On Tue, 31 Jul 2007 15:42:52 +0000, w0jrm wrote:
Drivers don't have to adjust their car's radio dial, because some Des
Moines stations are changing themselves.

A new digital signal is causing a few glitches for some car radios.

The problem started a few weeks ago. Fans of 93.3 FM and 102.5 FM
noticed their radio station presets would sometimes refuse to stay on
the right station.

Drivers would find that when they push their preset button that is set
to 93.3 FM, it stays for a second and then switches back to 102.5 FM
by itself.

I STRONGLY suspect that what's going on in Des Moines is an
improperly-configured RDS encoder. This is not directly related to the
new IBOC digital radio system in use here in the States, but it seems that
the IBOC digital encoders also include an RDS encoder for the station's
analog side.

RDS is a 57KHz subcarrier that transmits low-speed data. Information like
the name of the station, a "program identification" code that's invaluable
for DXers as it often maps directly to the call letters (continuous ID!),
sometimes the name of the song/artist that's on the air right now.

It also includes an "alternate frequency" field. This idea came from
Europe where a single station may have a number of transmitters on
different frequencies. The station places all its frequencies in the "AF"
field on all its transmitters. If the radio senses the signal it's tuned
to isn't coming in well, it tries the other frequencies in the AF list,
looking for a better choice.

Multiple-transmitter stations are MUCH less common in the U.S.. I would
imagine many station personnel don't understand what the Alternate
Frequency (AF) field is all about. My guess is that when KIOA-93.3
configured their equipment, they entered the frequency of different (but
commonly-owned) station KSTZ-102.5 in the AF field -- and didn't enter
their own frequency of 93.3. Presumably some models of radio interpret
this as indicating not that 102.5 is an *alternative* frequency of KIOA,
but the *only* frequency.

I've heard of similar problems in other cities. The IBOC digital
deployment is resulting in RDS installations in a LOT of stations that
haven't used it before, and there seem to be a LOT more RDS-enabled
receivers than there were a few years ago.