V O R S I S H D P 3 G U I
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HD P3 / Aug 2006
HD P3 Features for AM Broadcast
The HD P3 is an ideal AM broadcast airchain processor given the
inherent multiband and broadband capabilities and a number of key
AM-specific features:
Brick-wall Bandwidth Filters.
These are accessible on “Output”. Three bandwidths are available:
4.5kHz—common where 9kHz channelization is in use (outside the
Americas).
5kHz—required should an AM station be simultaneously broadcasting
IBOC, and also recently proposed as at least a “night-time”
bandwidth standard within the US, aimed at reducing
adjacent-channel chatter.
10kHz—the US NRSC AM bandwidth standard.
Emphasis
It is common to add a rising gain-vs.-frequency emphasis to combat the
inevitable high-frequency rolloff of the IF filters in AM receivers. The
NRSC standard, for use in conjunction with the above 10kHz brick-wall
filter, is for use of a modified 75uS pre-emphasis, conveniently available on
your local friendly HD P3’s “Input” or “Output”. The “modification”
consists of a gentle rolloff of the pre-emphasis slope approaching 10kHz,
and with the HD P3 may be nicely emulated by applying the low-pass filter
(“Filters”) set to a frequency of approximately 12kHz.
Truth be told, the niceties of matching exactly such a paper specification
get substantially blown away when complex processing is used, but arrang-
ing for a non-excessive rising trend in response either by pre-emphasis,
multiband processing, or EQ to counter receiver IF “droop” is a good idea;
the brick-wall filter will keep sideband energy happily within the mandated
spectral mask.
