Mobile Viewpoint Reviewed: A (Literal) Road Test
So, as with each of the technologies I have reviewed, the video signal into the system is then presented at the receiver and, despite the underlying use of IP streaming technology, Mobile Viewpoint and all cellmuxes should be thought of as a video signal acquisition technology, enabling TV playout centers or security operations centers to receive a video signal from almost anywhere in the mobile phone networks’ footprint.
How We Tested Mobile Viewpoint
Lets talk about the specifics of my test now, so you can get a feel for the use and performance of the rig.
First let me introduce to you my two testing partners: Tim Thompson of Link-Tim is an old friend of mine who has for many years worked with contribution feeds for video webcasting. Historically he (and often I) worked with VSAT IP links, being the most affordable way to bring ad-hoc IP to remote locations. Cellmux technology potentially represents something very disruptive to the VSAT IP, and in wider terms to the satellite video services sector. Now, instead of a truck and a half-hour dish alignment, and the tethering to the truck, operators can be online within 90seconds, can be completely mobile and since they are on-foot there is no van to get through the traffic jams building up around the event. Tim has been keen to test these systems to become familiar with when they are appropriate to put in front of his clients.
I also seconded a friend, professional photographer, and all-around polymath Michael O’Rourke (www.thelightwhispererphoto.com) as our cameraman for the day, and for his terrific eye for detail—better than Tim or me (who typically just focus on IP and routing, rather than picture quality).
We tested the system was working and then, leaving on the receiver (which always runs a DVR service, capturing up to 24 hours) connected to my domestic (Virgin cable) broadband line, we headed out, first to the local electronics store to buy a component or two I needed.
The idea was that we would take a long drive with the link running throughout. The aim was to record, back at my home office on the receiver, the live stream, and then to review this for quality on our return.
I only had a Sony Handycam available, and while the unit can handle 1080p source, I fed it an SD PAL Composite video source. The device has ?” audio jacks (in my mind a little unusual) but I only had RCA phone audio on my Handycam cable, so we elected to leave audio out.
We pushed the telescopic antenna out of the rear window of my car (Figure 3), with Tim in the back operating the unit and Michael holding the camera and responsible for things like ensuring a few shots of time keeping devices to allow us to sync our data later. I then drove us out of Brighton, through the suburban zone north of the city, out onto the dual carriageway, out to a major roundabout near a local town called Lewes, then back again to Brighton, returning around the outskirts to my home office. This is a journey that covers nearly every aspect of typical highway cellular coverage from dense population to relatively remote rural areas.
Figure 3. For our testing we placed the Mobile Viewpoint backpack in the backseat of a car, pushing the telescopic antenna out the window. (Photo 2012 by Michael O'Rourke, www.lightwhispererphoto.com)
To ensure we had detailed logs of velocity, GPS location tracking, and altitude I used an app I usually use for jogging (LogYourRun) to log our journey.
The key reason we wanted to setup this way was to (admittedly probably far less scientifically) replicate the testing that the BBC undertook when selecting the Mobile Viewpoint technology for the Olympic Relay coverage. They key performance indicator is not rocket science: we were looking for a measure of how continuous the link was.
Now I must get into some of the detail of the device options so that you can understand the assumptions and account for the inherent error yourselves.
One of the options available to the user is to vary the latency. To a savvy streaming audience (as I know you all are) this essentially means increasing the buffering over the link. While increasing the buffer reduces the number of dropouts—points in the transmission where the buffer is starved and the stream stops until the buffer is replenished—this also means that the time from input at the transmitter to output at the receiver is increased.
Tim and I come from a live sports and financial news streaming background, and typically these two areas are quite sensitive to latency. There are many scenarios that are less sensitive, but we wanted to see the system performing with the two-second latency setting, which actually delivered pretty much as promised; I have seen encoding platforms that claim X seconds of latency only to actually deliver many-times-X in reality.
It is important to note that Mobile Viewpoint and the BBC have developed their own buffering algorithm (‘CPT’), which gives then a staggering 2-minute link-latency. This has been specifically developed for the Olympic Torch relay. It’s a great model; if the signal drops away while the link is active, the backpack continues to create the high-quality—typically 2.5Mbps H.264 /AAC—stream, even if a lower rate is all that is available. When the signal picks up quality again then, any packets that have been re-requested in the previous two minutes are sent over and above the live 2.5Mbps feed. These issuing packets are sorted and inserted in the buffer on the receiver and the output signal is “reconstituted” back to the full 2.5Mbps signal. This means a continuous 2.5Mbps signal can be delivered even if the link drops out completely for many tens of seconds. If you can put up with the 2-minute latency, this is a unique and interesting feature, and clearly helped Mobile Viewpoint win the opportunity with the BBC.
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