Synchronization
From WikiRecording
In order for some types of audio devices to interoperate most effectively, particularly when it comes to digital devices, they must be synchronized.
Synchronization methods generally fall into one of three categories, Timecode (SMPTE), MIDI Clock, and Digital Clock, detailed below.
A basic generalization that can be made about all methods of synchronization is that there should only be one sender (sometimes called the master), but there can be multiple recipients of the synchronization source (sometimes called the "slave").
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Timecode
An electronic signal used to identify the precise location in time on media such as audio/video tape or tracks in a DAW. Also called SMPTE, for Society of Motion Picture and Television Engineers. SMPTE can be set at various framerates however 29.9 or 30 FPS with or without drop are most common.
Timecode is displayed in the format HH:MM:SS:Frames, where Frames is the frame rate format.
Timecode has three varieties: LTC, VITC, and MTC.
- LTC (Longitudinal Timecode)
The standard form of SMPTE, can be used to synchronize both audio and video information. LTC code is an audible signal (Square wave) that is recorded onto an available track on a tape deck (referred to as "striping the tape", DAW, or other recorder. On playback, other devices can listen to the LTC code on the track and synchronize to it. When someone refers to SMPTE they almost always refer to SMPTE in the form of LTC, otherwise it is specified as MTC or VITC.
- MTC (MIDI Timecode)
Typically used to synchronize audio/video to MIDI devices. MTC timecode is sent as a modified MIDI signal. Because of the way MIDI sends data the timing is not as exact at LTC. See the MIDI article for more information on MIDI and MIDI Latency.
- VITC (Vertical Interval Timecode)
Typically only used for video synchronization, with other VITC-compatible devices. Unlike MTC and LTC, VITC code is written to an unused line in the video signal rather than the audio signal. This gives it the advantage of being readable even if playback is paused.
Putting LTC code on a track is sometimes called striping the track.
MIDI Clock
MIDI Clock is used to synchronize multiple MIDI devices. Unlike timecode which is based on absolute time values, MIDI Clock is based on the tempo of the song, and sends exactly 24 clocks per quarter note. You can use this formula to calculate the number of clocks sent per minute at a given tempo:
So, at a tempo of 80 beats per minute, it sends roughly 1920 clock signals per minute. To determine the clock frequency in milliseconds:
At 80 bpm, a clock signal is sent every 31.25 milliseconds.
MIDI Clock should only be used for synchronizing MIDI devices. It is not precise enough for audio and video.
Digital Clock
Digital Clock uses the sample rate of the master device as its clock reference. So, for audio at 44.1 KHz, digital clock is precise to 1/44,100th of a second (approx. 0.02 ms). Higher sample rates mean correspondingly more accurate synchronization.
Digital clock signals can be sent using any of these formats:
- S/PDIF
Sony/Philips Digital Interface. Uses RCA or optical connectors. S/PDIF carries mono or stereo audio along with embedded clock information.
- AES/EBU
Audio Engineering Society/European Broadcast Union. Uses XLR connectors. Similar to S/PDIF in that it can carry mono or stereo audio along with clock information.
- ADAT
Alesis Digital Audio Technology. Uses fiber-optic cable. Can transmit up to eight mono audio signals, with embedded clock information.
- Word Clock
Uses 75-ohm coaxial cable with BNC connectors. Transmits clock signals only, with no audio.
References
Cubase SX/SL 3 Power! by Robert Guerin, Thomson Course Technology, ISBN 1592005373
