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MillimeterMatrox CompressHD Test Drive: Mac
Aug 24, 2009 12:00 PM, By Jan Ozer
Figure 1. Matrox CompressHD tempates in Apple Compressor’s settings window.
Overview
When you install CompressHD, Matrox loads six folders of CompressHD-specific functions into Compressor—Apple Devices, Blu-ray, Flash, Other Workflows, Sony PSP, and YouTube—which operate just like regular Compressor presets. You apply them by dragging them into the batch window, you can modify them in the Inspector window, and you can save custom presets. Unlike with previous versions of CompressHD, you don’t have to disable CompressHD to encode using a Compressor template. When you apply a Compressor template, Compressor uses the Apple codec; when you apply a Matrox template, Compressor uses CompressHD.
If you have a multiple-core computer set up as a cluster via Apple Qmaster, do not send the CompressHD job to the cluster; send it to the single computer. You can setup multiple-machine clusters with CompressHD, but each needs a separate CompressHD card.
Figure 2. A CompressHD Blu-ray template.
CompressHD has two types of templates with slightly different interfaces: one for Blu-ray, and the other for all other files. The Blu-ray template is video-only, and you use existing Compressor Dolby Digital templates for audio. Blu-ray encoding is generally straightforward, with the option to use context-adaptive binary arithmetic coding (CABAC) entropy encoding, which should increase compressed video quality and isn’t an option when producing H.264 with Compressor.
The only confusing bit is the option to use Simple GOP, Advanced GOP (short), and Advanced GOP (long). At Matrox’s advice, for the broadest compatibility, I used Simple GOP for all encoding, which is different from the default setting of many templates. Keep this in mind if you encounter difficulties with Blu-ray compatibility in the field.
Figure 3. My data rate is too low for this H.264 level, so CompressHD won’t let me save the preset.
CompressHD’s general encoding template has three tabs: one to specify the settings of your source file, one for video parameters, and one for audio. The encoder produces using the Main profile and monitors compliance with H.264 levels, and it will prevent you from encoding if your parameters exceed the spec.
This is ideal for devices, but probably not necessary for computers since the QuickTime, Flash, and Silverlight Players can play any level of the Main profile. Telestream Episode Encoder Pro takes a better approach, and it will automatically increase the level if other encoding parameters, such as resolution or data rate, exceed the then specified level. As a practical matter, CompressHD prevented me from encoding my comparative files at 1280x720p@800kbps, since the data rate was too low at level 4 and the resolution too high at level 3. I’ve produced using these parameters for more than two years and have never had a problem producing in other encoders or playing the files back in any player.
You can produce at any even-number resolution at 64x64 and higher. On the other hand, the release that I tested wouldn’t allow a GOP size of 300—any setting over 254 jumped the GOP size to 2048, which is too high. I also couldn’t set my target B-frame interval to my standard 3, as 1, 2, 5, 10, and 25 were my only options. Given the performance and quality, none of these are major options, just rough edges to be smoothed in the next release.
More seriously, CompressHD only operates in single-pass encoding mode and doesn’t have the ability to add keyframes at scene changes, a quality-enhancing feature supported by Compressor and most other encoding tools. As you’ll see, at some extreme scene changes, this resulted in a multiple frames of ugly video, and is a deficit that Matrox should resolve as soon as possible.
Continue the discussion on “Crosstalk” the Millimeter Forum.