So TomTom have tackled this head on with the release of a TomTom RDS TMC receiver which is compatible with the newer TomTom Go 510 / 710 and 910 units (not the older 300 / 500 / 700 or One devices). It is an external unit which is dash mounted and connects to the TomTom via a small cable with a shortish run. The unit is an FM receiver which looks for TMC data signals transmitted as silent FM data channels. It's designed for Europe wide coverage and in the UK it uses iTIS service in the UK which is broadcast from 40 classic FM transmitters around the country.

The idea behind the TomTom RDS TMC receiver is that the traffic data freely available from iTIS is decoded from the FM signal and displayed on the Go unit in an identical manner to the standard TomTom Traffic service. This option will enable a few new menu options on your TomTom that helps you firstly see that the unit is working and then allows you to react to the traffic info.

Incidents are displayed on screen as a roadwork's or traffic queue icon with the total expected delay time and you are offered the option to replan which uses the roadblock feature that many users rely on during daily commutes already. The traffic warning data isn't prefect as we found with our review of the main TomTom traffic service but it is well integrated with the navigation views and demands little of you while driving.

So our review is really about the TMC FM reception rather than the data itself which is already covered in our TomTom Traffic review. Clearly a one off fee of £50 for this TMC RDS unit is preferable to a monthly subscription fee and GPRS data costs, but it has to work reliably. With the unit connected to our test TomTom Go 510 we set out on some test routes on a day in the run up to Christmas, according to the radio and to Trafficmaster the M25 was pretty busy with roadwork's in the Holmesdale tunnel causing a queue of at least 5 miles, surely we'd get this alert?

We watched the TomTom's screen expecting a traffic icon to appear on our route, but all remained quiet in fact strangely so. We checked the Traffic menu where you can see that the receiver is tuned into a TMC signal and found that it had locked onto Heart FM 106.2 Mhz rather than Classic FM at around 100 Mhz. It transpires that the second provider of TMC data, Trafficmaster broadcasts alongside Heart on a silent data channel and our unit had locked onto this channel but would not use the data as it was invalid. After about 5 minutes we noted that the unit had retuned and was now showing a frequency near classic FM with a green icon suggesting data was being received.  Hopeful that some incidents would soon be flagged on our TomTom we pressed on until we were firmly stuck in what the radio RDS reports described as 8 miles of traffic.

Still we had no reports at all and decided to use plan B which involved using our older TomTom unit, LP's mobile with GPRS and a 1 month free trial of TomTom Traffic (cheers TomTom) to see what data was being sent. After a bit of a fiddle we had TomTom traffic up and running and soon saw a very different picture of the London area which looked more convincing than the picture of serenity on the Go 510. Our own jam we shown very clearly with an estimated 20 minutes delay which prompted our other TomTom unit o offer as an alternate route (sadly we were stuck fast).

We set about performing some tests to see if we had a dud TMC unit, these involved firing up the TomTom in the office with the unit on the first floor hanging out of a window. We can't be sure but it seems a safe assumption that the cable between the unit and the TomTom doubles up as the antenna. After a few minutes we started to get alerts coming through that were inline with those picked up via GPRS. The unit showed that it was locked to classic FM as before and the green icon was present, however data was clearly flowing this time.

Some desk research showed that the TMC data channels are far more susceptible to interference than the audio channels alongside them, so while we were listening to Classic FM that is no guarantee that the data channel is being received cleanly. The icon in the traffic menu seems to be present when the data carrier is detected not when data is successfully being decoded and there is no other form of indication, save from the actual alerts themselves.

We tried the now proven receiver on a few more journeys in the run up to Christmas, on a few occasions we did pick up some alerts on various routes however these rarely seemed to update which led us to doubt their accuracy and wonder about alerts that had been missed. That seems to be the flaw for the RDS TMC system, if you can't be sure that your are getting data then you doubt the system and it looses its value. Why TomTom have shipped a unit which relies on an internal antenna rather than allowing it to be connect to the one already in the cars radio escapes us. Yes this would be tricky to connect in some cars but at least you would get a strong signal of what turns out to be a weak data transmission.

The TomTom RDS TMC receiver may be under £50 and remove the worry of data charges but our review suggests that this is replaced with a worry about missed traffic alerts and leads us to recommend that for now the TomTom Traffic GPRS option is the better bet.

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Published - 30/12/2007

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