WCRB transition

[Ric Werme]

> On 18 Dec 2006 at 0:58, Larry Weil wrote:

>> At 12:50 AM -0500 12/18/06, A. Joseph Ross wrote:

>> >A friend of mine told me today that he was surprised when the car
>> >radio in his new Prius automatically reset itself to tune to 99.5 for
>> >WCRB.  What sort of mechanism will cause it to do that?
 
>> Probably RDS.
 
What's RDS?

Google got me to that web page John posted, perhaps the germane
feature is one of:

# AF: Alternate Frequencies. This allows a receiver to re-tune to a different
  frequency providing the same station when the first signal becomes too weak
  (e.g. when moving out of range). This is often utilised in car stereo
  systems.

# PI: Program Identification. 
  This is the unique code that identifies the station. Every station receives
  a specific code with a country prefix.

# PS: Program Service.
  This is simply an eight-character static display that represents the call
  letters or station identity name. Most RDS capable receivers display this
  information and, if the station is stored in the receiver's presets, will
  cache this information with the frequency and other details associated with
  that preset.

So maybe when the radio preset was made it recorded the call letters or
some other uniquely WCRB item and when 102.5 wasn't satisfactory, it was
redirected to or went looking for another WCRB source.

http://www.radio-electronics.com/info/broadcast/rds/rds.php suggests:

  Autotuning
  RDS brings intelligence into the tuning of a radio. The autotuning facility
  comes into its own on long journeys when the car moves from the service area
  of one transmitter to the next. Without RDS the radio has to be manually
  tuned to the next station. This is not always easy because it is difficult
  to reliably detect which is the strongest station.

  An RDS set will look for the Programme Identification or PI code. A national
  network will be broadcast from a large number of different transmitters
  around the country. The station or network eg Radio 4 will have its own PI
  code. When the radio moves out of the range of one transmitter the radio
  will seek the strongest signal which has the same PI code, allowing the
  radio to remain tuned to the same programme.

  When radios fitted with RDS store a station frequency, they also store the
  PI code along side it. This has the advantage that when the radio is turned
  on in a place outside the coverage area for the transmitter frequency which
  is stored then the radio will seek the strongest signal which has the
  correct PI code.

  Local radio stations also have a PI code. In view of the local nature of
  these stations the PI code works slightly differently.

  If the station has two or more transmitters then the PI code will operate in
  the normal way when it is range of these transmitters. However when the
  radio moves outside this coverage area it will retune to the strongest
  signal of the same type of station.

  The PI code consists of four characters. The first indicates the country of
  origin and for the UK this is C. The next one indicates the type of
  coverage. The figure "2" indicates a national station, and the final two
  characters are the programme reference. For example Radio 3 has the PI code
  C203 and BBC GLR has C311.

  Instant Tuning
  It takes a number of seconds for the radio to search for the strongest
  signal with the correct PI code. During this time the radio would mute
  itself and the listener would have an annoying gap in listening. To enable
  the set to tune itself very quickly from one transmission to the next each
  transmitter broadcasts a short list of frequencies of adjacent
  transmitters. This vastly reduces the amount of seeking which the radio set
  has to perform. In addition to this a second front end is often employed to
  constantly detect the strength of the alternative frequency
  transmissions. This results in much faster changes in setting - to the
  extent that the listener should not be able to detect when the radio changes
  from one transmitter to another.

  Another facility associated with tuning is called the Programme Service Name
  (PS). This enables the set to display the station name. This normally takes
  a second or two to come up on the display after the station has been tuned
  in. However it is a most useful facility with the ever-increasing number of
  stations on the air.

     -Ric Werme