In this article, we explain the concept of frame edge sync in Pro Tools. If you’ve heard this term mentioned and want to know what it is, why and when you might want it and what you’ll need in order to get it working, read on!

What Is Frame Edge Sync?

If you import a video of a supported codec into a Pro Tools session and start it playing, it usually plays back without too much of a problem (usually!). However, while it might run in reasonably close sync with the audio, it’s likely that the video will be slightly off by up to half a frame or so, either early or late. This can actually fluctuate in one direction or the other from one playback to the next, potentially causing perceivable discrepancies with the audio/video synchronisation.

Video files are made up of frames. The frame rate describes how many of these frames the video is comprised of per second. There are many frame rates out there but common ones include 24, 25 and 29.97fps. The duration of each of these frames is very broad when compared to digital audio samples. At 25 frames per second, each frame of video should be displayed for 40 milliseconds. Compare this to a single audio sample which, at 48kHz sample rate would last for just 0.020833 milliseconds. To put it another way, a video frame lasts for 1920 audio samples. Because of this, in situations where super accurate video and audio sync is required, we need to tell Pro Tools not only what the frame rate is but also when video frames start. This is the concept of frame edge sync.

Why And When Is It Needed?

Whether or not you need to worry about frame edge sync really depends on the type of work you do. If you work in music production, then it’s pretty unlikely you need to be concerned about it. Even in audio post, it may not be critical in all situations. For example, if you’re mixing corporate videos or doing sound design on short films, the playback synchronisation of video and audio in Pro Tools is probably more than good enough as it is. However, if you regularly work with ADR or if you’re a dialogue editor, then frame edge sync could be critical. Some people are more perceptive to sync variations than others, but if close lip sync or anything else which requires an absolute solid lock between audio and video is critical, then you really need frame edge sync.

What’s Required For Frame Edge Sync To Work?

A common reference signal needs to be distributed to all video devices in a system in order for frame edge sync to work. This signal takes the form of video reference. Also known as blackburst or house sync, video reference is a video signal comprised of black frames which run continuously at the frame rate of your project. This should be sent to Pro Tools itself and to any video peripherals in the system.

Typically, the video reference generator is a small box with dip switches on the side which can be set to output the required frame rate. The outputs are on BNC connectors. For this to work with Pro Tools, you’ll need a sync device. In the case of the Avid Sync HD, the video reference signal should be connected to the Video Ref input. If you also have a video output device like a Blackmagic PCIe card or one of the Avid boxes such as the DNxIV, then this needs to receive the video reference signal too. This can be directly from another output of the video reference generator or from the second Video Ref connector on the Sync HD, which will pass the signal through.

When using Avid Sync X, you won’t need a separate video reference generator because it can generate and output its own video reference signal. Of course, the use of a Sync HD or Sync X requires a Pro Tools HD Native or HDX system because it has to connect to the serial connector on the system. The HD Native Thunderbolt box has a serial connector on the back, as does the older HD Native PCIe card. In an HDX system, you should connect the serial cable from the Sync X or Sync HD to the HDX card. In a multi-card HDX system, the sync device should be connected to the first card.

Another requirement for frame edge sync is that your audio interfaces have DigiLink Mini connectors so they can be incorporated into the system and playback engine. Audio interfaces also need either Loop Sync or Word Clock to carry clocking signals to all audio devices in the system.

In short, the following are needed for frame edge sync:

• Pro Tools HD Native or HDX

• Sync HD or Sync X

• Video reference generator (When using Sync HD)

• Audio interfaces with DigiLink and Loop Sync or Word Clock

How Is Everything Configured?

The diagram here shows an example of the connections in a Pro Tools HDX system consisting of a single HDX card, Blackmagic video reference generator, Sync HD, Avid Artist DNxIV video output box and Pro Tools MTRX Studio audio interface. If using a Sync X instead of a Sync HD, the video reference generator can still be used, or video reference can be generated internally by the Sync X and output to video devices from one of its Video Ref Gen connectors.

Once these connections have been made and the sync device has been enabled in the Peripherals window in Pro Tools, you need to open the Session Setup window. In the case of Sync HD, the Clock Master should be set as Sync HD and the Clock Reference is then set to Video Reference. The Video Ref Format should be set to match the resolution and frame rate of your video reference signal and video file.

Where a Sync X is used, the settings are slightly different. Once Sync X is set as the Clock Master, the Video Ref Source should be set to either Internal or, if you still want to use an external video reference generator, Video Reference. As you can see from the screenshot below, Frame Edge Lock is mentioned in the Video Ref section of Session Setup. If Sync X is generating its own video reference signal, the green Generating indicator will light. Each connected video peripheral will also need to be configured in software to follow the video reference signal. Exactly how this is done depends on the hardware. 

This might seem obvious, but the Playback Engine needs to be set to HDX or HD Native (depending on your system) for frame edge sync to work. Some audio interfaces, such as Avid’s MTRX Studio with the optional Thunderbolt card, can be used outside of the HDX or HD Native playback engine when running in Thunderbolt mode. When used outside of the HDX or HD Native playback engine, audio interfaces will not lock to frame edge sync. One Avid interface, Pro Tools Carbon, doesn’t have DigiLink connectors and so can’t be incorporated into a system supporting frame edge sync. It’s worth noting that there are some third-party interfaces which will support frame-edge sync. One example is the Focusrite Red16 Line which has both DigiLink mini connections and Loop Sync.

Final Thoughts

If you’ve read this far and still find yourself wondering if you actually need frame edge sync, then it’s probably safe to assume that you don’t. It’s something which is nice to have when you’re working with video, and it’s critical in a few workflows but not essential in many situations.

While video playback can fluctuate without being locked to video reference, there are a few things you can do even without it to stand the best chance of fairly solid video playback. The most effective is to use video encoded as either Avid DNxHD or Apple Pro Res. These formats still compress the video, but they’re intra-frame codecs, meaning they treat and compress every frame individually. This is in contrast to inter-frame codecs such as H.264, which compress occasional frames (known as key frames) with full image information and following frames as P or B frames which record only differences in the image from one frame to another. Consequently, these inter-frame, or Long Group of Pictures (Long GOP) codecs, are harder to playback because the computer constantly has to refer to previous frames in order to get all of the information needed to make up the picture.

Computers running on Apple Silicon are more likely to playback H.264, H.265 and Apple Pro Res codecs more smoothly than previous generations because the M1 and M2 chips have hardware-accelerated video decoding on the chip specifically for these formats. Even so, the only way to guarantee a completely solid lock between audio and video in Pro Tools is frame edge sync!