Mac Pro vs. iMac: Video Compression

[ Please see my disclosure statement on product reviews. ]

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.

EXECUTIVE SUMMARY

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:

• A ten-minute audio and video episode from “2 Reel Guys” which included a reasonable amount of movement between frames.
• A two-and-a-half-minute audio and video talking head video from my YouTube channel.
• A twelve-minute audio and video screen capture from my current Compressor 4.1 training.
• A one-hour audio-only podcast from the Digital Production Buzz.

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:

• The default YouTube 720p setting to create a QuickTime movie
• The default Apple devices HD Best quality to create an MPEG-4 movie
• The default DVD compression setting for audio and video, which scaled the HD video to SD
• A custom QuickTime setting running in single-pass mode to test hardware acceleration
• A custom QuickTime setting running in multi-pass mode, which turns hardware acceleration off.
• A custom MPEG-4 setting running in single-pass mode to test hardware acceleration
• A custom MPEG-4 setting running in multi-pass mode, which turns hardware acceleration off.
• The default MP3 audio compression setting
• The default AAC audio compression setting

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

• When compressing audio, the Mac Pro was about 15% slower than the iMac.
• When using hardware acceleration, the Mac Pro was, on average, 29% slower than the iMac. (Speeds ranged from 2.8% faster to 73% slower.) This is due to the fact that the Mac Pro does not support hardware acceleration for compression.

The GOOD NEWS

• When compressing using multi-pass settings, the Mac Pro was, on average, 45% faster than the iMac. (Speeds ranged from 33% to 60% faster.)
• When compressing for YouTube, the Mac Pro was 37% faster.
• When compressing for Apple devices, the Mac Pro was, on average, 24% faster. (Speeds ranged from 12% to 35% faster.)
• The Mac Pro compressed MPEG-4 videos faster than QuickTime videos, when compared to the iMac.

Click the table to see a PDF of all my results.

Column definitions:

• The left column describes the specific test
• iMac 2013 are the timing results from the iMac computer
• File size is the size of the compressed files from that test, created by the iMac
• Mac Pro are the timing results from the Mac Pro computer
• File size is the size of the compressed files from that test, created by the Mac Pro
• Mac Pro Speed Difference is the difference in speed between the iMac and Mac Pro. Negative numbers indicate where the iMac is faster.
• HW Accel. iMac Speed Diff. is the difference in speed when hardware acceleration is enabled. This column compares differences in speed with the iMac between accelerated and non-accelerated compression for the QuickTime and MPEG-4 custom setitngs. Not all compression settings support hardware acceleration.

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.

• Mac Pro Single vs Multi Speed Diff. compares the difference in speed between single pass and multi-pass for QuickTime and MPEG-4 custom settings; as the Mac Pro does not support hardware acceleration.
• Notes indicates that the QuickTime compression setting created uncompressed audio, while the MPEG-4 setting created compressed audio. As this setting did not change between machines, I am ignoring audio compression when comparing results.

NOTES ON THE TEST

• iMac
• 3.4 GHz i7
• 16 GB RAM

I am working with a new Mac Pro, which is on loan from Apple. (You can read my first review of it here.)

• New Mac Pro
• 2.7 GHz 12-Core Xeon E5
• 32 GB RAM

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.

FINAL THOUGHTS

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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