{"id":36,"date":"2025-08-30T17:58:51","date_gmt":"2025-08-30T08:58:51","guid":{"rendered":"https:\/\/www.decteng.com\/en\/?p=36"},"modified":"2025-08-30T17:58:52","modified_gmt":"2025-08-30T08:58:52","slug":"wowza-streaming-engine-transcoder-cbr-bug-eng-2346","status":"publish","type":"post","link":"https:\/\/www.decteng.com\/en\/wowza-streaming-engine-transcoder-cbr-bug-eng-2346\/","title":{"rendered":"A Month-long Record of Registering Wowza Transcoder CBR Bug ENG-2346."},"content":{"rendered":"\n
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Start of digging : Huh? Where is it?<\/h2>\n

When the writer started to write the transcoder section of the Wowza Streaming Engine(hearafter Wowza), the writer encountered a confusing situation.<\/p>\n

Huh? Where is the VBR selection in the transcoding options?<\/blockquote>\n

Apart from the early days of handling satellite transmission systems or RealVideo, using VBR (Variable Bit Rate) configurations was almost never necessary. Most of the systems handled so far were based on CBR (Constant Bit Rate). In addition, the main approach has always been to avoid using Wowza’s transcoding feature whenever possible.<\/p>\n

That is why, even after using Wowza for such a long period, the absence of a VBR setting went completely unnoticed. (Yeah, true. It’s actually an excuse\u2026)<\/del><\/p>\n

However, the writer had not seen a system without VBR-CBR selection during worked with countless encoders and transcoders until now. It was a moment of confusion for the writer, who had intended to cover this aspect while writing about the Wowza transcoder.<\/p>\n<\/section>\n

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I knew it! Of course it exists!<\/h2>\n

The writer started investigation, but could not find significant information about VBR settings. While searching around, the writer eventually found a clue.<\/p>\n

Updated transcoder VBR target to peak bitrate range from varying values per-implementation to 110% everywhere
Wowza Streaming Engine release note<\/a><\/blockquote>\n

I knew it! It’s a transcoder, after all. There’s no way it wouldn’t have a VBR option!<\/strong>Referring to the release notes, the writer contacted Wowza technical support to ask how to change the transcoder’s operating mode, and received a reply a few hours later. The response instructed that, add the following parameters to the transcoder’s template file.<\/p>\n

<Parameter>\n    <Name>mainconcept.bit_rate_mode<\/Name>\n    <Value>2<\/Value> <!-- 0 = CBR, 1 = CQT, 2 = VBR, 3 = TQM -->\n    <Type>Long<\/Type>\n<\/Parameter><\/code><\/pre>\n
Happy about being able to continue the article, the writer added the parameters to the template file. Next, the writer attempted to capture the decoding bitrate in VLC to visually show the difference between CBR and VBR. And then, the real digging began.<\/strong><\/p>\n<\/section>\n
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The Real Digging: Huh? This Is odd.<\/h2>\n
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What changed?<\/h3>\n
In a one live application, the writer created two transcoder profiles as follows, and then inputted a 9Mbps UDP Multicast MPEG TS stream.\n
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  • Applied 3.65Mbps to the video bitrate of the default Wowza transcoding profile.<\/li>\n
  • Applied 3.65Mbps to the video bitrate of the default Wowza transcoding profile, and then applied the VBR parameters.<\/li>\n<\/ul>\nThe bitrates of the two output streams from the different transcoders should have measured differently. However, the bitrates of both output streams were same. Even after trying to change the mode to CBR, the result was the same too.<\/p>\n
    Thinking the settings might have been entered incorrectly, the writer contacted Wowza again with the bitrate data measured by ffprobe<\/code> and the transrate.xml<\/code> file used. A few days later, a reply with the following content arrived.<\/p>\n
    I\u2019d also suggest testing with a more dynamic source (fast movement, scene changes, etc.) to give the encoder more reason to vary the bitrate.<\/blockquote>\n
    The moment the author read the reply, a hunch kicked in: Uh-oh… this won\u2019t be easy<\/strong>.<\/p> \n\n<\/section>\n\t
    \n
    Open Your Eyes and See!<\/h3>\n
    When someone talking about the content issue, the best answer is a direct visual demonstration. Typically, dynamic content brings to mind things like concerts or sports broadcasts. The writer agrees, but there is another type of content that is just as dynamic: anime featuring transforming magical girls<\/strong>.<\/p>\n