Encoding for Advanced FF/RW

To use Advanced FF/RW, you must create compressed copies of the original source content files (sometimes called "trick mode speed files") for the different fast-forward or rewind speed values that may be requested by clients.

By default, when a user clicks the Fast Forward or Rewind button in Windows Media Player, the file plays in the desired direction at five times normal speed. Rate requests up to ten times normal speed can be sent by Windows Media Player if the user adjusts the slider in the Play Speed Settings Enhancements pane. Windows CE supports even higher rate requests.

If a client makes a rate request for which you have not encoded a corresponding trick mode speed file, the server fulfills the request by using standard trick mode instead. In typical video-on-demand scenarios, clients can be restricted to request trick mode functionality at predetermined speeds to use Advanced FF/RW.

Trick mode speed files are identical to the original content source file, except that excess frames are removed during encoding. For example, for clients that might request fast-forward at five times normal speed, a corresponding trick mode speed file is created by retaining every fifth frame as a key frame during the encoding process, while the rest of the frames are removed. By using only key frames in trick mode speed files, the fast-forward presentation in the client is smoother. Also, by using only 20 percent of the original frames in the trick mode speed file, the file size is reduced by approximately 80 percent, reducing the presentation time. If you play the trick mode speed file using Windows Media Player 9 Series or later, it will appear as if the original file is being played five times faster.

Video editing tools such as Adobe Premiere Pro should be used to create trick mode speed files. After creating a project with the appropriate settings (such as video size, frame rate, and so on), the source content file can be dropped onto a time line and the appropriate speed value applied to match a rate request. For example, you can use the following settings on the Speed/Duration tab in Adobe Premiere:

The file can then be exported as an Audio-Video Interleaved (AVI), which is then encoded to the corresponding normal and trick mode speed files in Windows Media Video (WMV) format.

For best results, make sure to observe the following guidelines while creating trick mode speed files:

• Use AVI files for your original source content.
• If your encoding software includes an option to import or include an encoding profile in the encoding process, you can save time by using an encoding profile to encode trick mode speed files for various speeds.
• Encode a normal speed content file to WMV format using the encoding profile of your choice. Set the key frame distance to an appropriate value for smooth playback on the client.
• Use the same encoding profile settings (audio/video bit rate, stream numbers, and buffers, video size and pixel aspect ratio, interlace/progressive mode, frame rate, packet size, and encoding buffer) and codecs to encode all of your trick mode speed files to WMV format. Unpredictable behavior may result if you change the settings. For example, the server may not be able to stream the files to the client or the client experience may vary when switching between the normal speed file and a trick mode speed file. However, you should set the key frame distance to zero. This setting allows the Windows Media server to send all frames as key frames, resulting in smooth playback on the client during fast-forward or rewind. Note, however, that the quality of a file is reduced when more key frames are used.
• Name your files so that it is easy to differentiate the normal speed file and trick mode speed files.

Normal and trick mode speed files created for Advanced FF/RW must be placed in a location that is accessible to the client. Clients are directed to trick mode speed files through either a fully qualified or relative URL contained in a tmi element in a client-side playlist (.asx). For more information, see Implementing Advanced FF/RW.

Related topics

• Understanding Advanced FF/RW