XAVC S vs Prores: Latency, Grading and Chroma Key

In this lesson we’ll compare the quality of the internal XAVC S codec against Prores HQ, Prores 422 and Prores LT to understand when to choose which. I’ll only be focusing on 1080p.

In this part we’ll study latency, grading and chroma key. I’m going to be comparing XAVC S with Prores HQ, 422 and LT, so we know where each one stands. We’ll be testing all of the above codecs at 24p.

Latency

There is a one-frame delay between the internal XAVC S recording and the Prores versions. This latency is obviously from the camera via HDMI – first of all due to the inherent latency in HDMI, and secondly because it has to trigger the Shogun to record as well.

In a 24p timeline, one frame is the equivalent of 42 milliseconds. In practical use, it is a non-issue.

Grading stress test

Here are full frames (JPEGS, click to enlarge) of a double-Color Finesse layer on the same Cine1 image from the previous lesson. The first is Prores HQ from the Shogun:

GradedShogun

This is the XAVC S frame:GradedXAVCS

There is only a slight difference, especially in the green channel. It is not significant. This means, for light to medium grading with just one recompression step, XAVC S is perfectly fine.

For heavier grades or multiple compression iterations, Prores HQ should be used.

Chroma key stress test (including motion)

This comparison is quite telling (click to enlarge):

ComparisonHQXAVCS

The green screen stress test is the easiest way to judge footage. You prop up a poorly lit green screen, and wave a hairy object in front of it. Then, use a really good keyer (I used Keylight in After Effects, color picked at the exact same pixel) to study the images in various stages of motion.

Here’s what the original scene looked like:

SceneChroma

The Prores HQ version is miles better in the keying department. I have included versions for 422 and LT as well, but they are really unnecessary. I have spent countless hours keying images in heavily compressed 4:2:0, and trust me, if you want to shoot good chroma key, shoot Prores HQ.

My recommendations are in the next lesson.

Click on the link below to the next lesson or head over to the main menu (above). If you need help with something, feel free to send me an email. I’ll try to get back to you as soon as possible.