addprobe?

gistfile1.txt

        private bool Blocked
        {
            set {
                if (vis_resampler == null)
                    return;
                Pad queue_sink = vis_resampler.GetStaticPad ("src");
                queue_sink.SetBlocked (value, new PadBlockCallback (BlockCallback));
            }
        }

        private void BlockCallback (Pad pad, bool blocked)
        {
            if (!blocked) {
                // Set thawing mode (discards buffers that are too old from the queue).
                vis_thawing = true;
            }
        }