Provided by: libltc-doc_1.3.0-1_all 
NAME
ltc.h - libltc - en+decode linear timecode
SYNOPSIS
#include <endian.h>
#include <stddef.h>
Data Structures
struct LTCFrame
struct LTCFrameExt
struct SMPTETimecode
Macros
#define LTC_FRAME_BIT_COUNT 80
Typedefs
typedef unsigned char ltcsnd_sample_t
typedef long long int ltc_off_t
typedef struct LTCFrame LTCFrame
typedef struct LTCFrameExt LTCFrameExt
typedef struct SMPTETimecode SMPTETimecode
typedef struct LTCDecoder LTCDecoder
typedef struct LTCEncoder LTCEncoder
Enumerations
enum LTC_TV_STANDARD { LTC_TV_525_60, LTC_TV_625_50, LTC_TV_1125_60, LTC_TV_FILM_24 }
enum LTC_BG_FLAGS { LTC_USE_DATE = 1, LTC_TC_CLOCK = 2, LTC_BGF_DONT_TOUCH = 4,
LTC_NO_PARITY = 8 }
Functions
void ltc_frame_to_time (SMPTETimecode *stime, LTCFrame *frame, int flags)
void ltc_time_to_frame (LTCFrame *frame, SMPTETimecode *stime, enum LTC_TV_STANDARD
standard, int flags)
void ltc_frame_reset (LTCFrame *frame)
int ltc_frame_increment (LTCFrame *frame, int fps, enum LTC_TV_STANDARD standard, int
flags)
int ltc_frame_decrement (LTCFrame *frame, int fps, enum LTC_TV_STANDARD standard, int
flags)
LTCDecoder * ltc_decoder_create (int apv, int queue_size)
int ltc_decoder_free (LTCDecoder *d)
void ltc_decoder_write (LTCDecoder *d, ltcsnd_sample_t *buf, size_t size, ltc_off_t
posinfo)
void ltc_decoder_write_float (LTCDecoder *d, float *buf, size_t size, ltc_off_t posinfo)
void ltc_decoder_write_s16 (LTCDecoder *d, short *buf, size_t size, ltc_off_t posinfo)
void ltc_decoder_write_u16 (LTCDecoder *d, unsigned short *buf, size_t size, ltc_off_t
posinfo)
int ltc_decoder_read (LTCDecoder *d, LTCFrameExt *frame)
void ltc_decoder_queue_flush (LTCDecoder *d)
int ltc_decoder_queue_length (LTCDecoder *d)
LTCEncoder * ltc_encoder_create (double sample_rate, double fps, enum LTC_TV_STANDARD
standard, int flags)
void ltc_encoder_free (LTCEncoder *e)
void ltc_encoder_set_timecode (LTCEncoder *e, SMPTETimecode *t)
void ltc_encoder_get_timecode (LTCEncoder *e, SMPTETimecode *t)
int ltc_encoder_inc_timecode (LTCEncoder *e)
int ltc_encoder_dec_timecode (LTCEncoder *e)
void ltc_encoder_set_frame (LTCEncoder *e, LTCFrame *f)
void ltc_encoder_get_frame (LTCEncoder *e, LTCFrame *f)
int ltc_encoder_get_buffer (LTCEncoder *e, ltcsnd_sample_t *buf)
ltcsnd_sample_t * ltc_encoder_get_bufptr (LTCEncoder *e, int *size, int flush)
void ltc_encoder_buffer_flush (LTCEncoder *e)
size_t ltc_encoder_get_buffersize (LTCEncoder *e)
int ltc_encoder_reinit (LTCEncoder *e, double sample_rate, double fps, enum
LTC_TV_STANDARD standard, int flags)
void ltc_encoder_reset (LTCEncoder *e)
int ltc_encoder_set_bufsize (LTCEncoder *e, double sample_rate, double fps)
int ltc_encoder_set_volume (LTCEncoder *e, double dBFS)
void ltc_encoder_set_filter (LTCEncoder *e, double rise_time)
int ltc_encoder_encode_byte (LTCEncoder *e, int byte, double speed)
void ltc_encoder_encode_frame (LTCEncoder *e)
void ltc_frame_set_parity (LTCFrame *frame, enum LTC_TV_STANDARD standard)
int parse_bcg_flags (LTCFrame *f, enum LTC_TV_STANDARD standard)
ltc_off_t ltc_frame_alignment (double samples_per_frame, enum LTC_TV_STANDARD standard)
Detailed Description
libltc - en+decode linear timecode
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:
Robin Gareus robin@gareus.org
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
Raw 80 bit LTC frame
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:
unsigned int binary_group_flag_bit0:1 indicate user-data char encoding, see table
above - bit 43
unsigned int binary_group_flag_bit1:1 indicate timecode is local time wall-clock, see
table above - bit 58
unsigned int binary_group_flag_bit2:1 indicate user-data char encoding (or parity with
25fps), see table above - bit 59
unsigned int biphase_mark_phase_correction:1 see note on Bit 27 in description and
ltc_frame_set_parity .
