North American cable television frequencies
In North American cable TV networks, the radio frequencies used to carry signals to the customer are allocated to standardarized channel numbers listed in the CEA standard 542. Cable channel frequencies are generally different from over-the-air broadcasting frequencies. Since the cable network is a closed system, frequencies used for over-the-air services such as mobile radio, cellular telephone, or aircraft communications can be assigned to carry television programming.
The assignment of channel frequencies must on the one hand reduce the effects of distortion and mutual intereference generated within the cable television distribution system, and on the other hand maintain compatibility with the customer's connected equipment.
Slight frequency offsets are applied in some systems so that any signal leakage out of the cable distribution plant is less likely to cause objectionable interference to over-the-air users of the same frequencies.
Harmonically related carriers (HRC) is a system for assigning television channel numbers to bands of frequencies over a cable TV network. All video carrier frequencies are on exact multiples of the channel bandwidth frequency (6 MHz in the case of System-M used in most countries of the Americas, other TV systems worldwide use 7 MHz or 8 MHz).
William Grant, in his book, Cable Television, 2nd edition, GWG Associates, Schoharie, NY, 1988, page 82, states:
"By harmonically relating the carrier frequencies themselves it is ... possible to improve system performance. This does not reduce the beats produced, but positions them within the system transmission spectrum, such that they are more tolerable. In effect, all signal carriers are spaced precisely at 6 MHz apart, and thus, all beats generated are at 6 MHz increments."
"Since the television signals are vestigial sideband modulation, if the beat products can be manipulated to fall on or near the RF carriers themselves, they are much less offensive."
Incrementally related carriers (IRC) is a system for assigning television channel numbers to bands of frequencies over a cable TV network. The IRC plan attempts to minimize distortion products by deriving all video carrier signals from a common source. The IRC system assigns channel frequencies (for the North American NTSC-M system) spaced 6 MHz apart. In an IRC (Incrementally Related Carrier) system, the VHF channels are at their off-air frequencies except for channels 5 and 6, which will be 2 MHz higher than usual.
- In North American cable television the IRC frequency plan would be:
IRC 2 54-60 MHz 3 60-66 MHz 4 66-72 MHz 1 44-50 MHz1 72-78 MHz (A-8) 5 76-82 MHz 78-84 MHz (A-7) 6 82-88 MHz 84-90 MHz (A-6)
North America cable television frequencies
These frequencies are used for both (NTSC-based analog television) and (QAM-based digital television). Band plans for North American cable television systems are standardized in EIA standard 542-B.
- NOTE: Frequencies given are for luminance carriers. For channel center frequencies, add 1.75 MHz.
|Physical Channel||Video Carrier (MHz)||QAM Carrier (MHz)||Audio Carrier (MHz)|
|Subband CATV "T" Channels|
|1||73.25 (A-8)||75.00||77.75 (A-8)|
|5||77.25 or 79.25 (A-7)||79.00 or 81.00 (A-7)||81.75 or 83.75 (A-7)|
|6||83.25 or 85.25 (A-6)||85.00 or 87.00 (A-6)||87.75 or 89.75 (A-6)|
Channels T-7 through T-14 are sub-band channels and are not used for normal television channel distribution. These channels are used for sending video to the cable television headend, such as from Public, educational, and government access (PEG) stations through a cable TV system. They are also used by cable modems for sending upstream data to the headend's CMTS.
Channel 1 frequency assignments, where provided, are non-standard. If a Channel 1 were inserted (A-8) between Channels 4 and 5, Channel 5 would need to move 2 MHz off-frequency, thereby pushing Channel 6 into spectrum needed for FM stereo radio by the same 2MHz. Any assignment placing Channel 1 in its historical location before VHF Channel 2 would also be problematic, as the last cable reverse channel (T-14) now occupies frequencies from the defunct terrestrial Channel 1. Most systems that provide a (named) "Channel 1" will therefore either alias "1" to some higher converter channel (such as 101) or to a digital virtual channel.
Cable channels 2 through 13 operate on the same frequencies as broadcast television (the VHF band). They were assigned by the FCC. The other channels were assigned by cable television operators.
Cable channels 65 through 94 and 100 through 125 operate on approximately the same frequencies as broadcast television (the UHF band). Ultraband 65 and up will appear to be UHF TV 14 and up on most non-cable-ready analog television receivers; as each ultraband channel is exactly 2 MHz below a standard UHF TV channel, a slight fine-tuning of mechanical UHF tuners is all that is required to shift this block squarely onto the UHF dial.
Cable channels 95 through 97 (90-108 MHz) operate on the same frequencies as FM radio, so cable companies offering FM radio will not show TV programming on these channels.
Cable channels 98 and 99 (A2 and A1, 108-120 MHz), if used, have appeared as channel 00 and 01 respectively on some converter boxes.
Many cable providers currently do not distribute any video content above channel 139, or about 900 MHz. As of 2009, it is common for TVs in North America with built-in tuners to not search analog or digital channels above channel 139. Most TVs made before 2005 do not include a QAM tuner, and only have an analog cable tuner which often cannot tune beyond channel 125. With the addition of services such as premium HD content, cable providers such as Cox Communications and Insight Communications have begun to roll out digital cable services which use frequencies up to 1GHz (analog channel 158), while also dropping the analog channel formats.
Digital cable channels
Digital television receivers with integrated QAM tuners use ATSC conventions for numbering channels; unlabelled channels would appear as a main channel number, separated by a dot or dash, then a digital subchannel number. For instance, this may display as a physical converter channel number, a dot or dash, then a virtual channel number as it appeared on a digital cable set-top box. As there is no requirement that any specific convention be used to number digital or virtual channels, numbering is at the discretion of the cable operator. Digital cable channels are often numbered starting at 100 or 200, but these are virtual channel numbers and do not correspond to used frequencies.
- Television channel frequencies
- Moving image formats
- Broadcast television systems
- Walter S. Ciciora (ed), Modern cable television technology: video, voice, and data communications , Morgan Kaufmann, 2004 ISBN 1-55860-828-1, page 399
- Walter S. Ciciora, Modern cable television technology: video, voice, and data communications Morgan Kaufmann, 2004 ISBN 1-55860-828-1, pages 397-402