Matrox CompressHD Test Drive: PC
Last time out, I promised a look at the Canon Vixia HFS10 camcorder for this edition, but I couldn't get enough lab testing done to feel comfortable writing about this wonderful little camcorder. Give me a few weeks. Anyway, in this issue, I'll substitute a review of Matrox's CompressHD H.264 encoding accelerator card for Windows.
By way of background, I looked at CompressHD for the Mac a few months ago, and found a very compelling value proposition: The hardware was better and faster than Apple Compressor at all tested configurations. In Windows, using the same exact hardware, the value proposition is narrowerprimarily because Adobe Media Encoder, which uses MainConcept's H.264 codec, is both faster and higher-quality than Compressor, which uses Apple's own H.264 codec. Since the Windows version of CompressHD is designed to be used within Adobe Media Encoder, the starting bar is higher, and it's tougher to make a strong impression.
To summarize my findings, on Windows, CompressHD is best suited for rendering HD input to high-resolution, high-data-rate formats such as Blu-ray or 720p files to upload to YouTube. Files produced at this resolution were equivalent in quality to those produced by Adobe Media Encoder, and they generally encoded significantly faster. In addition, I was able to load the Blu-ray files that I produced into Adobe Encore and compile a Blu-ray disc with no problem.
In contrast, I would avoid the Windows version of CompressHD for producing H.264 files for streaming; CompressHD couldn't produce files that met the target data rate of my standard 500kbps H.264 file, which Adobe Media Encoder easily could. Even at the higher data rate, the low-bit-rate files produced by CompressHD were substantially lower in quality than those produced by Adobe Media Encoder. In addition, with a minimum audio data rate of 128kbps, CompressHD is not well-tuned for streaming, since 64kbps, or even 32kbps, can often suffice for speech-only files.
On the Mac, CompressHD met my streaming target and produced good-quality output, so I expected the Windows version, which uses the same hardware, to get there as well. If you're reading this review sometime in mid-2010 or beyond, you should probably assume that Windows quality has improved, and see if you can find a more recent review assessing CompressHD's H.264 quality in streaming configurations.
CompressHD costs $495 and is based upon a chip from ASIC vendor Ambarella. In Adobe Premiere Pro, you access CompressHD through the traditional Export > Media function, choosing either Matrox MP4 or Matrox Blu-ray in the Format list. Matrox provides a number of useful presets, but it encodes only using a single-pass, constant-bit-rate (CBR) encoding technique. Once you choose a preset, your configuration options are limited to setting the video data rate, choosing the H.264 level and entropy encoding algorithm, and adjusting the GOP structure.
Compression novices who simply want to choose a preset and go will like this approach. On the other hand, experienced compressionists who want to glimpse under the hood may be frustrated by the inability to discern details such as which H.264 profile CompressHD uses with their various templates.
In addition, from where I sit, there's no reason to not let me adjust the audio data rate, but since you won't be using this encoder for streaming anyway, it really doesn't matter for this release. I'll note for the record that you can't extend GOP duration longer than 128 frames, though again, this is only critical in streaming configurations for which this tool isn't well suited.
I tested CompressHD on two HP workstations; the first was a liquid-cooled, 3.2GHz dual-processor, quad-core Z800 (Intel Nehalem) workstation running 64-bit Vista. (Yeah, I know that liquid-cooled is irrelevant from a performance perspective, but I never get tired of mentioning it.) The other was a 2.83GHz dual-processor, quad-core xw6600 (Intel Xeon Q6850) workstation running 32-bit Windows XP.
Working from multiple HD input formats, CompressHD decreased rendering time by up to 54 percent on the Z800 and up to 60 percent on the older xw6600. As mentioned, however, CompressHD was unable to meet the 500kbps target with my standard test file, which makes the encoding time in that test somewhat irrelevant, as did the comparatively poor quality. As my deadline drew near, I threw in a couple of encode-to-1080i-Blu-ray tests on the Z800, just to see if encoding at the higher resolution would affect the comparisons. It didn't on the Z800, and I suspect that it would have little impact on the slower HP workstation either.
In higher-bit-rate tests, such as producing at 640x360 at 3Mbps for uploading to YouTube, CompressHD's quality was indistinguishable from that of Adobe Media Encoder, as shown in Figure 3. As you would expect, at our 15Mbps target for Blu-ray, quality was also indistinguishable from that of Adobe. You can see the Encore project containing files from both AME and CompressHD in Figure 4.
On the Mac, CompressHD was a one-size-fits-all solution for H.264 encoding. In Windows, CompressHD is a fast, high-quality, and easy-to-use solution for high-data-rate encoding for Blu-ray and other full-quality HD outputs. Streaming producers seeking to accelerate their encoding, however, should look elsewhere.