unsigned int col_frame:1 colour-frame: timecode intentionally synchronized to a colour
TV field sequence
unsigned int dfbit:1 indicated drop-frame timecode
unsigned int frame_tens:2 SMPTE framenumber BCD tens 0..3.
unsigned int frame_units:4 SMPTE framenumber BCD unit 0..9.
unsigned int hours_tens:2 SMPTE hours BCD tens 0..2.
unsigned int hours_units:4 SMPTE hours BCD unit 0..9.
unsigned int mins_tens:3 SMPTE minutes BCD tens 0..6.
unsigned int mins_units:4 SMPTE minutes BCD unit 0..9.
unsigned int secs_tens:3 SMPTE seconds BCD tens 0..6.
unsigned int secs_units:4 SMPTE seconds BCD unit 0..9.
unsigned int sync_word:16
unsigned int user1:4
unsigned int user2:4
unsigned int user3:4
unsigned int user4:4
unsigned int user5:4
unsigned int user6:4
unsigned int user7:4
unsigned int user8:4
struct LTCFrameExt
Extended LTC frame - includes audio-sample position offsets, volume, etc
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:
float biphase_tics[LTC_FRAME_BIT_COUNT] detailed timing info: phase of the LTC signal;
the time between each bit in the LTC-frame in audio-frames. Summing all 80 values in
the array will yield audio-frames/LTC-frame = (off_end - off_start + 1).
LTCFrame ltc the actual LTC frame. see LTCFrame
ltc_off_t off_end the sample in the stream corresponding to the end of the LTC frame.
ltc_off_t off_start the approximate sample in the stream corresponding to the start of
the LTC frame.
int reverse if non-zero, a reverse played LTC frame was detected. Since the frame was
reversed, it started at off_end and finishes as off_start (off_end > off_start).
(Note: in reverse playback the (reversed) sync-word of the next/previous frame is
detected, this offset is corrected).
ltcsnd_sample_t sample_max the maximum input sample signal for this frame (0..255)
ltcsnd_sample_t sample_min the minimum input sample signal for this frame (0..255)
double volume the volume of the input signal in dbFS
struct SMPTETimecode
Human readable time representation, decimal values.
Examples:
example_encode.c, ltcdecode.c, and ltcencode.c.
Data Fields:
unsigned char days day of month 1..31
unsigned char frame sub-second frame 0..(FPS - 1)
unsigned char hours hour 0..23
unsigned char mins minute 0..60
unsigned char months valid months are 1..12
unsigned char secs second 0..60
char timezone[6] the timezone 6bytes: '+HHMM' textual representation
unsigned char years LTC-date uses 2-digit year 00.99.
Macro Definition Documentation
#define LTC_FRAME_BIT_COUNT 80
Typedef Documentation
typedef long long int ltc_off_t
sample-count offset - 64bit wide
typedef struct LTCDecoder LTCDecoder
Opaque structure see: ltc_decoder_create, ltc_decoder_free
typedef struct LTCEncoder LTCEncoder
Opaque structure see: ltc_encoder_create, ltc_encoder_free
typedef struct LTCFrame LTCFrame
see LTCFrame
typedef struct LTCFrameExt LTCFrameExt
see LTCFrameExt
typedef unsigned char ltcsnd_sample_t
default audio sample type: 8bit unsigned (mono)
typedef struct SMPTETimecode SMPTETimecode
see SMPTETimecode
Enumeration Type Documentation
enum LTC_BG_FLAGS
encoder and LTCframe <> timecode operation flags
Enumerator
LTC_USE_DATE
LTCFrame <> SMPTETimecode converter and LTCFrame increment/decrement use date, also
set BGF2 to '1' when encoder is initialized or re-initialized (unless
LTC_BGF_DONT_TOUCH is given)
LTC_TC_CLOCK
the Timecode is wall-clock aka freerun. This also sets BGF1 (unless
LTC_BGF_DONT_TOUCH is given)
LTC_BGF_DONT_TOUCH
encoder init or re-init does not touch the BGF bits (initial values after
initialization is zero)
LTC_NO_PARITY
parity bit is left untouched when setting or in/decrementing the encoder frame-
number
enum LTC_TV_STANDARD
the standard defines the assignment of the binary-group-flag bits basically only 25fps is
different, but other standards defined in the SMPTE spec have been included for
completeness.
