Transit Technology
As digital technology continues to wiggle its way into ever increasing A/V systems, it’s intriguing to find out applications where analog composite video is still the best value and selection for the job. One recent example of this involves a specifically challenging installation of audio and video surveillance systems on thirty-four light rail vehicles belonging to the GCRTA -- Greater Cleveland Rapid Transit Authority. While there can be no such thing as a routine rail car install, the one of a kind requirements and complicated logistics for this particular project rapidly narrowed the field of interested integrators. Among other requirements, the project called for each of 34 LRVs -- Light Rail Vehicles to be equipped with wireless color video transmission and recording, wireless intercoms, and an automated switching and control infrastructure. The integrator, Doan Pyramid LLC, chose Extron to provide all of the matrix switching and audio amplification for this project.
The Communication Challenge
Every morning, rail yard workers gather trains by linking up to three LRVs based on the anticipated passenger volume for a particular route. Unlike most rail cars, LRVs are designed to work as stand alone trains or to work as part of a modular train comprised of up to three units. Because of this modular design, the LRVs are not equipped with front and rear inter-compartment doors, so there is no way to walk between them. There is also no built in provision for inter-compartment communication. A passenger isolated in LRV #2 or #3 had no way of communicating with the operator in LRV #1.
The Security Challenge
The GCRTA also requested that the new A/V system integrate a security system in which video feeds from all eight cameras in each LRV are recorded onto dedicated DVRs. Between the safety and security revoyage, the GCRTA posed multiple A/V challenges that Doan Pyramid LLC was eager to solve.
The Solutions
Due to the trains’ modular design, there was no way to route cable between the LRVs. Electrical connections resided within the train couplings, but there was no room to integrate A/V communications into this system. At this point, with no cabling options, digital video transmission was ruled out. Douglas Fortney, Doan Pyramid's Lead Designer on this project, was forced to think about what could be done with analog composite video and wireless A/V transmission. “We [Doan Pyramid] love challenging wireless video projects like this, but there was more to this than assembling a secure, wireless video system," Fortney says. "We had to ensure uninterrupted communication between vehicles traveling through all types of transmission conditions and capture all audio and video from up to 24 cameras per train to an on-board security system.”
One of the first hurdles was planning for high-bandwidth video transmission. “We chose MPEG2 compression because, even though it requires more bandwidth than some other CODECs, it has no latency,” Fortney says. Zero latency was essential because this project required real-time communication via the cameras and intercom stations. The only way to make this wireless network successful was to manage bandwidth and control the number of signals sent at once. "We had to do a lot of work to get the packet size correct so that the packets didn't collapse the wireless network," Fortney says. "We finally dialed it all in and, boy, was that a moment to celebrate."
Enter the MAV Plus
“We used the MAV Plus to help control bandwidth requirements for A/V over the wireless network,” says Fortney. “That way, we were able to transmit only the video and audio signal we needed, and nothing more.” When a passenger presses the intercom button, the train’s control system identifies the camera tied to that call box, and the MAV Plus routes the selected video and audio directly to the operator if the passenger is in the same train car as the operator. If the passenger is in a different LRV, video and audio are routed to the local MPEG2 encoder, and transmitted wirelessly to the operator in Car #1.
The Quad Display
Because each LRV is equipped with eight separate cameras and space is limited, Fortney designed the operator’s touchpanel with a quad picture display. When the train is stopped and the doors open, the operator sees only the exterior door cameras; once the train is in motion, the operator’s view switches to the inside cameras. Fortney and his team designed the system so that the video signals are first routed from the MAV Plus through the PIP processor, and then the output of the processor is then routed back into the MAV Plus as a separate, combined input signal. “This allowed the PIP to ‘live’ inside the MAV Plus,” Fortney says. “We send the output of the PIP processor back into the matrix so that the PIP view could be sent where it’s needed.”
Fortney elaborates on the quad-display signal distribution: “In a typical three-LRV train configuration, we've all three 16x16 MAV Plus units, one per LRV, managing the four pictures in the quad PIP processor which are routed to the operator’s touchpanel controller. The operator can sometimes touch any one of those four pictures and the view he touches will appear fullscreen. When this happens, the MAV Plus switches from sending the PIP signal to just sending the full screen video of the chosen view.” This all happens without interrupting the DVR, which is recording directly from the MAV Plus on eight dedicated outputs, one for each of the LRV’s cameras. “This is a good example of why the MAV Plus is a crucial component inside the system. We’re able to manage our wireless network bandwidth efficiently, without compromising the security of the passengers or the quality of the recorded video signal.”
Intercom Amplification
When it came to voice communication, Fortney's team knew that standard intercoms wouldn't suffice in this environment, so they designed enhanced call boxes. These call boxes house special microphones and dynamic, high-powered speakers designed to overcome the ambient noise aboard the trains. Extron MPA 181T power amplifiers provide intercom application. "We chose Extron amplifiers because we needed close to 95 dB SPL in order for the voices to be heard over the custom call box speakers. The compactness of the MPA amps, their low power consumption, and minimal heat output made them ideal for this application."
There was also an issue of echoes produced between the speakers and the microphone. The outboard DSP handles this challenge. "The microphone signals from the call boxes are sent to the acoustic echo and noise cancellation unit, then on to the Extron MPA amplifier, and that’s what’s driving the speakers at the volumes we need," Fortney says.
The camera mounted above the call box provides real time video to the train operator who can rapidly assess the situation based on both audio and visual information inside the LRV. At one time a passenger initiates a call, the operator's quad view display converts to a sole video display showing the feed from the call box camera. During the communication, a red light on the call box lets the passenger know that video is being sent to the operator. "This helps reassure passengers that the operator is fully aware of any situation inside the LRV," Fortney says.
The Bottom Line
When asked how the system works, and if the GCRTA is glad with the results, Fortney gives an unequivocal "Yes." He adds, "We are just finishing the last train now, and the very first one we completed has been in service for about one year. It's been performing very, very well, and the GCRTA is quite pleased."