Mon Dec 18 08:24:34 EST 2006
> 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.
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
# 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
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.
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.
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.
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