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What is this Maximum bit rate for a .mp4 format file whose bit rate mode is Constant?

Media information displayed for MP4 (Using MediaInfo Tool)

ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : Baseline@L3.0
Format settings, CABAC : No
Format settings, ReFrames : 1 frame
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Bit rate mode : Constant
Bit rate : 1 500 Kbps
Maximum bit rate : 3 961 Kbps
Display aspect ratio : 4:3
Frame rate mode : Constant
Frame rate : 29.970 fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.163


In this case where the bit rate mode is set to Variable, is the Bit rate field where the value is displayed as 309 is its average bit rate?

Media information displayed for M4V (Using MediaInfo Tool)

ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : Baseline@L3.0
Format settings, CABAC : No
Format settings, ReFrames : 1 frame
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Bit rate mode : Variable
Bit rate : 309 Kbps
Display aspect ratio : 16:9
Frame rate mode : Variable
Frame rate : 23.976 fps
Minimum frame rate : 23.810 fps
Maximum frame rate : 24.390 fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.229
Writing library : x264 core 120

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1 Answer 1

When MediaInfo reports Bit rate, it'll report the bit rate the actual media stream specifies. So, if it says 1,500 and a constant bit rate was used, then this is the rate that was set by the encoder.

On the other hand, when a variable bit rate mode was used for encoding, the encoder cannot supply you with a predefined average. So, MediaInfo will report the measured average. It does that by analyzing the stream size and the duration (see File__Analyze_Streams_Finish.cpp in its source code), in a very generic fashion.

When it gives you a Maximum bit rate for VBR, the meaning is pretty evident. It also reports Maximum bit rate in the case of CBR streams. Unfortunately I haven't been able to find out what it means exactly—the source code is a little convoluted—but it depends on the specific media codecs or containers used.

Could be one of these:

  • the result of a calculation, searching for a peak bit rate (very unlikely, haven't checked the full source code)
  • a piece of information extracted from the MOOV atom of the MP4 container
  • the MPEG-2 descriptor defined in ISO/IEC 13818-1 section 2.6.26:

    The value indicates an upper bound of the bitrate, including transport overhead, that will be encountered in this program element or program

  • the maximum bit rate supported by a certain container, e.g. in MXF
  • the maximum bit rate supported by a certain codec profile / level combination


By the way: The MP4 (MPEG-4 Part 14) container has no restrictions on bit rates of the codecs used as far as I know. MP4 is not a video codec. If you're talking about "MP4" as in MPEG-4 Part 10 AVC video (h.264), then the maximum bit rate specified by the High 10 profile at level 5.2 is 720,000 kBit/s (reference).

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