The Other Side of Compression

A Quick QuickTime Pro Tutorial

Most of the settings for the QT codecs are handled within their configuration panels. Here’s a rundown of the steps you need to take in order to compress these clips in QT Pro.

1. Open the source movie with File Open Movie.
2. Choose Export Movie to QuickTime Movie and click on Options to bring up the Movie Settings panel.
   1. Select the Video Settings button to access the Compression Settings dialog box used for each of the codecs
   2. Choose the final frame size of the clip under the Video Size button.
   3. The Audio checkbox should be inactive if there is no audio on the source clip.
   4. Check "Prepare for Internet Streaming" and choose Fast Start.
   5. Hit OK to close this panel.
3. Enter a file name and type in the Open Movie panel and Hit OK to begin compressing.

   
   Chapter 3: DivX Networks' DivX 4.0.1

   
   DivX 4.0.1, DivX Networks

   PROS	CONS
   Two-pass VBR (variable bit rate) mode can maximize quality at lower bit rates.	Two-pass VBR processing is cumbersome in general-purpose encoding applications.
   Supports de-interlacing software.	Data rate control is difficult in VirtualDub.
   Post-processing available in the decoder for improved quality on playback.	Post-processor does not apply to video encoded at lower frame sized (i.e., 160x120 at 30Kbps) where it is needed most.

   From the founders of the controversial DivX (wink, wink) has sprung a new codec with a new company name and strategy. DivX 4.0.1 creates fully compliant MPEG-4 simple profile video streams that are targeted for long-form, video on demand needs of producers looking to capitalize on the streaming media business potential of their content.

   DivX Networks, as the parent company of DivX and the Open Mayo project, is coordinating its codec development efforts with hosting services and digital rights management, as well. It offers the codec and its own "Playa" application free on its Web site, although the encoded streams can also play back in Windows Media Player (6.4 or 7), RealPlayer on most Windows platforms, and Linux and Pocket PCs.

   During our tests, we found the highest-quality DivX movies were achieved using a two-pass VBR mode and the slowest performance/quality option. Here’s how it’s done, starting in the DivX Codec configuration menu:

   1. Select the "2-pass, first pass" option in the Variable Bit Rate mode pulldown.
   2. Enter the target bit rate and the maximum key frame interval, adjusted for the frame rate you are trying to achieve as noted in the preface above.
   3. Under the two-pass settings, enter a file name for the log file that will be created during the first pass. Make sure it points to the directory where your encoded clip will go.
   4. Click OK twice to close the Video Compression dialog.
   5. Select File Save as AVI and enter a filename for the first-pass output. To use the batch-processing feature of VirtualDub, you must also check the "Add operation to job list and defer processing" option at the bottom of the Save dialog box.
   6. Go back to the DivX Codec Configuration dialog and select the "2-pass, second pass" option for VBR. Click OK twice to close the compression settings windows.
   7. Return to File Save as AVI and enter a new filename for the second-pass results, and check the deferred processing box if you are batch processing.
   8. Open the File Job Control window in VirtualDub to see two jobs posted. The first will create a temporary AVI file and the log file that will be used to create the final encode in the second pass.

   For tighter control over bit rate and parameters, the single-pass encoding options can be applied as well. Quality-based one-pass encoding has no bit rate control and so does not fit into our testing strategy.

   We set the maximum key frame interval to be 10 times the frame rate, but natural key frames are also inserted by the codec, especially in the type of content we tested. The CPU usage and de-interlace options were also left off. In VirtualDub, the resize filters are applied prior to encoding, so using the built-in de-interlacing feature of DivX would have no effect on the 320x240 images being fed to the codec.

   A unique feature of this codec is its post-processing filters built into the decoder. There are six levels that are set using the slider, and corresponding to the following processes:

   0 – No post processing
   1 - Deblocking in the horizontal direction, luminance only
   2 - Deblocking in the vertical direction, luminance only
   3 - Deblocking in the horizontal direction, chrominance also
   4 – Deblocking in the vertical direction, chrominance also
   5 – De-ringing, luminance only
   6 – De-ringing, chrominance also

   We found no need for the De-ringing filters, but De-blocking at level 2 worked well on the Anim 252Kbps clip. The help file that comes with the encoder has a good explanation and example of how these work. They are ActiveX filters applied during decoding, and can be modified in the "Playa" application under the Playback->Properties window, or in Windows MediaPlayer 6. The company cautions that these functions are very CPU intensive, but we didn’t notice any performance issues when testing playback on a 300MHz Celeron laptop.