Editing MIDI timecode

{{Short description|SMPTE timecode as MIDI messages}}
{{More citations needed|date=November 2024}}
{{Use British English|date=December 2024}}

'''MIDI time code''' ('''MTC''') embeds the same timing information as standard [[SMPTE timecode]] as a series of small 'quarter-frame' [[MIDI]] messages. There is no provision for the user bits in the standard MIDI time code messages, and [[:en:SysEx#System Exclusive messages|SysEx]] messages are used to carry this information instead. The quarter-frame messages are transmitted in a sequence of eight messages, thus a complete timecode value is specified every two frames. If the MIDI data stream is running close to capacity, the MTC data may arrive a little behind schedule which has the effect of introducing a small amount of jitter. In order to avoid this it is ideal to use a completely separate MIDI port for MTC data. Larger full-frame messages, which encapsulate a frame worth of timecode in a single message, are used to locate to a time while timecode is not running. <ref>{{Cite web |title=SMPTE & MTC (MIDI Time Code) |url=https://www.soundonsound.com/techniques/smpte-mtc-midi-time-code |access-date=2024-11-29 |website=www.soundonsound.com}}</ref>

Unlike standard SMPTE timecode, MIDI timecode's quarter-frame, and full-frame messages carry a two-bit flag value that identifies the rate of the timecode, specifying it as either:
* 24 frame/s (standard rate for [[Frame rate|film work]]) 
* 25 frame/s (standard rate for [[PAL]] video)
* 29.97 frame/s (drop-frame timecode for [[NTSC]] video)
* 30 frame/s (non-drop timecode for NTSC video) <ref>{{Cite web |title=MIDI Time Code and MIDI Clock Explained |url=https://www.dummies.com/article/technology/software/music-recording-software/general-music-recording-software/midi-time-code-and-midi-clock-explained-179973/ |access-date=2024-11-29 |website=dummies |language=en}}</ref>

MTC distinguishes between film speed and video speed only by the rate at which timecode advances, not by the information contained in the timecode messages; thus, 29.97 frame/s dropframe is represented as 30 frame/s dropframe at 0.1% pulldown. <ref>{{Cite web |last=Schütte |first=Benjamin |last2=Bachmann |first2=Cristina |title=Timecode Standards |url=https://archive.steinberg.help/nuendo/v10/en/cubase_nuendo/topics/synchronization/synchronization_timecode_standards_c.html |access-date=2024-11-29 |website=archive.steinberg.help |language=en-US}}</ref>

MTC allows the synchronisation of a [[MIDI sequencer|sequencer]] or [[Digital audio workstation|DAW]] with other devices that can synchronise to MTC or for these devices to 'slave' to a tape machine that is striped with SMPTE. For this to happen a SMPTE to MTC converter needs to be employed. It is possible for a tape machine to synchronise to an MTC signal (if converted to SMPTE) if the tape machine is able to 'slave' to incoming timecode via motor control, which is a rare feature.

==Time code format==
The MIDI time code is 32 bits long, of which 24 are used, while 8 bits are unused and always zero.  Because the full-time code messages require that the most significant bits of each byte are zero (valid MIDI data bytes), there are really only 28 available bits and 4 spare bits.

Like most audiovisual timecodes such as [[SMPTE time code]], it encodes only time of day, repeating each 24 hours.  Time is given in units of hours, minutes, seconds, and [[Film frame|frame]]s.  There may be 24, 25, or 30 frames per second.

Unlike most other timecodes, the components are encoded in straight binary, not [[binary-coded decimal]].

Each component is assigned one byte:
; Byte 0 : <code>0rrhhhhh</code>: Rate (0–3) and hour (0–23).
:* <code>rr = 00</code>: 24 frames/s
:* <code>rr = 01</code>: 25 frames/s
:* <code>rr = 10</code>: 29.97 frames/s (SMPTE [[drop-frame timecode]])
:* <code>rr = 11</code>: 30 frames/s
; Byte 1 : <code>00mmmmmm</code>: Minute (0–59)
; Byte 2 : <code>00ssssss</code>: Second (0–59)
; Byte 3 : <code>000fffff</code>: Frame (0–29, or less at lower [[frame rate]]s)

=== Full time code ===
When there is a jump in the time code, a single full-time code is sent to synchronize attached equipment.  This takes the form of a special global system exclusive message:
: <code>F0 7F 7F 01 01 hh mm ss ff F7</code>
The manufacturer ID of <code>7F</code> indicates a real-time universal message, the channel of <code>7F</code> indicates it is a global broadcast.  The following ID of <code>01</code> identifies this is a time code type message, and the second <code>01</code> indicates it is a full-time code message.  The 4 bytes of time code follow.  Although MIDI is generally little-endian, the 4 time code bytes follow in big-endian order, followed by a <code>F7</code> "end of exclusive" byte.

After a jump, the time clock stops until the first following quarter-frame message is received.

=== Quarter-frame messages ===
When the time is running continuously, the 32-bit time code is broken into 8 4-bit pieces, and one piece is transmitted each quarter frame.  I.e. 96—120 times per second, depending on the frame rate. Since it takes eight quarter frames for a complete time code message, the complete SMPTE time is updated every two frames. A quarter-frame message consists of a status byte of 0xF1, followed by a single 7-bit data value: 3 bits to identify the piece, and 4 bits of partial time code.  When time is running forward, the piece numbers increment from 0–7; with the time that piece 0 is transmitted is the coded instant, and the remaining pieces are transmitted later.

If the MIDI data stream is being rewound, the piece numbers count backward.  Again, piece 0 is transmitted at the coded moment.

The time code is divided little-endian as follows:
{|class="wikitable"
|+MIDI time code pieces
! Piece # || Data byte || Significance
|-
! 0
| <code>0000&thinsp;ffff</code> || Frame number lsbits
|-
! 1
| <code>0001&thinsp;000f</code> || Frame number msbit
|-
! 2
| <code>0010&thinsp;ssss</code> || Second lsbits
|-
! 3
| <code>0011&thinsp;00ss</code> || Second msbits
|-
! 4
| <code>0100&thinsp;mmmm</code> || Minute lsbits
|-
! 5
| <code>0101&thinsp;00mm</code> || Minute msbits
|-
! 6
| <code>0110&thinsp;hhhh</code> || Hour lsbits
|-
! 7
| <code>0111&thinsp;0rrh</code> || Rate and hour msbit
|}

==See also==
*[[AES-EBU embedded timecode]]
*[[Burnt-in timecode]]
*[[CTL timecode]]
*[[DIN sync]]
*[[Linear timecode]]
*[[MIDI beat clock]]
*[[Rewritable consumer timecode]]
*[[Vertical interval timecode]]

==References==
{{reflist}}

==External links==
* [https://web.archive.org/web/20120212181214/http://home.roadrunner.com/~jgglatt/tech/mtc.htm MIDI Time Code information]
* [https://web.archive.org/web/20110629053759/http://web.media.mit.edu/~meyers/mcgill/multimedia/senior_project/MTC.html MIDI time code specification] 12 Feb 1987
* [http://www.somascape.org/midi/tech/spec.html#syscommsgs Guide to the MIDI Software Specification]

{{DEFAULTSORT:Midi Timecode}}
[[Category:MIDI standards]]
[[Category:Timecodes]]
[[Category:SMPTE standards]]