Comparing Loop Mode and Run/Stop Mode with the Phantom Flex4K
High-speed cameras like the Phantom Flex4K store data in a very different way than what you might be used to from the likes of an ARRI Alexa or even a DSLR. Those cameras are designed to roll for long periods of time, and so must record sustainably: if an Alexa can safely store 120 frames per second, it will only record at 120 frames per second. Anything higher - say, recording 150 frames per second in a burst, is considered a bug because it would quickly overload the camera and stop the recording.
To compensate for their slow storage, most cameras save their images in a compressed format like ProRes, XAVC, or h.264. Mirrorless cameras are especially prone to this; SD cards are not very fast in the grand scheme of things. This compression allows for more frames per second, because each individual frame is ‘smaller,’ but puts more strain on the processors that now have to keep up with that compression on the fly. This is why more expensive cameras often record higher frame rates in RAW than in ProRes - the latter requires processing, and the amount they can process is much lower than the amount they can store.
The High-Speed Workaround
To record its incredibly high maximum frame rates, the Phantom Flex4K procrastinates on both processing AND storage concerns. When shooting in Loop mode, it is essentially recording to a buffer of RAM (incredibly fast, but small, short-term storage) instead of the final storage medium. Each video frame replaces the oldest one still remaining, continuously turning over until you press the record button. At that point, depending on your settings, the camera will record a few more seconds of video before screeching to a halt.
Once the action is over, the Phantom Flex4K takes a few seconds to catch up on all of the processing it has been putting off. In RAW, this mostly means just bundling up all of the frames and handing them off to the CineMag. For ProRes, the full compression begins. You will notice this takes much longer. Then, it goes about the relatively slow process of storing all of this processed video. Once that is completed, the camera dumps everything out of the RAM and starts taking in new video frames all over again.
Shooting in this mode allows for some absolutely enormous frame rates - 990fps in Cinema 4K, 2564fps in 1080p, over 5000fps in 720p, and, for the true speed-demons, 10,000fps in 480p resolution. Assuming you have the 64gb model, each of those combinations of resolution and frame rate earns you about 5 seconds of record time. To effectively store and play back this mass of footage, you will likely need SSDs in a RAID configuration and thunderbolt connections. These kinds of drives run for about $2000 right now - and you'll want another one for backup. But hey, what's another $4,000 when you've already bought a $100k camera?