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There are a variety of excellent performance reviews of the new Mac Pro on a variety of sites, so I decided to compare the Mac Pro with an iMac from a different perspective: video compression. What I learned surprised me, as you’ll see in this article.
The purpose of this test was to judge compression speed, not image quality, in an effort to compare these two systems; though compressed image quality seemed comparable between the two systems.
When running Apple Compressor 4.1 as a bench-mark, the new Mac Pro is faster for some compression tasks and significantly slower for others when compared to a recent model iMac. If video compression is your primary use for a new computer, you may be better off buying a top of the line iMac.
Take a look at the table below. Different compression tasks yield significantly different completion speeds. Select the system that meets the needs of the compression tasks you need to accomplish.
A NOTE ON HARDWARE ACCELERATION
One of the speed advantages of the iMac is that it uses an Intel technology called “QuickSync.” This is a special processor “engine” inside many consumer-grade Intel CPUs that accelerates H.264 compression for certain encoding settings; for example, when compressing for Apple devices, QuickTime or MPEG-4 movies using the H.264 codec. The Mac Pro Xeon CPU is considered “workstation-grade,” and doesn’t provide this hardware acceleration. This explains why the iMac is faster when encoding in single-pass mode, which enables hardware acceleration, but slower in multi-pass mode, which disables hardware acceleration.
Hardware acceleration is a two-edged sword. It is MUCH faster than software encoding. However, it only yields image quality and file sizes equal to single-pass encoding. This will often be fine for movies that don’t contain a lot of movement, such as screen captures; or movies where getting it done fast is better than image quality, such as news or digital dailies. However, hardware compression is generally not the best choice for movies with lots of movement between frames or where you need the highest image quality with the smallest file size.
WHAT I DID
I ran a series of 21 compression tasks on both a current model iMac and new Mac Pro, noting how long the compression took and the difference in file sizes created. I used four test files:
All videos were 720p ProRes 422 or ProRes 4444 files with uncompressed audio. The audio podcast was in uncompressed WAV format.
I created nine compression test settings:
All settings matched between the two computers. Both Compressor and Mavericks were running the latest version. The Mac Pro had its latest firmware update installed.
Here are the settings I used for the custom QuickTime setting: H.264 codec, 2000 kbps data rate, frame reordering on, keyframes every 90 frames.
Here are the settings I used for the custom MPEG-4 setting.
WHAT I LEARNED
Compression speeds varied depending upon the length and complexity of the source files, though compressed file sizes were essentially the same between the two computers (which I would expect). All files were stored and saved to the desktop.
NOTE: As measured by the Blackmagic Design Speed Test (BMD), the Mac Pro was roughly 5 times faster at reading and writing to the desktop than the iMac. This speed differential does not seem to be significant in compression.
The BAD NEWS
The GOOD NEWS
Click the table to see a PDF of all my results.
NOTE: To compare the differences in hardware acceleration between the iMac and Mac Pro, look in the Mac Pro Speed Difference column. In all but one case, the Mac Pro is slower when hardware acceleration is turned on than the iMac.
NOTES ON THE TEST
I am working with a new Mac Pro, which is on loan from Apple. (You can read my first review of it here.)
I used the same compression settings on both computers. Timings were measured by Compressor and displayed in the Completed tab. One job was fully complete before the next job started. Two jobs never ran at the same time.
Compressor was run in single instance mode, which is its default setting. Though I didn’t test for this specifically, I discovered that for short movies, single instance mode is about 20% faster than multiple instance mode. This difference disappears as the duration of the source media increases.
NOTE: Running Compressor in multiple instance mode does not guarantee faster performance. In general, I recommend leaving Compressor in its default setting with multiple instances are turned off.
Here’s an article that explains the difference between single-instance and multiple-instance mode and when to use which.
With the exception of compressing for DVD no files were resized and no filters were applied. All source files were copied to the desktop of the computer, and all compressed files were also stored to the desktop of the test computer. No network drives, or direct attached drives, were used for any part of this test.
Audio file sample rates were converted from 48 kHz to 44.1 kHz.
The same compression settings were used between the three video tests. The only difference was in the source media.
The only difference I made between the single-pass and multi-pass compression settings was checking, or unchecking, the multi-pass check box.
I was totally surprised by these findings. Until we start to see applications optimized to take advantage of the power of the Mac Pro, if video compression is your key task, a high-end iMac is your best choice.
As always, let me know what you think.
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