Specifications

Chapter 34: Synchronization Concepts 541
SMPTE and ISO/EBU Time Code
The “Where are we?” question refers to relative
position. To describe position, many profes-
sional audio, video, and multimedia devices and
programs use SMPTE (Society of Motion Picture
& Television Engineers) time code. In Europe, a
standard called ISO (International Standards Or-
ganization) time code, formerly called EBU (Eu-
ropean Broadcasters Union) time code, is gener-
ally used. These time code types are almost
exactly the same in terms of how they are repre-
sented electronically. However, ISO/EBU time
code works at a fixed rate of 25 fps (frames per
second) and SMPTE includes provisions for sev-
eral different frame rates. For the purposes of ex-
planation, this chapter will generally refer to
frame-measured timing information as “SMPTE
time code.” This description should be under-
stood to include SMPTE and ISO/EBU time code.
SMPTE Time Code Methods
There are two basic techniques used to record
SMPTE time code onto magnetic tape: LTC (Lin-
ear Time Code) and VITC (Vertical Interval Time
Code). LTC is recorded or generated on an audio
channel or a dedicated time code track of the au-
dio or video device. VITC is recorded within the
video signal in the video “blanking area” of each
video frame. VITC cannot be recorded on audio
tracks, so it has no application when working
with audio tape recorders, but it does offer fea-
tures for post production (such as still frame and
slow speed time code address reading).
There is also a non-SMPTE form of time code
called MIDI Time Code (MTC) that some devices
use to send timing information.
SMPTE Time Code Units
Time code describes locations in terms of hours,
minutes, seconds, frames, and subframes
(1/100th of a frame). The frame is used as a unit
of time measurement due to SMPTE time code’s
origin in film and video applications (see also
“Frame Count and Frame Rate” on page 541.)
Depending on the SMPTE frame rate, one frame
is equal to 1/24th, 1/25th, 1/29.97th, or 1/30th
of a second. For example, a video tape time code
reading of “01:12:27:15” would tell us that we
were at a position of one hour, twelve minutes,
twenty-seven seconds, and fifteen frames. How-
ever, this time address alone does not tell us
frame rate information.
Because SMPTE stores an absolute time reference
on the tape in the form of time code, any loca-
tion on that tape can be precisely located by de-
vices that read time code. Once the time code
has been recorded or striped on a tape, it pro-
vides a permanent positional reference that al-
lows Pro Tools to link the playback of an event
to an exact tape location. For example, with
time code synchronization, a gun shot sound ef-
fect can be played at the precise instant that the
gun’s flash appears on-screen.
Frame Count and Frame Rate
It is important to remember the difference be-
tween frame count and frame rate.
Frame count is the amount of frames that the
time code counts before ascending to the next
second count. Standard frame counts are:
Frame counts per frame rate
Frame Count fps
frames 0–23 24
frames 0–24 25
frames 0–29 30