# Synchronization Concepts

As soon as you start handling audio on more than one device, it is important to understand and to think about synchronization : how to get the devices to have the same sense of time and speed.

However, there are two fundamentally different kinds of synchronization:

## Sample Clock

As outlined in the introductory concepts section, digital audio is created by taking a "sample" of an analog signal level on a periodic basis, say 48000 times per seconds (the "sample rate"). A dedicated clock (the "sample clock") ((actually, an oscillating crystal, but technology people call such things clocks)) "ticks" at that rate, and every time it does, a new sample is measured. The way the clock is used to convert digital audio back to an analog signal (i.e. to be sent to some loudspeakers) is more complex, but the clock is still an absolutely fundamental part of the mechanism.

Whenever you connect two digital audio devices together in order to move audio data from one to the other, you must ensure they share the same sample clock . Why is this necessary? The oscillating crystals used for the sample clock are generally very stable (they always tick at the same speed), but there are always minute differences in the speed that any two clocks tick at. When used by themselves, this makes no difference, but connect two digital audio devices together and these minute differences will eventually accumulate over time. Eventually, one of the devices will be trying to read a sample "in the middle" of the other device's tick, and the result is a small click or pop in the audio stream.

## Timeline Sync

The concept of a timeline comes up over and over again when working with a digital audio workstation, and also with video editing systems. By "timeline" we mean nothing more than some way to define a "name" for the point where certain sounds (and/or visual images) occur. When you work in Ardour's editor window, the rulers near the top provide one or more timelines in different units. You can look at the editor window and say "this sound starts at 1 minute 32 seconds" or "this tracks fades out starting at bar 13 beat 22".

But what happens when you want to share a timeline between two different devices? For example, you may want to run a hardware video editor in conjunction with ardour, and always have the visual and audio playback be at the same point "in time". How do they each know what "in time" means? How do they know where the other one is? A mechanism for answering these questions provides timeline synchronization .

Timeline synchronization is entirely different from sample clock synchronization. Two devices can share a sample clock, but never use timeline information. Two devices can be sharing timeline information, but run on different sample clocks - they might not even have sample clocks if they are analog devices.

## Word Clock

"Word Clock" is the name given to a signal used to distribute the "ticks" of a sample clock to multiple devices. Most digital audio devices that are intended for professional use have a word clock connector and a way to tell the device to use either its internal sample clock (for standalone use), or to use the word clock signal as the sample clock. Because of the electrical characteristics of the signal, it is very important that any length of cable used to distribute word clock is "terminated" with a 75 ohm resistor at both ends. Unfortunately, some devices include this terminator themselves, some contain a switchable resistor and some do not. Worse still, the user manuals for many devices do not provide any information on their termination configuration. It is often necessary to ask the manufacturer in cases where it is not made very obvious from marking near the word clock connectors on the device.

## Timecode

"Timecode" is a signal that contains positional or "timeline" information. There are several different kinds of timecode signal, but by far the most important is known as SMPTE. Its name is an acronym for the Society for Motion Picture T?? Engineering, and timecode is just one of the standards they defined, but its the most well known. Because of its origins in the film/video world, SMPTE is very centered on the time units that matter to film/video editors. The base unit is called a "frame" and corresponds to a single still image in a film or video. There are typically on the order of 20-30 frames per second, so the actual resolution of SMPTE timecode is not very good compared to audio-based units where there are tens of thousands of "frames" per second.

## SMPTE

SMPTE defines time using a combinations of hours, minutes, seconds, frames and subframes, combined with the frame rate. In a film/video environment, SMPTE is typically stored on the film/video media, and sent from the device used to play it. There are different ways of storing it on the media - you may come across terms like LTR and VTC - but the crucial idea to grasp is that the film/video has a timecode signal "stamped" into it, so that it is always possible to determine "what time it is" when any given image is visible.

SMPTE timecode is sent from one system to another as an analog audio signal. You could listen to it if you wanted to, though it sounds like a generally screeching and unpleasant noise. What the SMPTE standard defines is a way to encode and decode the hrs:mins:secs:frames:subframes time into or from this audio signal.

## MTC

The other very common form of timecode is known as "MTC" (MIDI Time Code). However, MTC is actually nothing more than a different way to transmit SMPTE timecode. It uses the exact same units as SMPTE timecode, but rather than send the signal as audio MTC defines a transmission method that uses a MIDI cabable and a data protocol. MTC consumes a measurable, but small, percentage of the available bandwidth on a MIDI cable (on the order of 2-3%). Most of the time, it is wise to use a single cable for MTC and MMC (MIDI Machine Control) and not share it with "musical" MIDI data (the kind that an instrument would send while being played).

## JACK Transport

For Ardour and other programs that use JACK, there is another method of doing timeline synchronization that is not based on SMPTE or MTC.