. The BCD digits are loaded 'least significant bit first' (libltc takes care of the architecture specific alignment).

32 bits are assigned as eight groups of four USER-BITS (also sometimes called the 'Binary Groups'). This capacity is generally used to carry extra info such as reel number and/or date. The User Bits may be allocated howsoever one wishes as long as both Binary Group Flag Bits are cleared.

The function ltc_frame_to_time can interpret the user-bits as SMPTE Date+Timezone according to SMPTE 309M-1999. similarly ltc_time_to_frame will do the reverse.

The last 16 Bits make up the SYNC WORD. These bits indicate the frame boundary, the tape direction, and the bit-rate of the sync tone. The values of these Bits are fixed as 0011 1111 1111 1101

The Bi-Phase Mark Phase Correction Bit (Bit 27 or 59) may be set or cleared so that that every 80-bit word contains an even number of zeroes. This means that the phase of the pulse train in every Sync Word will be the same.

Bit 10 indicates drop-frame timecode. The Colour Frame Flag col.frm is Bit 11; if the timecode intentionally synchronized to a colour TV field sequence, this bit is set.

Bit 58 is not required for the BCD count for HOURS-TENS (which has a maximum value of two) and has not been given any other special purpose so remains unassigned. This Bit has been RESERVED for future assignment.

The Binary Group Flag Bits (bits 43 and 59) are two bits indicate the format of the User Bits data. SMPTE 12M-1999 defines the previously reserved bit 58 to signals that the time is locked to wall-clock within a tolerance of ± 0.5 seconds.

SMPTE 12M-1999 also changes the numbering schema of the BGF. (BGF1 was renamed to BGF2 and bit 58 becomes BGFB1)

To further complicate matters, the BGFB assignment as well as the biphase_mark_phase_correction (aka parity) bit depends on the timecode-format used.

         25 fps   24, 30 fps
 BGF0      27        43
 BGF1      58        58
 BGF2      43        59
 Parity    59        27

The variable naming chosen for the LTCFrame struct is based on the 24,30 fps standard.

The Binary Group Flag Bits should be used only as shown in the truth table below. The Unassigned entries in the table should not be used, as they may be allocated specific meanings in the future.

                                                BGF0      BGF1    BGF2
      user-bits                     timecode    Bit 43   Bit 58  Bit 59 (30fps, 24 fps)
                                   |        |   Bit 27   Bit 58  Bit 43 (25fps)
 No User Bits format specified     |   ?    |     0       0        0
 Eight-bit character set (1)       |   ?    |     1       0        0
 Date and Timezone set             |   ?    |     0       0        1
 Page/Line multiplex (2)           |   ?    |     1       0        1
 Character set not specified       |  clk   |     0       1        0
 Reserved                          |   ?    |     1       1        0
 Date and Timezone set             |  clk   |     0       1        1
 Page/Line multiplex (2)           |  clk   |     1       1        1

(1) ISO/IEC 646 or ISO/IEC 2022 character set. If the seven-bit ISO codes are being used, they shall be converted to eight-bit codes by setting the eighth bit to zero. 4 ISO codes can be encoded, user7 and user8 are to be used for the first code with LSB 7 and MSB in 8. the remaining ISO codes are to be distributed in the same manner to user5/6 user3/4 and user1/2 accordingly.

(2) The Page/Line indicates ANSI/SMPTE-262M is used for the user-bits. It is multiplex system that can be used to encode large amounts of data in the binary groups through the use of time multiplexing.

libltc does not use any of the BGF - except for the Parity bit which can be calculated and set with ltc_frame_set_parity. Setting and interpreting the BGF is left to the application using libltc. However libltc provides functionality to parse or set date and timezoe according to SMPTE 309M-1999.

further information: http://www.philrees.co.uk/articles/timecode.htm and http://www.barney-wol.net/time/timecode.html

Data Fields:

struct LTCFrameExt

Note: For TV systems, the sample in the LTC audio data stream where the LTC Frame starts is not necessarily at the same time as the video-frame which is described by the LTC Frame.

off_start denotes the time of the first transition of bit 0 in the LTC frame.

For 525/60 Television systems, the first transition shall occur at the beginning of line 5 of the frame with which it is associated. The tolerance is ± 1.5 lines.

For 625/50 systems, the first transition shall occur at the beginning of line 2 ± 1.5 lines of the frame with which it is associated.

Only for 1125/60 systems, the first transition occurs exactly at the vertical sync timing reference of the frame. ± 1 line.

Examples:
ltcdecode.c.

Data Fields:

struct SMPTETimecode

Examples:
example_encode.c, ltcdecode.c, and ltcencode.c.

Data Fields:

Macro Definition Documentation

Typedef Documentation

Enumeration Type Documentation

Function Documentation

Author