Stream info API indicating B-frames when no B-frames present?

[bchah]

(Edited: I realized this isn't coming from the Alert module, it's in the stream info API! Sorry!)

We inspect the stream info API to let our users know if there is a problem with the incoming stream (/v1/vhosts/vhost/apps/app/streams/stream). It is wonderful.

But sometimes I notice we get hasBframes=true even though the stream does not contain them. Here is a stream sample coming from the Blackmagic Web Presenter. It plays back great in OME but the stats API tells us there are B-frames the whole time.

BlackmagicNoBFrames.mp4

Here is the analysis by MediaInfo of the recorded stream:

General
Complete name                            : /Users/brian/Movies/BlackmagicNoBFrames.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 10.7 MiB
Duration                                 : 20 s 683 ms
Overall bit rate mode                    : Variable
Overall bit rate                         : 4 342 kb/s
Frame rate                               : 30.000 FPS
Writing application                      : Lavf59.16.100

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : Main@L4
Format settings                          : CABAC / 1 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 1 frame
Format settings, GOP                     : M=1, N=60
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 20 s 600 ms
Bit rate mode                            : Variable
Bit rate                                 : 4 224 kb/s
Maximum bit rate                         : 12.5 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 30.000 FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.068
Stream size                              : 10.4 MiB (97%)
Codec configuration box                  : avcC

And the Stream info as reported by OME API:

{
  "createdTime": "2025-03-05T08:21:34.132-05:00",
  "sourceType": "SRT",
  "sourceUrl": "SRT://192.168.65.1:41204",
  "tracks": [
    {
      "id": 1,
      "name": "Data",
      "type": "Data"
    },
    {
      "id": 256,
      "name": "Video",
      "type": "Video",
      "video": {
        "bitrate": "4288081",
        "bitrateAvg": "4288081",
        "bitrateConf": "0",
        "bitrateLatest": "3975376",
        "bypass": false,
        "codec": "H264",
        "deltaFramesSinceLastKeyFrame": 9,
        "framerate": 30.025630950927734,
        "framerateAvg": 30.025630950927734,
        "framerateConf": 0,
        "framerateLatest": 29.970029830932617,
        "hasBframes": true,
        "height": 1080,
        "keyFrameInterval": 59,
        "keyFrameIntervalAvg": 59.53333282470703,
        "keyFrameIntervalConf": 0,
        "keyFrameIntervalLatest": 60,
        "width": 1920
      }
    },
    {
      "audio": {
        "bitrate": "130915",
        "bitrateAvg": "130915",
        "bitrateConf": "0",
        "bitrateLatest": "130712",
        "bypass": false,
        "channel": 2,
        "codec": "AAC",
        "samplerate": 48000
      },
      "id": 257,
      "name": "Audio",
      "type": "Audio"
    }
  ]
}

[Corrodias]

I don't know much about this, but I might as well try as much as I can to be helpful, since you haven't gotten a response, yet.

How are you feeding the video into the server? If you're using live streaming via a program like OBS, it may be adding its own b-frames when re-encoding, and so you may need to adjust its settings.

[bchah]

@Corrodias thank you for contributing!

In the MediaInfo analysis we see that the GOP structure shows as M=1. That means there are only I and P frames, not B-frames.

When streaming from OBS using B-Frames, here is the difference where we see M=2 (that means there is 1 B-frame between reference frames - e.g. I B P B P I)

StreamWithBFrames.mp4

General
Complete name                            : /Users/brian/Downloads/StreamWithBFrames.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 7.01 MiB
Duration                                 : 22 s 400 ms
Overall bit rate                         : 2 625 kb/s
Frame rate                               : 30.000 FPS
Writing application                      : Lavf59.16.100

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4
Format settings                          : CABAC / 4 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 4 frames
Format settings, GOP                     : M=2, N=29
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 22 s 400 ms
Source duration                          : 22 s 367 ms
Bit rate                                 : 2 384 kb/s
Maximum bit rate                         : 2 893 kb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 30.000 FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.038
Stream size                              : 6.37 MiB (91%)
Source stream size                       : 6.64 MiB (95%)
Color range                              : Full
Color primaries                          : BT.709
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.709
mdhd_Duration                            : 22400
Codec configuration box                  : avcC

[Corrodias]

Alright, since it's going through OBS, which encodes its own output stream from the canvas, I believe it's OBS's output settings that are important here, not the source video.

If you're using the Nvidia hardware encoder, you can specify the number of b-frames near the bottom of the encoder settings, and it should be set to 0.

If you're using the x264 software encoder, then in my experience, you must select the "baseline" profile in order for it not to include b-frames.

[bchah]

Thank you @Corrodias this is all good info about OBS but the topic here is that OME mis-identifies the stream as containing B-Frames when it does not. In my case I am streaming from the Blackmagic Web Presenter (a hardware encoder) rather than OBS.

By the way, when using x264 with OBS you can also use main profile, or any profile with 'zerolatency' tuning option and those will skip B-Frames too 👍🏻

[Corrodias]

Oh, sorry for the misunderstanding, and thank you for that tip. When you said "When streaming from OBS using B-Frames", I thought you meant that that was how you were streaming and what you were having trouble with. Carry on. 😅 Unfortunately, I lack the experience with Blackmagic and OME needed to assist in this scenario.

[bchah]

@Keukhan is this something you might be able to help with?