Enumerator
LTC_TV_525_60
30fps
LTC_TV_625_50
25fps
LTC_TV_1125_60
30fps
LTC_TV_FILM_24
24fps
Function Documentation
LTCDecoder* ltc_decoder_create (int apv, int queue_size)
Create a new LTC decoder.
Parameters:
apv audio-frames per video frame. This is just used for initial settings, the speed is
tracked dynamically. setting this in the right ballpark is needed to properly decode
the first LTC frame in a sequence.
queue_size length of the internal queue to store decoded frames to SMPTEDecoderWrite.
Returns:
decoder handle or NULL if out-of-memory
Examples:
ltcdecode.c.
int ltc_decoder_free (LTCDecoder * d)
Release memory of decoder.
Parameters:
d decoder handle
Examples:
ltcdecode.c.
void ltc_decoder_queue_flush (LTCDecoder * d)
Remove all LTC frames from the internal queue.
Parameters:
d decoder handle
int ltc_decoder_queue_length (LTCDecoder * d)
Count number of LTC frames currently in the queue.
Parameters:
d decoder handle
Returns:
number of queued frames
int ltc_decoder_read (LTCDecoder * d, LTCFrameExt * frame)
Decoded LTC frames are placed in a queue. This function retrieves a frame from the queue,
and stores it at LTCFrameExt*
Parameters:
d decoder handle
frame the decoded LTC frame is copied there
Returns:
1 on success or 0 when no frames queued.
Examples:
ltcdecode.c.
void ltc_decoder_write (LTCDecoder * d, ltcsnd_sample_t * buf, size_t size, ltc_off_t posinfo)
Feed the LTC decoder with new audio samples.
Parse raw audio for LTC timestamps. Once a complete LTC frame has been decoded it is
pushed into a queue (ltc_decoder_read)
Parameters:
d decoder handle
buf pointer to ltcsnd_sample_t - unsigned 8 bit mono audio data
size
Examples:
ltcdecode.c.
void ltc_decoder_write_float (LTCDecoder * d, float * buf, size_t size, ltc_off_t posinfo)
Wrapper around ltc_decoder_write that accepts floating point audio samples. Note:
internally libltc uses 8 bit only.
Parameters:
d decoder handle
buf pointer to audio sample data
size number of samples to parse
posinfo (optional, recommended) sample-offset in the audio-stream.
void ltc_decoder_write_s16 (LTCDecoder * d, short * buf, size_t size, ltc_off_t posinfo)
Wrapper around ltc_decoder_write that accepts signed 16 bit audio samples. Note:
internally libltc uses 8 bit only.
Parameters:
d decoder handle
buf pointer to audio sample data
size number of samples to parse
posinfo (optional, recommended) sample-offset in the audio-stream.
void ltc_decoder_write_u16 (LTCDecoder * d, unsigned short * buf, size_t size, ltc_off_t
posinfo)
Wrapper around ltc_decoder_write that accepts unsigned 16 bit audio samples. Note:
internally libltc uses 8 bit only.
Parameters:
d decoder handle
buf pointer to audio sample data
size number of samples to parse
posinfo (optional, recommended) sample-offset in the audio-stream.
void ltc_encoder_buffer_flush (LTCEncoder * e)
reset the write-pointer of the encoder-buffer
Parameters:
e encoder handle
LTCEncoder* ltc_encoder_create (double sample_rate, double fps, enum LTC_TV_STANDARD standard,
int flags)
Allocate and initialize LTC audio encoder.
calls ltc_encoder_reinit internally see, see notes there.
Parameters:
sample_rate audio sample rate (eg. 48000)
fps video-frames per second (e.g. 25.0)
standard the TV standard to use for Binary Group Flag bit position
flags binary combination of LTC_BG_FLAGS
Examples:
example_encode.c, and ltcencode.c.
int ltc_encoder_dec_timecode (LTCEncoder * e)
Move the encoder to the previous timecode frame. This is useful for encoding reverse LTC.
uses ltc_frame_decrement() internally.
int ltc_encoder_encode_byte (LTCEncoder * e, int byte, double speed)
Generate LTC audio for given byte of the LTC-frame and place it into the internal buffer.
see ltc_encoder_get_buffer and ltc_encoder_get_bufptr
LTC has 10 bytes per frame: 0 <= bytecnt < 10 use SMPTESetTime(..) to set the current
frame before Encoding. see tests/encoder.c for an example.
