by, 06.24.2012 at 03.47 AM (852 Views)
The new Rand McNally TND720, like it's older siblings the 710 and the smaller 510, has included the life time traffic by Navteq. This is also the same provider for the Garmin Nuvi465LMT(LT) and the 560LMT(LT) and most likely the new Cobra 8000 as well. And I suppose it's a wonderful feature and a good selling point IF you wish to spend the extra 80 bucks (for the Rand McNally receiver).
RBDS is the american moniker for Radio Broadcast Data System which basically, is a digital packet the rides in a side-band of an FM carrier transmitted by a station in the FM broad cast band (88MHz-108MHz). These packets contain channels which can carry several different flavors of information including traffic information assuming such services are available. That's the basic of the system. A google search can bring up a far more in depth look into the subject then I am willing to get into here. The important thing though, is to remember it rides on a FM stereo broadcast carrier-the same one bringing all that head bang'n rock into your truck's cab. And because it is FM, it is subject to the same rules that all FM carrier signals are subject to including the biggest issue - line of sight (los for short).
All FM ( frequency modulation) are subject to line of sight- it doesn't take much to degrade a receiver. From low power radios to GPS satelight signals, all FM can be blocked with very little effort. RBDS is no different. Which is the most serious set back in my opinion. A " footprint" is what engineers call the transmitter's signal radious- simpley, the are that is effectively reached by the transmitter. If one takes the time to look up Navteq's traffic website, you will see a list of cities- click on a city and you see a blob of color surrounding the city center- supposedly the footprint for the RBDS signal. It isn't. And that's important to note. Because the actual transmitter sites are not repersented meaning that footprint repersented is not the actual footprint for the signal. Indeed, unless one is very familar with the city, one doesn't know which FM broadcasters are carrying the signal and the actual location of the towers. And this has very apparent, very observable effect- the traffic warnings and alerts disappear from the screen ( in the RM's case. Garmin show's how old the last received data is). You could be on end of the city and the transmitters are on the other end. los blocked by building and terrain. Remember, the receiver has to pull the TMC ( traffic message channel) from the over all RBDS packet. The actual stereo channel is actually more energetic then the sub-carriers so while you may hear the music, your device may not "hear" the TMC. The receiver in the GPS ( I assume because this is propritary engineering by the manufacter) is eithor scanning the FM band or, more then likely, scanning an updated RBDS TMC carrier list for each city that has it. Navteq again, provides no data on that. But your receiver has to scan for the data packets and get enough of a signal over whatever the defined threashold is to decode the packet. Can't decode? No information.
Okay, maybe a little bit too technical. Bottem line time. I have experienced both strong and eratic, spotty reception of traffic data one two seperate devices ( two different manufactors). Lots of factors but the bottem line-except in one case the day before I posted this- one almost had to be right on the city to get reception. Don't expect the world of these receivers. Expect fair to good performance within the boundries of the town but expect the signal to drop not too long after leaving the town. Signal wise anyway.
Now that the hard part is over- here's something a little more easy to understand. Garbage in, garbage out. The traffic data just doesn't appear out of the ether by magic, some one ( and maybe something) has to enter the information in. This information can come from a lot of sources, from monitors and traffic sensors to live observations. Not every city that has TMC will have thier town wired the same way so one might not get the same or as accurate data as an that is wired ( like the LA area in California- really really good example of a wired area). And still someone has to enter this in or make sure the automated stuff is going into the feed- information can get delayed or skipped all together-and, well, if you can't see it then it doesn't help you does it. Another problem is the traffic itself. Familar with LA? By the time you actually get to where the "problem" area is, especially during rush hour, the displayed issue might be long over-or, maybe one that wasn't reported is now in progress.
So what is my opinion? I think there is a lot of poetential there and as it stands now, is a nice reference to use but certainly nothing to rely on very heavilly. In DC the other day, I didn't get anything till dang near the I695 around Baltimore but, it held till south of the DC area on 95. In contast, comming through Knoxville yesterday-hardly kept a steady lock- very spotty and I was downtown. Your results maybe better or worse, no two receivers react exactly the same way. But that generally is my take on this RDS traffic stuff.