[BC] IBOC AM Quality

[Barry McLarnon]

On Monday 26 March 2007 12:33, Mark Humphrey wrote:
> On 3/26/07, Mike Erickson <wirelessmedia at gmail.com> wrote:
> > I have seen the BA and Accurian drop in severe multipath situations,
> > similar to what you describe.  I don't know how much better your
> > listening experience would have been.
>
> Does anyone happen to know offhand the "guard interval" specification
> for OFDM carriers in the iBiquity FM system?  This is directly related
> to the amount of multipath delay it can handle without inter-symbol
> interference.
>
> My personal experience has been that FM HD Radio works well in urban
> multipath situations (because the delays are relatively short) but I
> haven't observed good results while driving in mountainous terrain.
> Recently, I spoke with a station engineer in the Bay Area who has been
> using the FM system for a few years; he tends to agree this is a
> shortcoming.

The guard interval in the FM system is 150 microseconds, though it apparently
falls apart when the multipath delay exceeds half that amount.  If 
synchronized repeaters are used, the specs recommend spacing them such that 
receivers never see a spread of more than 75 us between the signals.  Still, 
75 us should be enough to handle all but the most extreme single-transmitter 
multipath situations.  Another possibility is that the synchronization 
strategy used in the receiver demod is less than optimal, and it breaks down 
in some multipath situations.  Developing synchronization algorithms is a bit 
of an art form, and from my experience with testing Eureka DAB receivers, I 
can testify that not all digital receivers are born equal!  Some tests with a 
multipath simulator can really separate the men from the boys.  And unlike 
Eureka system, the IBOC system uses coherent demodulation, which may make 
mobile reception in difficult multipath scenarios more dicey.

Barry

-- 
Barry McLarnon  VE3JF  Ottawa, ON