Recently, my colleague
Gumm and I
tested the robustness
of NTIA-approved DTV
converter boxes when
subjected to a single
undesired ATSC signal
on what were formerly
called “UHF taboo” channels. The FCC believed that DTV signals
would not be subject to any interference
from other than co-channel and (first) adjacent
channels. A desired ATSC signal on,
for example, Ch. 28 would not be
jammed by another ATSC signal on Chs.
30, 32, 33, etc. This left former NTSC
channels in the UHF band available for use
by broadcasters displaced when Chs.
52–69 were auctioned. So far that repacking
seems to have worked out well.
However, in the next repacking,
up to 800 broadcasters assigned Chs.
31–51 will have to either find a still-vacant
channel or share the 19.39 Mbps capacity
of a 6 MHz channel below Ch. 31 or
simply cease transmitting.
BEGS THE QUESTION...
Will this second repacking work? It all
depends on how many broadcasters displaced by this repacking decide to seek
a vacant channel in what is left of the TV
spectrum after the next repacking. If, say,
600 broadcasters seek a vacant channel,
they will be looking for a channel that is
vacant and probably has taboo relationships
to existing stations in that market.
Suppose, for example, Ch. 28 is
presently vacant and that there is no Ch.
27 or 29 allotment within the FCC-prohibited
distances from the broadcaster’s
present transmitter. There certainly will be
stations in his market on Ch. 30 and above, or on channels below 27.
Will their signals cause interference to
reception of a DTV signal broadcasting on
Ch. 28? Or, will a signal on Ch.
28 cause interference to existing stations
in the community? If the displaced broadcaster’s
transmitter happens to be co-sited
with the other broadcasters, interference
will generally not result.
But will each displaced broadcaster be
able to find a vacant channel? He will have
to be co-sited with the stations on those
UHF taboo channels because some receivers would otherwise suffer interference.
Not all broadcasters are presently co-sited
with each other. The FCC rules have
no restrictions except those which protect
against co-channel or adjacent channel
DTV-DTV interference. So it would appear
that any site could be used for a channel,
which would not create co-channel or adjacent
Our tests demonstrated that DTV-DTV
interference due to taboo channel relationships
between desired (D) and undesired
(U) ATSC signals will happen.
|Table 1: De-sensitization of receivers tuned to Ch. 28 by one undesired ATSC signal on Ch. 30
Table 1 shows the de-sensitization occurring
to Ch. 28 (N) from a DTV signal
on Chs. 30 (N+2) to 38 (N+10).
We found interference from all, with Ch.
30 being the worst. De-sensitization in
Table 1 is the difference between the minimum
usable D signal power with the interference
at the stated level and the minimum
usable D signals power when there is
no interference or multipath.
For example, at U = –20 dBm, the best
unit suffered 3 dB of desensitization while
the worst unit suffered 26.5 dB of desensitization.
As the noise-limited D signal
power (channel N) for our ATSC signal is
about –85 dBm, with one U signal on N+2
at–20 dBm, the minimum required D signal
power is 26.5 dB above –85 dBm or –58.5
dBm where the U signal power is –20 dBm.
This is a desired-to-undesired power
ratio (D/U) of –38.5 dB. If these transmitters
are co-sited, and well engineered, such
large power differences would not exist
and there would be no interference.
Wideband RF AGC circuits sense the
total signal power at the RF input to the
mixer. The gain of the RF amplifier is controlled
to maintain the signal level at the
mixer constant so that the mixer would
not be overloaded and generate nonlinear
In this way, interference by U signals on
a channel adjacent to the D channel was
greatly improved. This was important because
it allowed the use of first adjacent
channels in the same community, which
had not been possible with analog signals.
A desired DTV signal can be received
when it is as weak as –85 dBm absent interference
and multipath. The ATSC believes
that the strongest DTV signals at receiver
inputs is about –8 dBm. In our experiments,
we were able to approach this limit
as Table 1 shows (–11 dBm).
DTV receivers may be subject to U signals
on multiple UHF taboo channels. This
is exactly the problem that broadcasters
may find after the FCC repacks the TV
spectrum. Table 1 clearly demonstrates that
channels such as N+/–2 or +/–3 should
not be allocated in the same community as
Ch. N in the UHF band. With four U
signals of about the same power, the desensitization
increases by 4.7 dB.
Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via email