Hybrid Digital Radio Power Increase

[Garrett Wollman]

<<On Sun, 31 Jan 2010 14:03:00 -0500, Larry Weil <kc1ih@mac.com> said:

> Last week the FCC approved an increase in the power level a station 
> can use for it's HD sidebands.  I haven't heard all the details, so 
> I'm not sure if this applies to FM stations only or to both AM and FM?

FM stations only.

> I know in many if not most cases the stations will have to purchase 
> new equipment to implement the higher HD power levels, so I guess it 
> may be a while before these increases actually happen.  Does anyone 
> know of any station that has the equipment and is planning to 
> increase their HD power soon, or perhaps already has done so?

Not in Boston IIRC, but there are a number of stations which
participated in the NPR Labs testing program which are presumably
equipped to do -10 dBc injection since they were testing it earlier.
Some stations will have enough transmitter headroom to make it with
their existing transmitter.  In general, the stations that will have
the most difficulty are the ones using high-level combining.  The
combiner losses are going to be ten times higher,[1] so they would
probably want (or need) to switch to space combining (i.e., using
separate antennas or an isolated dual-input antenna).  For some
stations, there will be issues of electrical capacity -- IBOC
transmitters have to run as class-A amplifiers, which are much less
efficient than the class-C amplifiers used in analog FM transmitters
(45% versus 85%).[2]

And, of course, whether you're doing -10 dBc or -20 dBc, exciter
reliability is still a problem.  The HD exciter is completely
proprietary to iBiquity -- even in the Harris, Nautel, BE, and
Continental exciters, the HD implementation is sole-sourced by
iBiquity -- and they have not proven enormously reliable in the field.

-GAWollman

[1] In the standard high-level design, if I recall correctly, you're
losing 1% of the analog and 90% of the digital at the combiner's
reject port.  Even if the combiner can take ten times the power on the
digital input port, the digital transmitter would have to ten times as
big -- which is to say, it would have to make as much output power as
the analog transmitter does.  You could design a combiner with
different reject levels, but then you'd have to increase the TPO on
the analog to compensate, which (for a full-power class-C) is likely
to be very expensive.

[2] Which is why so many stations chose to do high-level combining in
the first place.  At -20 dBc, the losses are not so horrible (for a
typical class-C with 35 kW TPO, you're only wasting 6.65 kW or 17% of
your RF output, as compared to low-level combining where you lose much
less RF but pay much higher power bills (79 kW(E), given the
efficiencies above, versus 49 kW(E) for high-level) to run the
transmitter in class A.  Much cheaper in the long run to use
space-combined or completely separate antennas.