I was noodling around in Compressor 4’s presets, and noticed the presets for HTTP Live Streaming. “HMMM,” I thought, “I wonder if Apple chunks the files and creates the required manifest files?” (Didn’t in my tests). Then I pondered, “I wonder if Apple’s presets match their recommendations in Technical Note TN2224?” So, I opened all the presets, recorded their encoding parameters and compared them to the Tech Note. That’s when things got interesting and I found (what I think are) errors in two of the presets.
Here’s the relevant screen grab from the Tech Note:
Here are the numbers that I compiled:
Some minor differences in resolution (428×240 as compared to 416×234), but no big deal. The numbers that were curious, however, are the circled ones. For example, why in the Cellular Low preset would the frame rate be 15 and the key frame interval 30? That computes to a key frame interval of two seconds, while all other presets use three seconds (45 keyframe / 15 fps = 3; 90 keyframe / 30 fps = 3). One of the tenants of encoding for adaptive streaming is to use an identical key frame interval in all files, which appears to be in error here.
Here’s the actual preset from Compressor to verify my observations.
Looking at the chart from the Tech Note, I would guess that the frame rate is supposed to be 10, which would give you a three-second key frame interval. In the Corrected Cellular Low preset that you can download below, I made that fix.
The other issue is why would Apple produce two presets, Cellular High and Cellular Low, at exactly the same data rate? Here’s the Cellular High preset from Compressor.
If you look at the Description line, you’ll see that the total was intended to be 500 kbps, not 216 kbps. The obvious fix is to boost the data rate to 460 kbps for a total of 500 kbps. A bit higher than the 400 kbps in the Tech Note, but better than the original preset. However, at 460 kbps for a 428×240@15 fps file, you get a rediculously high bits-per-pixel value of .299, which is probably why the equivalent preset in the Tech Note was encoded at 29.97 fps. So, in the Corrected Cellular High preset you can download below, I changed the data rate to 456 kbps and the frame rate to 29.97.
To use the presets, download them and drag them into the Custom folder in Compressor’s Settings window. If you’re encoding for adaptive streaming, substitute these for the presets that Apple provided.