The default output signal is @ -3dBFS (38..218 at 8 bit unsigned). see also
ltc_encoder_set_volume
if speed is < 0, the bits are encoded in reverse. slowdown > 10.0 requires custom buffer
sizes; see ltc_encoder_set_bufsize
Parameters:
e encoder handle
byte byte of the LTC-frame to encode 0..9
speed vari-speed, < 1.0 faster, > 1.0 slower ; must be != 0
Returns:
0 on success, -1 if byte is invalid or buffer overflow (speed > 10.0)
Examples:
example_encode.c.
void ltc_encoder_encode_frame (LTCEncoder * e)
Encode a full LTC frame at fixed speed. This is equivalent to calling
ltc_encoder_encode_byte 10 times for bytes 0..9 with speed 1.0.
Note: The internal buffer must be empty before calling this function. Otherwise it may
overflow. This is usually the case if it is read with ltc_encoder_get_buffer after calling
this function.
The default internal buffersize is exactly one full LTC frame at speed 1.0.
Parameters:
e encoder handle
Examples:
example_encode.c, and ltcencode.c.
void ltc_encoder_free (LTCEncoder * e)
Release memory of the encoder.
Parameters:
e encoder handle
Examples:
example_encode.c, and ltcencode.c.
int ltc_encoder_get_buffer (LTCEncoder * e, ltcsnd_sample_t * buf)
Copy the accumulated encoded audio to the given sample-buffer and flush the internal
buffer.
Parameters:
e encoder handle
buf place to store the audio-samples, needs to be large enough to hold
ltc_encoder_get_buffersize bytes
Returns:
the number of bytes written to the memory area pointed to by buf.
Examples:
example_encode.c.
size_t ltc_encoder_get_buffersize (LTCEncoder * e)
Query the length of the internal buffer. It is allocated to hold audio-frames for exactly
one LTC frame for the given sample-rate and frame-rate. ie. (1 + sample-rate / fps) bytes
Note this returns the total size of the buffer, not the used/free part. See also
ltc_encoder_get_bufptr
Parameters:
e encoder handle
Returns:
size of the allocated internal buffer.
Examples:
example_encode.c.
ltcsnd_sample_t* ltc_encoder_get_bufptr (LTCEncoder * e, int * size, int flush)
Retrieve a pointer to the accumulated encoded audio-data.
Parameters:
e encoder handle
size if set, the number of valid bytes in the buffer is stored there
flush call ltc_encoder_buffer_flush - reset the buffer write-pointer
Returns:
pointer to encoder-buffer
Examples:
example_encode.c, and ltcencode.c.
void ltc_encoder_get_frame (LTCEncoder * e, LTCFrame * f)
Low-level access to the encoder internal LTCFrame data
Parameters:
e encoder handle
f return LTC frame data
void ltc_encoder_get_timecode (LTCEncoder * e, SMPTETimecode * t)
Query the current encoder timecode.
Note: the decoder stores its internal state in an LTC-frame, this function converts that
LTC-Frame into SMPTETimecode on demand. see also ltc_encoder_get_frame.
Parameters:
e encoder handle
t is set to current timecode
int ltc_encoder_inc_timecode (LTCEncoder * e)
Move the encoder to the next timecode frame. uses ltc_frame_increment() internally.
Examples:
example_encode.c, and ltcencode.c.
int ltc_encoder_reinit (LTCEncoder * e, double sample_rate, double fps, enum LTC_TV_STANDARD
standard, int flags)
Change the encoder settings without re-allocating any library internal data structure
(realtime safe). changing the fps and or sample-rate implies a buffer flush, and biphase
state reset.
This call will fail if the internal buffer is too small to hold one full LTC frame. Use
ltc_encoder_set_bufsize to prepare an internal buffer large enough to accommodate all
sample_rate, fps combinations that you would like to re-init to.
The LTC frame payload data is not modified by this call, however, the flag-bits of the
LTC-Frame are updated: If fps equals to 29.97 or 30000.0/1001.0, the LTCFrame's 'dfbit'
bit is set to 1 to indicate drop-frame timecode.
