Better Video Through Compression

AbelCine recently hosted the VII Evolution Tour, an event series for photojournalists making the transition to video production. The tour helps attendees identify and overcome the challenges of going from stills to motion production. A topic that was covered during this event was the use of external recorders. In one of her workshops, photojournalist and filmmaker Jessica Dimmock talked about her transition and showed off her camera rig. She uses a Canon EOS C100 along with an Atomos Ninja Blade external recorder. Many attendees asked why she used that particular combination, and Jessica explained that the resulting ProRes compressed video is better quality than the internal camera recording and in a format that editors like. After the workshop, a few attendees asked me to explain in more detail the benefits of using an external recorder. Keep reading to learn more about the advantages of external ProRes recorders, and more about video compression, in general, along with plenty of visual examples.

WHAT’S THE DIFFERENCE?

Most video cameras and DSLRs have an internal recording format designed to get you the most footage possible on a media card. For example, the Canon EOS C100 uses an AVCHD format running at a maximum of 28 Mbps, which is very lightweight, resulting in about 12 GB per hour of footage. Compare that to the popular Apple ProRes 422 HQ compression, which comes in at 220 Mbps or over 100 GB per hour. That’s a big difference in file size, so you can see why many cameras go for the more compressed format. But that big difference in file size does give you a noticeable improvement in video quality—you just need to know where to look to find it.

I wanted to visualize the difference between the internal compression on these cameras and an external ProRes recording. I combined the Canon EOS C100 Mark II with two external recorders to give me ProRes 422 HQ, as well as an uncompressed recording. I used an Atomos Ninja Blade for the ProRes and the Blackmagic Design HyperDeck for a completely uncompressed recording. The uncompressed recording served as a baseline of what the camera could output without any compression added. With this setup on my cart, I grabbed a few video clips, recording to the camera and both recorders simultaneously.

Above is the result of the three recordings side by side, and it’s hard to tell the difference until you zoom in.

At 400%, you can see there’s a lot of detail lost in the internal recording. The leaves in the trees are noticeably blurrier. But the ProRes and uncompressed recordings look about the same, so let’s look a bit deeper.

To see the exact differences between the three recordings, I brought them into Adobe’s After Effects and built a difference map. The colors in the image represent the differences between the two videos—the more colors you see, the more degradation there is. The internal comparison has some obvious difference, especially on edges, but the ProRes recording is completely black, meaning it has a very minimal difference compared to the uncompressed recording. By gaining it up 10x, you can really see the difference between the internal and ProRes recordings.

A LITTLE BIT ON COMPRESSION

A lot of work goes into making video small. The first is blocking. Video compression blocks together similar frequencies, colors and textures that are next to each other. Say you’re shooting a blue sky that’s all basically the same color blue. The compression will lump together that blue sky into large blocks. This saves a lot of space, as not every pixel has to be saved. In the image to the right, we can see this compression happening—it’s the colored noise that you see between edges. That information has been lost, so it shows up in the difference map. Generally, as data rates go up, the blocks get smaller in size—that’s why ProRes has notably less quality loss.

Another compression technique that we can clearly see in our example is color subsampling, or the 4:2:2, 4:2:0 thing. The C100 outputs 4:2:2 video for external recording, but records in 4:2:0 internally. Many people think that color subsampling makes an image less colorful, which isn’t the case. What it’s really doing is reducing the color resolution of an image. A 4:2:0 and 4:2:2 image will share the same basic black-and-white data, but the 4:2:0 image has half the color information stored in it. We can see this in the difference map as glowing edges. They show up in the difference map because the edges are transitions from one color to another, and we threw away half the color, so these edges just aren’t as good as they used to be. This is why 4:2:0 isn’t great for greenscreen work where we want clean edges.

Hopefully, this gives you a better understanding of what you lose on the compressed recording and what you gain with an external ProRes recorder.

WHY NOT GO UNCOMPRESSED?

A question that many of you are probably wondering now is, why not just go uncompressed all the time? Well, the answer is pretty straightforward—file size. The internal recorded clip came in at 170 MB/s, the ProRes clip was 1.5 GB/s, and the same uncompressed clip was over 10 GB/s. That’s a huge difference in file size, so uncompressed isn’t really an option for most of us.

BUT IT’S JUST FOR THE WEB

I’ve often heard the comment that recording high quality doesn’t matter because the project is "just for the web." It’s true that web compression is pretty extreme, but starting with higher-quality material always yields a better result on the web. To show this, I’ve compressed both the original internal recording and the ProRes recording into a web compression used for YouTube and Vimeo. Then, I generated the same difference map as before.

As you can see in the images above, the ProRes web compression is considerably closer to the original uncompressed video. So it’s pretty clear that starting with a higher-quality video gives you better results on the web.

EXTERNAL RECORDER OPTIONS

There are many great external recorders out there today that can record in ProRes and other high-quality formats. A couple of great options include the Atomos line of recorders, from the HDMI-only Ninja line to the 4K recording Shogun. Convergent Design has their Odyssey line of products, with the Odyssey7Q/7Q+ giving you all the way up to 4K ProRes recording. And Sound Devices has their PIX and new PIX-E 4K ProRes recorders. All of these are great options, each with their own advantages. These recorders can work with a wide variety of cameras, from old to new, and can all be used to extend the life of your camera. Give us a call at AbelCine to discuss the best solution for you.

Andy Shipsides is a N.Y.-based Camera Technology Specialist and AbelCine’s Director of Education. Learn more at training.abelcine.com.

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