What are transcoding and transrating?

Transcoding is the process of converting one analog-to-analog or digital-to-digital file to another file format. Frequently, video and audio files are transcoded when the original file formats are not supported in a system or then may be transcoded because newer formats have become available.

Transcoding is often a lossy process, introducing a loss in quality each time a file is transcoded. Transcoding can be lossless if the input is losslessly compressed and the output is either losslessly compressed or uncompressed.

As Wowza Streaming Engine™ software only reflects streams and does not typically transcode video and audio codecs unless the Wowza Transcoder is involved, the encoding of a live stream or file for on-demand play is determined by and set in the encoder.

Transcoding is not to be confused with packetizing. Wowza Streaming Engine software packetizes streams for each playback client differently, but in this process the video and audio codecs are unchanged from the source.

Using the Wowza Transcoder, video and audio tracks can be transcoded to formats supported by Wowza playback clients. The transcoder is also used to transrate, that is, to encode to lower bitrate versions of the source stream. This is a process that is commonly used to encode keyframe-aligned sets of live streams to support ABR streaming. Refer to the Wowza Transcoder documentation for a current list of video and audio codecs that are supported for decode (incoming live streams) and encode (play).
 
Transrating is the process of changing a video file from one bitrate to another. This is often done for adaptive bitrate systems via an encoder to create the different versions of the video. Transrating converts a file to a different bitrate, but does not change the video or audio encoding  format.

The Wowza Transcoder is used to transcode audio and video codecs that are not supported by the clients that the Wowza Streaming Engine™ software supports, and/or to transrate several renditions at different bitrates for ABR (Adaptive bitrate) streaming. The key advantage to using the transcoder to transcode server-side for ABR streaming is the guarantee of keyframe alignment of each rendition, which is necessary for switching during play. Moreover, not having to produce each rendition on the encoder side saves uplink bandwidth. This provides support for a higher-quality source stream where bandwidth is limited.

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