Unless the LTC_BGF_DONT_TOUCH flag is set the BGF1 is set or cleared depending on
LTC_TC_CLOCK and BGF0,2 according to LTC_USE_DATE and the given standard. col_frame is
cleared and the parity recomputed (unless LTC_NO_PARITY is given).
Parameters:
e encoder handle
sample_rate audio sample rate (eg. 48000)
fps video-frames per second (e.g. 25.0)
standard the TV standard to use for Binary Group Flag bit position
flags binary combination of LTC_BG_FLAGS
Examples:
ltcencode.c.
void ltc_encoder_reset (LTCEncoder * e)
reset ecoder state. flushes buffer, reset biphase state
Parameters:
e encoder handle
int ltc_encoder_set_bufsize (LTCEncoder * e, double sample_rate, double fps)
Configure a custom size for the internal buffer.
This is needed if you are planning to call ltc_encoder_reinit() or if you want to keep
more than one LTC frame's worth of data in the library's internal buffer.
The buffer-size is (1 + sample_rate / fps) bytes. resizing the internal buffer will flush
all existing data in it - alike ltc_encoder_buffer_flush.
Parameters:
e encoder handle
sample_rate audio sample rate (eg. 48000)
fps video-frames per second (e.g. 25.0)
Returns:
0 on success, -1 if allocation fails (which makes the encoder unusable, call
ltc_encoder_free or realloc the buffer)
Examples:
ltcencode.c.
void ltc_encoder_set_filter (LTCEncoder * e, double rise_time)
Set encoder signal rise-time / signal filtering
LTC signal should have a rise time of 40us +/- 10 us. by default the encoder honors this
and low-pass filters the output depending on the sample-rate.
If you want a perfect square wave, set 'rise_time' to 0.
Note ltc_encoder_reinit resets the filter-time-constant to use the default 40us for the
given sample-rate, overriding any value previously set with ltc_encoder_set_filter
Parameters:
e encoder handle
rise_time the signal rise-time in us (10^(-6) sec), set to 0 for perfect square wave,
default 40.0
Examples:
ltcencode.c.
void ltc_encoder_set_frame (LTCEncoder * e, LTCFrame * f)
Low-level access to the internal LTCFrame data.
Note: be careful to about f->dfbit, the encoder sets this [only] upon initialization.
Parameters:
e encoder handle
f LTC frame data to use
void ltc_encoder_set_timecode (LTCEncoder * e, SMPTETimecode * t)
Set the encoder LTC-frame to the given SMPTETimecode. The next call to
ltc_encoder_encode_byte or ltc_encoder_encode_frame will encode this time to LTC audio-
samples.
Internally this call uses ltc_time_to_frame because the LTCEncoder operates on LTCframes
only. see als ltc_encoder_set_frame
Parameters:
e encoder handle
t timecode to set.
Examples:
example_encode.c, and ltcencode.c.
int ltc_encoder_set_volume (LTCEncoder * e, double dBFS)
Set the volume of the generated LTC signal
typically LTC is sent at 0dBu ; in EBU callibrated systems that corresponds to -18dBFS. -
by default libltc creates -3dBFS
since libltc generated 8bit audio-data, the minimum dBFS is about -42dB which corresponds
to 1 bit.
0dB corresponds to a signal range of 127 1..255 with 128 at the center.
Parameters:
e encoder handle
dBFS the volume in dB full-scale (<= 0.0)
Returns:
0 on success, -1 if the value was out of range
Examples:
ltcencode.c.
ltc_off_t ltc_frame_alignment (double samples_per_frame, enum LTC_TV_STANDARD standard)
LTCFrame sample alignment offset.
There is a relative offset of the LTC-Frame start and the TV-frame. The first bit of a LTC
frame corresponds to a specific line in the actual video frame. When decoding this offset
needs to be subtracted from the LTC-frame's audio-sample-time to match the TV-frame's
start position.
For film frames or HDV the offset is zero.
Parameters:
samples_per_frame audio-samples per timecode-frame (eg. 1920 = 48000/25)
standard the TV standard
Returns:
offset in samples
int ltc_frame_decrement (LTCFrame * frame, int fps, enum LTC_TV_STANDARD standard, int flags)
Decrement the timecode by one Frame (1/framerate seconds) and set the Frame's parity bit
accordingly (see ltc_frame_set_parity)
Parameters:
frame the LTC-timecode to decrement
fps integer framerate (for drop-frame-timecode set frame->dfbit and round-up the fps).
standard the TV standard to use for parity bit assignment if set to 1 the 25fps
standard is enabled and LTC Frame bit 59 instead of 27 is used for the parity. It only
has only has effect flag bit 4 (LTC_NO_PARITY) is cleared.
