Digital Video Gateway
ASE’s client, Petards Joyce-Loebl ltd (PJL), had an existing train-based CCTV recording system utilising proprietary protocols and storage media layout. PJL had the code for the recorders and their own cameras. All camera images were provided in 4CIF MJPEG frames, which were stored to disk as they were received. The video recorder was also able to forward unmodified frames as they were received to a simple driver’s monitor in the cab at the front of the train.
ASE’s project was required to meet several objectives:
- The end client (train manufacturer Bombardier (BT)) wanted the CCTV system to communicate directly with Bombardier bespoke controller units using IPTCOM; utilising the existing BT train Ethernet backbone.
- Because the on-train Ethernet network has limited bandwidth, the size of the bandwidth allocated to the CCTV system was limited to 30Mbit/s over any segment of the network.
- The CCTV system was required to handle 12 recording units, recording 87 concurrent cameras at 4CIF resolution at 12 frames per second, simultaneously streaming to 10 4CIF displays on monitors distributed around the train. (E.g. to cabs and guards areas.)
- The system also had to provide a tiled image of 12 external cameras on two tile-monitor display in two 3×2 grids.
- The monitors had to support either MPEG-4 or JPEG displays.
- The system had to be presented with a safety case to SIL-1 for the timely display of the DOO images to the driver. The real-time delay was required to be less than 300 milliseconds from live movement to display upon the driver’s monitors, with the ability to easily see a spherical object of 37 mm diameter in front of any door to the train; effectively a four-pixel height object.
After some consideration, ASE elected to develop a dual-redundant video gateway unit, which would be running on an off-the-shelf single board computer solution. Two units would be needed in each consist, or train unit. The resulting Digital Video Gateway (DVG) utilised a Fedora Linux kernel, certified using evidence and representation from ASE as suitable for SIL-1 use. The video pipelines were based on the GStreamer system, with a number of bespoke components added to provide GStreamer access to ASE’s own Linux communication libraries and the tiling feature of the system.
ASE designed the DVGs to be located on the same network switches as the driver’s monitors in order to help to reduce the segment bandwidth. The existing recorder units had to be modified to forward more than one screen of information at any time up to a maximum of 6 4 CIF streams. The DOO displays did not require 4CIF images; hence the possibility of saving bandwidth, but the video recorders did not have the processing power to provide any scaling facilities. To keep the display bandwidths within the allocated budget , ASE modified the camera code to produce 4CIF interleaved images, which meant that the DVR (Digital Video Recorder) could reduce the bandwidth required for DOO images by sending only the even frames. This meant sending a 2CIF image across the network, saving 50% of the bandwidth, without major processing overheads in the DVR. Further reductions in network bandwidth were achieved by routing these 2CIF images to the “local” DVG which cropped and then scaled the images down to a tile size of 2/3 CIF, for retransmission (if necessary) to the DVG at the front of the train.
The DVG utilised both the proprietary CCTV system protocol and the Bombardier’s IPTCOM protocol and were dual redundant, but not with a master slave concept. A train request was always satisfied by the DVG that was closest to the destination display monitor, unless that unit failed during transmission, or was failed at the time of the request, in which cases the alternative unit would process the request. The final display images sent to the driver’s monitor or guard’s monitor were sent as MJPEG frames with an RTP stream, controlled by RTCP feedback. The redundant switch-over system was able to detect the failure of a DVG and replace the stream to the driver so quickly that the driver had no knowledge that the stream had been interrupted, other than via, an alarm, which was sent through the train’s diagnostics system. The key point was that the failure could be dealt with at an appropriate time and would not affect the dispatch of the train at the time of the failure.
ASE were presented with the requirements, and given full access to the code of the cameras and recorders within the existing CCTV system, and then designed, implemented, tested, and approved the entire system to SIL-1 for all the software development. The DVG was developed to SIL-1 EN50128, while the camera and video recorder were rated at SIL-0. The overall system was certified to SIL-1 for the functionality defined.
On the final system, each DVG used a Core Duo Intel processor running at 1.7GHz in a sealed IP66 box designed by our client. The average latency through the system was 176 milliseconds, with the maximum frame delay occasionally hitting 260 milliseconds. ASE’s involvement in this project took 9 months from start to finish.
The system is currently in use for cab forward-facing cameras, passenger compartment cameras and Driver-Only Operation (DOO) door monitoring on the Class 379 Electrostar Units and the recorded video has been used in judicial representations in the UK courts.