Framerate issue with v4l2sink

[Lenz, Mika]

We are streaming data from appsrc into a v4l2sink. Therefore, we are using v4l2loopback on /dev/videoX. The camera has a fix resolution of 640x480 and a variable framerate. The framerate is set up at the beginning with a library provided by the manufacturer. The data is 16 Bit Gray LE.
The problem is, that Gstreamer always wants to put 33 frame per second into the sink. So, if we choose 60fps, it's 33fps and if we choose 30fps, it isn't working. Every value bigger than 33fps is working, but only 33fps in real and every value lower than 33 fps is not working at all.
Before this, we already set the framerate and pushed data into a multifilesink or a udpsink with correct frame rate at the end. So, the initialization should not be the problem. Is there anything special to remind working with v42sink and v4l2loopback?
This is from our application, I removed error checks etc:
cb_need_data (GstElement *appsrc,
          guint       unused_size,
          gpointer    user_data)
{
  static GstClockTime timestamp = 0;
  GstBuffer *buffer;
  GstFlowReturn ret;

  GstMapInfo map;
  eDALProxy640USBErr eReturnCode;

  buffer = gst_buffer_new_and_alloc(IRIMAGE_NBPIXELS*2); //allocate 2 Bytes for every pixel
  if(gst_buffer_map (buffer, &map, GST_MAP_WRITE)) {
while(true)
         {
             eReturnCode = Proxy640USB_GetImage(m_Handle, (short unsigned int*)map.data, paMeta, GETIMAGE_TIMEOUT);
             if (eReturnCode == eProxy640USBSuccess)
                    break;
         }
  }

  GST_BUFFER_PTS (buffer) = timestamp;
  GST_BUFFER_DURATION (buffer) = gst_util_uint64_scale_int (1, GST_SECOND, 33);

  timestamp += GST_BUFFER_DURATION (buffer);
  g_signal_emit_by_name (appsrc, "push-buffer", buffer, &ret);
  gst_buffer_unmap(buffer, &map);
  gst_buffer_unref(buffer);
}


gint main (gint argc, gchar *argv[])
{
sprintf(DEVICE, "/dev/video0");
// GStreamer
  GstElement *pipeline, *converter, *videosink, *videosrc, *appsrc;
  gst_init (&argc, &argv);
  loop = g_main_loop_new (NULL, FALSE);

// LWIR device
  m_Handle = NULL;
  eAGCProcessingValue agcVal = eAGCEnhanced;
  eDALProxy640USBErr eReturnCode;
  eReturnCode = Proxy640USB_GetModuleCount(&iCount);

// Connect to LWIR
eReturnCode = Proxy640USB_ConnectToModule(0, &m_Handle);

// Set Processing depending of parameters (NUC/AGC/Shutterless/Framerate)
  Proxy640USB_SetNUCProcessing(m_Handle, bBP, nuc);
  Proxy640USB_SetShutterLessProcessing(m_Handle, bSL);
  Proxy640USB_SetFloatFeature(m_Handle, efFrameRate, FRAMERATE);
  Proxy640USB_SetAGCProcessing(m_Handle, agcVal);
  Proxy640USB_SetUIntFeature(m_Handle, eiVideoUSBOutActivation, 1);

// Setup Pipeline
  pipeline = gst_pipeline_new ("LWIR-streamer");
  appsrc = gst_element_factory_make ("appsrc", "source");
  videosrc = gst_element_factory_make("videotestsrc", "source");
  converter = gst_element_factory_make ("videoconvert", "converter");
  videosink = gst_element_factory_make ("v4l2sink", "videosink");
  g_object_set (videosink, "device", DEVICE, NULL);

// Setting Capabilities
  g_object_set (G_OBJECT (appsrc), "caps",
        gst_caps_new_simple ("video/x-raw",
                     "format", G_TYPE_STRING, "GRAY16_LE",
                     "width", G_TYPE_INT, IRIMAGE_WIDTH,
                     "height", G_TYPE_INT, IRIMAGE_HEIGHT,
                     "framerate", GST_TYPE_FRACTION, FRAMERATE, 1,
                     NULL), NULL);

  gst_bin_add_many (GST_BIN (pipeline), appsrc, converter, videosink,NULL);
  gst_element_link_many (appsrc, converter, videosink, NULL);

// Setup appsrc to cd_need_data
  g_object_set (G_OBJECT (appsrc),
        "stream-type", 0,
        "format", GST_FORMAT_TIME, NULL);
  g_signal_connect (appsrc, "need-data", G_CALLBACK (cb_need_data), NULL);

  /* play */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);
  g_main_loop_run (loop);
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