flags binary combination of LTC_BG_FLAGS - here only LTC_USE_DATE and LTC_NO_PARITY
are relevant. if the bit 0 is set (1) interpret user-data as date and decrement date
if timecode wraps at 24h. (Note: leap-years are taken into account, but since the year
is two-digit only, the 100,400yr rules are ignored. '00' is assumed to be year 2000
which was a leap year.) bit 3 (8) indicates that the parity bit should not be touched
Returns:
1 if timecode was wrapped around at 23:59:59:ff, 0 otherwise
int ltc_frame_increment (LTCFrame * frame, int fps, enum LTC_TV_STANDARD standard, int flags)
Increment the timecode by one Frame (1/framerate seconds) and set the Frame's parity bit
accordingly (see ltc_frame_set_parity)
Parameters:
frame the LTC-timecode to increment
fps integer framerate (for drop-frame-timecode set frame->dfbit and round-up the fps).
standard the TV standard to use for parity bit assignment if set to 1 the 25fps
standard is enabled and LTC Frame bit 59 instead of 27 is used for the parity. It only
has only has effect flag bit 4 (LTC_NO_PARITY) is cleared.
flags binary combination of LTC_BG_FLAGS - here only LTC_USE_DATE and LTC_NO_PARITY
are relevant. If the bit 0 (1) is set (1) interpret user-data as date and increment
date if timecode wraps after 24h. (Note: leap-years are taken into account, but since
the year is two-digit only, the 100,400yr rules are ignored. '00' is assumed to be
year 2000 which was a leap year.)
Returns:
1 if timecode was wrapped around after 23:59:59:ff, 0 otherwise
void ltc_frame_reset (LTCFrame * frame)
Reset all values of a LTC FRAME to zero, except for the sync-word (0x3FFD) at the end. The
sync word is set according to architecture (big/little endian). Also set the Frame's
parity bit accordingly (see ltc_frame_set_parity)
Parameters:
frame the LTCFrame to reset
void ltc_frame_set_parity (LTCFrame * frame, enum LTC_TV_STANDARD standard)
Set the parity of the LTC frame.
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 in every
Sync Word will be the same.
This is merely cosmetic; the motivation to keep the polarity of the waveform constant is
to make finding the Sync Word visibly (on a scope) easier.
There is usually no need to call this function directly. The encoder utility functions
ltc_time_to_frame, ltc_frame_increment and ltc_frame_decrement include a call to it.
Parameters:
frame the LTC to analyze and set or clear the biphase_mark_phase_correction bit.
standard If 1 (aka LTC_TV_625_50) , the 25fps mode (bit 59 - aka
binary_group_flag_bit2) is used, otherwise the 30fps, 24fps mode (bit 27 --
biphase_mark_phase_correction) is set or cleared.
void ltc_frame_to_time (SMPTETimecode * stime, LTCFrame * frame, int flags)
Convert binary LTCFrame into SMPTETimecode struct
Parameters:
stime output
frame input
flags binary combination of LTC_BG_FLAGS - here only LTC_USE_DATE is relevant. if
LTC_USE_DATE is set, the user-fields in LTCFrame will be parsed into the date variable
of SMPTETimecode. otherwise the date information in the SMPTETimecode is set to zero.
Examples:
ltcdecode.c.
void ltc_time_to_frame (LTCFrame * frame, SMPTETimecode * stime, enum LTC_TV_STANDARD
standard, int flags)
Translate SMPTETimecode struct into its binary LTC representation and set the LTC frame's
parity bit accordingly (see ltc_frame_set_parity)
Parameters:
frame output - the frame to be set
stime input - timecode input
standard the TV standard to use for parity bit assignment
flags binary combination of LTC_BG_FLAGS - here only LTC_USE_DATE and LTC_NO_PARITY
are relevant. if LTC_USE_DATE is given, user-fields in LTCFrame will be set from the
date in SMPTETimecode, otherwise the user-bits are not modified. All non-timecode
fields remain untouched - except for the parity bit unless LTC_NO_PARITY is given.
int parse_bcg_flags (LTCFrame * f, enum LTC_TV_STANDARD standard)
Parse Binary Group Flags into standard independent format: bit 0 (1) - BGF 0, bit 1 (2) -
BGF 1, bit 2 (4) - BGF 2
Parameters:
f LTC frame data analyze
standard the TV standard to use -- see LTCFrame for BGF assignment
Returns:
LTC Binary Group Flags
Author
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