Last month we discussed
that the FCC
must work under in repacking
spectrum. This month,
I will venture out on a
limb and show how it
could be done under
those restrictions in a
way that minimizes interference to ATSC
reception after the Great Repacking. No, I
don’t know anything about how the FCC
plans to repack. I’ll start from the beginning.
Before most of you were born, the FCC
introduced frequency offsets of the visual
carriers of TV stations to minimize the visibility
of co-channel interference (CCI). The
beat between analog TV signals is the difference
in visual carrier frequencies. If these
beat frequencies were to change so that the
beat, while still audible, would apparently
vanish, they would reappear with a slight
additional change in the frequency offset.
Imagine three cities approximately 100
miles apart. At times, their reception of
analog TV was marred by CCI. The FCC set
offsets at –10,000 Hz, zero Hz and +10,000
Hz between visual carrier frequencies of
the stations comprising trios (Fig. 1a). The
beats went away with the introduction of
visual carrier frequency offset. Sadly, offsetting
of the ATSC pilot carriers has no effect
on DTV reception. This was proved at
the ATSC in 1995.
Let’s revisit those three stations (A, B
and C) now transmitting ATSC signals. With
all DTV systems, CCI and adjacent channel
interference (ACI) have the nasty effect of
blocking reception of picture, sound and
data. The viewer sees a flat blue field, and
hears nothing. No clue is provided.
|Figure 1a: Frequency offset of visual carriers
to minimize co-channel Interference
between analog TV signals
|Figure 1b: Channel offset to eliminate co-channel Interference,
One way to repack the TV spectrum
would be to create blocks of
contiguous channels (Fig. 1b).
The first block of contiguous
channels is allocated to the
community with station A.
The second block of contiguous channels
is allocated to the community with station
B, and the third block is allocated to the
third city in this trio.
If the UHF TV band is reduced to 18
or more channels, each community could
have up to six UHF channels. With digital
transmission standards there would be no
CCI. But we’ve just made ACI worse. This
block of six contiguous channels will overload
some receivers and reception would
fail due to the fact that the NTIA-approved
converter boxes and modern DTV receivers
we have tested are not perfectly linear.
They generate third-order intermodulation
(IM3) products when overloaded.
Take Block A for example—Channels 14,
15, 16, 17, 18, 19 and 20. Each contiguous
pair of channels could be subject to ACI.
But the FCC regulates ACI, so perhaps there
won’t be a problem.
Now consider the
plight of Channels 15–
19. Ch. 15 gets ACI from
Chs. 14 and 16. The sideband
power dumped into receivers
tuned to Ch. 15
increases by up to 3 dB. A
3 dB increase in the signal
power at the mixer
input would result in a
9 dB increase in third-order
except that all NTIA-approved
and modern DTV receivers
we have tested have
wideband RF automatic
gain control (AGC). This sort
of RF AGC was known in the
mid-1930s in high-end “all
wave radios,” but never used in TV receivers
The second signal forces the gain of the
RF amplifier to decrease in order to hold
the total signal power at the mixer constant.
This is no coincidence; there is no
known alternative to wideband RF AGC to
meet the FCC specifications regarding ACI.
So I believe all ATSC receivers will continue
to have some form of wideband RF AGC.
However to make this scheme work all transmitters
serving a given community should be
co-sited. Are all transmitters co-sited? In some
communities the answer is “yes,” but it is “no”
elsewhere and the elsewhere will not want to
be moved to an antenna farm.
So why does this scheme require co-siting?
It is the age-old near/far problem. A station
received from afar puts a weak signal into
the receiver, while a transmitter close to the
receiving site on an adjacent channel may
overload the receiver because the undesired
signal (U) is too strong relative to the desired
(D) signal; the D/U is too negative for the receiver
Transmitters not now co-sited present a formidable
set of problems. Perhaps there is a way
around this co-siting problem.
My second try allocates blocks as follows: Community A
gets Channels 14, 17, 20, 23, 26 and 29. Community B of this
trio of communities gets Channels 15, 18, 21, 24, 27 and 30.
The third member of this trio gets Channels 16, 19, 22, 25,
28 and 31 as shown in Fig. 2. Farewell to ACI. But consider
now that while we have avoided both CCI and ACI, there is
still the taboo channel interference problem.
The FCC’s Sixth Report and Order said that taboo channel
interference to DTV reception was almost impossible
so it is not covered by commission rules. This is why FCC/
OET Bulletin # 69 says nothing about TCI.
|Figure 2: Two examples (a & b) of staggered channel blocks to reduce noise Interference between DTV signals
For example, consider Channels 16 and 19. A third-order
intermodulation product they may generate in an overloaded
receiver falls upon Channel 22. Did I say overloaded receiver?
Yes, unless these transmitters are co-sited, the near/
far problem may exist at some residential sites within this
This problem can be solved with co-siting as our experiments
with both converter
boxes and with
modern ATSC receivers
have shown. The term
“co-siting” can have several
meanings. Ideally it
means the same site, the
same tower and the same
I believe the FCC
considers two stations
to be co-sited if their towers are less than 8 km apart. But
interference could also be mitigated by reducing the number
of third-order products that fall within the UHF spectrum
after repacking. This means staggering these channels.
If the UHF TV spectrum after repacking is Channels 14–32
the lowest IM3 = 2*14 – 32 = Ch. –4, which doesn’t exist.
The highest IM3 is 2*32–14 = Ch. 50, which will no
longer be a TV channel. With triple beats (TB)—the other
form of third-order distortion, and by far the most potent
source of interference when there are six signals—the
lowest TB = 14 + 15 – 32 = –3. No problem. The highest
TB is 31 + 32–14 = Channel 49, which no longer exists as
a TV channel.
Stanley Knight, a regular reader of this column, has contributed
to our research on this problem. Table 1 by Mr.
Knight shows two of the many combinations of channels
that are non-uniformly staggered. This table explains what
I mean by staggering the channels of a block.
|Table 1: Staggered Channel Plans A and B
There are many staggered combinations of channels
to be considered. Minimizing the noise in the locally used
bock of channels is the name of the game. Knight is working
on software to do just that.
The noise in a given channel consists of IM3 and TB.
Both of these third-order distortion products occupy
three contiguous channels. Each side channel is given
a weight of 1. The center channel of an IM3 is given a
weight of 4. Each TB is weighted 10 while its side channels
are weighted 2.5. These weighted sums for each local
channel are added and compared to alternative staggering
schemes to find the least noise for each channel. That
is a work in process.
What I find exciting is that I believe this scheme complies
with the constraints imposed by Congress in its 2012
legislation to authorize the FCC to auction spectrum. I
would like to hear from broadcasters and their consulting
engineers about this scheme, or about alternatives to
it. I do not intend to propose this or any other scheme to
the FCC. Only stakeholders should address this matter, I
Charles Rhodes is a consultant in the field of television
broadcast technologies and planning. He can be reached
via e-mail at firstname.lastname@example.org.