convert-frame signal invalid

[Harry]

Using GStreamer 1.4.5 on Windows, and trying to extract the current frame as an image, without success.
My pipeline contains playbin as the only element, and it works fine for playing & pausing, but convert-frame fails.

My code is based on several examples I found and is similar to this:

GstCaps   *caps = gst_caps_from_string("image/png");
GstSample *sample = 0;

gst_element_set_state(pipeline, GST_STATE_PAUSED);
gst_element_get_state(pipeline, NULL, NULL, GST_CLOCK_TIME_NONE);

g_signal_emit_by_name(pipeline, "convert-frame", caps, &sample, NULL);

I get the error :

GLib-GObject-WARNING **: gsignal.c:3408: signal name 'convert-frame' is invalid for instance '086CD928' of type 'GstPipeline'

I get the same error when emitting the "convert-frame" signal to playbin and also to its video-sink.

Why is the "convert-frame" signal invalid ? To which element would it be valid ?