Comcast Moto STD HRC or IRC ?

Darrylhifi

Having a Comcast Moto HD Box, On my new DLP , in the setup menu it is asking me what type of cable system do I have. Can someone answer this question and explain to me what these terms mean as I am unfamiliar with them ? Thanks !

kdog044

Here's some information I found when looking for the same thing. From I was told by Comcast they use HRC but you may want to find out if it is different in your area. It may have to do with their use of Channel 1 for OnDemand.

Here's the history as I understand it:


In the early days of the cable TV industry, only Channels 2-13 were
used. Just as in the over-the-air broadcast world, second-harmonic
distortion was a potential problem; however, the fact that Channels
2-13 were split into two separate octaves rendered this problem moot:
all second harmonics fell outside of the bands of interest.


But as the industry grew, more channel space was needed, so channels
14-22 were added in the midband (120-174 MHz). Once those were used,
more channels were added in the superband (above channel 13).


These new channels were vulnerable to, and created, all sorts of
distortion products that fall in other channels. These products are
classified as follows:


SECOND ORDER DISTORTION: F1 +/- F2 (including second
harmonics). Example: the sum of the channels 3 and 4
visual carriers:


61.25 + 67.25 = 128.5 MHz.


This combination produces a spurious signal at 128.5
MHz, which falls 1.25 MHz above the visual carrier of
cable channel 15, right in the middle to the video
sidebands, where it causes rolling horizontal lines in
the picture.


THIRD ORDER DISTORTION: F1 +/- F2 +/- F3 Example:
channels 7, 8 and 9 visual carriers:


175.25 + 181.25 - 187.25 = 169.25 MHz


This combination produces a distortion product at 169.25
MHz, approximately at the visual carrier frequency of
cable channel 22. But (assuming that the three carriers
are generated by independent oscillators), this product
will not fall precisely on the visual carrier, so it
produces a "thumbprint" in the desired picture. If the
oscillators drift, the thumbprint dances around.


These problems became particularly severe as amplifier cascades became
longer: the more amplifiers in a cascade, the worse the distortion at
the end of the line. (The design goal for most cascades was a maximum
of 20 amplifiers, but I once heard of a cable system in California
that had a 67-amp cascade!)


To solve (or at least hide) these problems, various schemes were
developed for locking the visual carrier frequencies together at the
headend. The goal was to force the distortion products caused by the
interaction of visual carriers to fall precisely on top of other
visual carriers, effectively masking them.


Two schemes were developed:


- INCREMENTALLY RELATED CARRIERS (IRC)
(not to be confused with Internet Relay Chat)


This scheme phaselocks all visual carriers to a master oscillator
operating at F0 = 6.0000 MHz according to the formula


F = F0 * N + 1.2625


where N is an integer and 1.2625 is a constant (it was originally
1.25, but it was offset to 1.2625 to avoid conflict with aeronautical
communications -- but that's a different story).


Thus:


Channel 2 visual falls at 55.2625 N = 9
Channel 3 visual falls at 61.2625 N = 10
Channel 4 visual falls at 67.2625 N = 11
Channel 5 visual falls at 79.2625 N = 13
Channel 6 visual falls at 85.2625 N = 14


This scheme solves the third-order distortion problem, although it
doesn't solve the second-order problem.


Note that this scheme moves Channels 5 and 6 up by 2 MHz.
Consequently, this scheme only works if special arrangements are made
to accommodate this shift. Some cable operators provided special IRC
converters; others just left 5 and 6 vacant. Some "cable-ready" TV
sets were equipped with obscure little switches (or menu options) that
made the shift.


The converter that PAT described here several years ago was apparently
able to receive Channels 5 and 6 at both standard and IRC frequencies.
It received standard 5 and 6 on positions labeled 5 and 6, but it
received IRC 5 and 6 on positions labeled something else (55 and 56 as
I recall). Because the standard channels overlap the IRC channels,
it's not possible to use both. And that's why PAT was told that he
couldn't use Channels 55 and 56 if he used Channels 5 and 6.


Note that this scheme leaves a 6-MHz gap (72-78 MHz) between Channels
4 and 5. Which just happens to equal one television channel. And
that's cable Channel 1, with a visual carrier at 73.2625 MHz.


- HARMONICALLY RELATED CARRIERS (HRC)
(not to be confused with Hillary Rodham Clinton)


This scheme phaselocks all visual carriers to a master oscillator
operating at F0 = 6.0003 MHz +/- 1 Hz (that's right: plus-or-minus ONE
HERTZ) according to the formula


F = F0 * N


where N is an integer. The master oscillator frequency was originally
6.0000, but it was offset to 6.0003 to avoid conflict with
aeronautical communications -- again, that's a different story.


Thus:


Channel 2 visual falls at 54.0027 N = 9
Channel 3 visual falls at 60.0030 N = 10
Channel 4 visual falls at 66.0033 N = 11
Channel 5 visual falls at 78.0039 N = 13
Channel 6 visual falls at 84.0042 N = 14


This scheme solves both the second order and the third-order
distortion problems.


Note that this scheme moves everything down by (about) 1.25 MHz,
except for Channels 5 and 6 which move up by 0.75 MHz. Like IRC, this
system only works if special arrangements are made to accommodate the
shift. Some cable operators provided special HRC converters, and some
cable-ready TV sets were equipped with switches or menu options.


This scheme also leaves a 6-MHz gap (approximately 70.75- 76.75 MHz)
between Channels 4 and 5. So again, we have cable Channel 1, this
time at 72.0036.


In recent years, the use of fiber optics in cable TV networks has
dramatically reduced the need for long amplifier cascades (some
networks now have cascades as short as two amplifiers). This in turn
has virtually eliminated the need for IRC and HRC frequency schemes.
So most cable TV systems now use the "standard" frequency allocation
scheme: cable channels 2-13 fall at the same frequencies as broadcast
channels 2-13. And "cable channel 1" has been relegated to the
dustbin of ancient history.

__________________
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Darrylhifi

So Essentially what you are saying is Hillary Rodham Clinton was a pioneer in distortion free television ? LOL. Thanks for the info. Actually Comcast is telling me HRC as well. Girl put me on hold and asked a "tech" so sounds like its a legit answer. Thanks again.