Is It Necessary To Video Sync Two Dslr Cameras

Though many of us are familiar with cameras having two ports―a timecode in/out port and a genlock port―we may not consider why both might be needed. Perhaps nosotros dismiss genlock equally merely a legacy holdover from the times before switchers had built-in frame synchronizers. The terms "sync" and "timecode" are oftentimes used interchangeably, and the fact that timecode tin be used to sync devices just compounds this confusion. Sync (genlock) is like a beat that calls out when a field (as well as a line with tri-level) occurs, while timecode indexes each frame (or the equivalent period of fourth dimension for an audio recorder) then that information technology can be identified in uniquely postal service.

"... timecode solitary, while sufficient to provide syncing points for post production, may not exist a reliable fashion to keep devices in sync with each other."

If nosotros need to synchronize multiple cameras, or a camera and audio recorder, nosotros tend to just use timecode. The problem is, timecode alone, while sufficient to provide syncing points for post production, may non be a reliable way to keep devices in sync with each other.

With timecode, unless the cameras or recorders are re-jam-synced frequently, their timecode will start to migrate apart. The problem is that the quartz crystal (or other piezo-electrical cloth) clocks used in many cameras and quite a few audio recorders are too imprecise for critical applications. This means that 2 different clocks may have two different opinions on how long a 2nd is, causing recordings to drift apart over time. Depending on the camera or recorder, the timecode may drift every bit much as a field in a non-trivially brusk infinite of time (30 minutes in some cases). Even a field offset is enough that audio and film volition become visibly out of sync when combining the 2 elements later, in post.

In calorie-free of this limitation, several responses come to mind:

Genlock the devices, forcing them to "burn down" at the aforementioned instant
Rather than jam-syncing, constantly feed in timecode from a central source
Fix it in postal service, perhaps using a sync software

What is genlock?

To start, information technology is worth revisiting genlock. Genlock (generator locking) appeared in the broadcast world as a way to synchronize multiple video sources, including cameras, VTRs, and external feeds, and then that their field rates are all in phase with each other. This was required to prevent and so-called "jumping," an antiquity that occurs, when switching in live productions, if the all of the video sources are not in step with each other. For a long time information technology was necessary to genlock each device―camera or deck―by sending a black burst or composite signal into it, originating from a common house clock. These days, many switchers handle frame sync on their own by using a frame buffer that tin can concur the field (or frame if the indicate is progressive) momentarily until it falls into alignment with the program feed.

In the Hard disk world genlock is still with the states, but using a blended signal as a reference has largely been supplanted by tri-level, which emits pulses that clock both the frame rate and the line rate.

Because it synchronizes frames, genlock can be used to keep multiple devices from drifting apart. Simply it is tricky because, dissimilar timecode, the camera or recorder must always exist hardwired to the source. On top of that, where different length cables are used, each photographic camera must be calibrated to have into business relationship the length of the cable run. In a fixed studio setting this isn't a large problem, only it tin can be in the field. And it is particularly problematic in cinema production where the setup might change with every shot.

Incidentally, some other application for which genlock is used is to ensure both sensors on two-camera 3D rigs fire at the same fourth dimension. A mutual misconception is that that you can simply link the two cameras together, feeding the composite output of one photographic camera into the genlock input on the 2nd. While this works in theory, the take chances of miscalibrating the receiving camera means it is a much safer option to use a dissever sync box that is wired to both cameras with cables of identical length.

What well-nigh wireless timecode?

A popular, if dubious, fashion to sync multiple devices in the field is to continuously feed timecode into them using a wireless audio transmitter and receiver. Much like black burst being a type of composite signal, timecode such as SMPTE 12M LTC can be passed as an analog audio bespeak, and produces a well-know "telemetry" racket if played through speakers. Since we live in Age of the App, in that location are many app-driven solutions that promise timecode sync over Wi-Fi. This can work, but the inherent unreliability of wireless technologies comes in to play; cheap wireless systems may even digitally process the signals, introducing delay. Also, when sound or video is existence sent wirelessly, a loss of signal will be noticed right away―people on set are watching that. But if the timecode signal drops, the camera simply reverts to its internal clock and the trouble may not get noticed until it is too late. Plus, using audio hardware, not to mention a Wi-Fi network, rather than dedicated timecode hardware, adds variables that may chemical compound troubleshooting when things practice―every bit they inevitably will―become wrong.

What about syncing in software?

If the footage is being recoded, it'due south going to need to be edited anyway, then why non only use an awarding similar Cherry Giant PluralEyes or Terminal Cut Pro X's born facility to sync sound and video? If the textile is sufficiently broken up or if y'all are able to cutting up the clips into less than 30-minute segments, this will probably work, at to the lowest degree inside a half-frame margin of error.* But for an all-solar day live event, during which each camera is being recorded separately, this solution would create a lot of extra work. It would be overnice, if timecode is available anyway, to be able to utilize that exclusively when syncing cameras and audio in post.

The ideal solution, when all cameras can't be difficult-wired to a reference source as in a TV studio, is to connect each individually to a reliable sync device such as an Ambient Recording Lockit Box or a Sound Devices recorder with sync output. These devices apply high-precision, temperature-compensated crystal oscillator (TCVCXO) engineering science that boasts less than one frame of drift per day. For situations in which timecode has to be relayed wirelessly, at that place is also wireless hardware based on the same TCVCXO applied science which, while not as reliable as a difficult-wired solution, is the next best thing since it is optimized to send timecode data rather than natural sound in the frequency range of the human voice.

To sum it up, true synchronization without drift requires 2 things: It requires a reliable timecode source to avoid migrate, and it requires genlock to make sure the fields or frames are hitting the same vanquish. In some cases, sync problems tin can be fixed in mail service. But this can be time consuming, and the relative cost of getting it correct on-fix may win at the end of the 24-hour interval. The bad news is with the reduced need for genlock in broadcast, fewer and fewer cameras have sync ports, peculiarly at the prosumer level. Eventually, nosotros may be stuck with Wi-Fi merely, hopefully, by then cameras will feature better internal clocks and then that an 60 minutes in photographic camera A will match an hour in camera B―even if their timecodes aren't perfectly aligned.

*A video field in NTSC areas lasts just shy of one/lx of a second. Meanwhile, an audio sample is typically either 48 kHz or 96 kHz. That'due south 48 or 96 k samples per second versus 60 video samples per 2d for video. Unfortunately most NLEs just let y'all adjust audio with precision of ane frame, or at best, one field. This means you volition never go sound that wasn't already in sync precisely without resorting to dedicated tools.

Source: https://www.bhphotovideo.com/explora/video/tips-and-solutions/timecode-versus-sync-how-they-differ-and-why-it-matters

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