invalid FPS detected from an AppSrc that is generating FLV

[Brian Dilley]

I have an app that implements the RTMP protocol from an ingestion/server
perspective.  This app properly implements the RTMP protocol and assembles
messages for the other parts of my app to handle.  Currently I'm taking all
of the video and audio messages and writing their payloads to an AppSrc in
FLV format.

My app src is created like this:

        GstElement* ret = make_element("appsrc", name);
        GstCaps* parsedCaps = gst_caps_from_string("video/x-flv");
        g_object_set(G_OBJECT(ret),
                     "caps",            parsedCaps,
                     "is-live",         true,
                     "block",           false,
                     "format",          GST_FORMAT_BYTES,
                     "duration",        GST_CLOCK_TIME_NONE,
                     "stream-type",     GST_APP_STREAM_TYPE_STREAM,
                     "max-bytes",       200000000,
                     nullptr);
        gst_caps_unref(parsedCaps);

I initially write an FLV header and an onMetaData tag to the appsrc giving
it details like framerate, bitrate, etc.  Then for every video and audio
message coming over the RTMP wire I write a corrosponding FLV tag to the
app src.

The app source feeds into a decodebin and then further down to some other
stuff.  The problem that I'm having is that the decodebin detects the
framerate of the FLV incorrectly.  It's doing it at ~17 fps when it's
really at ~30fps.  This results in the stream having the appearance of
being much slower than it actually is.  I'm curious if I'm potentially
missing something obvious with decodebin.

During debugging I wrote the FLV data to a file as well.  I found that
playback of that file in vlc was also incorrectly assuming ~17fps however
FFPlay (ffmpeg) played the file correctly (and ffprobe showed the correct
framerate information).

Incase it's helpful, follows is the rest of my pipeline, each of the bins
get stitched together in real time as the pipeline comes to life
(this->input is the AppSrc element, and in this case this->output happens
to be an rtmpsink):

    string decoderBinDesc = std::string("")
            + "queue name=input "
            + "   ! decodebin name=output ";

    string videoFilterBinDesc = std::string("")
            + "videoconvert name=input "
            + "   ! video/x-raw,format=" + frameFormat + " "
            + "   ! streamhubvideofilter name=output ";

    string videoEncoderBinDesc = std::string("")
            + "videoconvert name=input "
            + (!enableHwAccel
                ? "   ! x264enc bitrate=6000 pass=0 speed-preset=1
option-string=\"subq=4:bframes=2:b_pyramid=normal:weight_b\" "
                  "   ! video/x-h264,stream-format=avc,profile=main "
                : "   ! nvh264enc preset=low-latency-hp "
                  "   ! video/x-h264,profile=main ")
            + "   ! h264parse name=output ";

    string audioEncoderBinDesc = std::string("")
            + "audioconvert name=input "
            + "   ! audio/x-raw,channels=2 "
            + "   ! avenc_aac "
            + "   ! audio/mpeg,channels=2 "
            + "   ! aacparse name=output ";

    string muxerBinDesc = std::string("")
            + "multiqueue name=input max-size-bytes=0 "
            + "   ! flvmux name=output streamable=true ";

        this->decoderBin        = parse_bin(decoderBinDesc, false,
"DecoderBin");
        this->videoFilterBin    = parse_bin(videoFilterBinDesc, false,
"VideoFilterBin");
        this->videoEncoderBin   = parse_bin(videoEncoderBinDesc, false,
"VideoEncoderBin");
        this->audioEncoderBin   = parse_bin(audioEncoderBinDesc, false,
"AudioEncoderBin");
        this->muxerBin          = parse_bin(muxerBinDesc, false,
"MuxerBin");
        this->input             = inputFactory("input");
        this->output            = outputFactory("output");
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