Comcast digital channel assignments

L

lesdotcom

Thread Starter
Member
May 11, 2006
5
0
Does anyone know what the digital channel frequencies are for comcast? I found many lists of analog frequency assignments but have yet to find any digital channels listed. Is this standardized? I need to install a filter to make room for my local modulators and don't want to inadvertently remove any channels I watch or filter out my comcast phone service. So far I have tried filtering above channel 78 (the last analog channel I have) and lost the majority of my digital channels.

Les
 
M

Mike_CA

Active SatelliteGuys Member
Sep 27, 2006
17
0
Once you go digital, it would be extremely hard to put any type of filter on the line.

The cable system is either a 550 Mhz, 750 MHz, or 860 MHz system. Usually everything below 54 MHz is phone, Internet, and On Demand. Analog channel 2 is at 54 MHz and every analog channel after that is 6 MHz apart up to channel 100. If there are empty analog channel numbers, those frequency bands may be used by digital channels. Only one analog channel can fit into a 6 MHz band but if it is used by digital channels, a band can supports multiple channels up to a total of 38.6 mb/s on a 256QAM system. Typically digital SD channels use between 2.5 mb/s to 3.5 mb/s so approxiamtely 10-16 digital SD channels can fit into one 6 MHz band. HD channels are anywhere from 10 mb/s to 19 mb/s so either 2 or three HD channels can fit into a 6 MHz band. Also HD and digital SD channels can be intermixed into one 6 MHz band.

Above channel 100 are the remainder of the digital channels. The STB aquires a map from the headend to map the digital channels to the displayable channel on the STB. Since there are various size cable systems and mapping can be done by the STB, there isn't any fixed channels when it comes to digital. It would be nearly impossible to have fixed channel numbers like analog due to the large number of digital channels which would probably cause conflicts if they were fixed.

If you have a Motorola 6200, 64xx, 34xx, or possibly a standard digital STB (I can't tell you how to do it though with a standard STB) you can determine where each of the digital channels lies with the cable system. Perform the following to determine the band for the channel.

  • Select the channel that you want to determine which band that channel resides in.
    Power down the STB/DVR and hit Select/OK within 2 seconds.
    A diagnostic screen will appear.
    Down arrow to d06 Current Channel Status.
    Click Select/OK.
    The Inband Frequency for the channel will be displayed.
    Use this formula to determine the band. (Inband frequency - 54)/6+2.
As an example and Inband Frequency of 717 will produce the following.

(717-54)/6+2 = 112 (the band or physical channel number).

You can possibly determine if you are on a 550 Mhz, 750 MH or 860 MHz cable system. If you not see bands or physical channels above 84 you are on a 550 MHz system. If you see channels above 117 your are on an 860 MHz system but if see bands only below 117 but also above 84 you are on a 750 MHz system.

Even if you we able to determine where your channels resided today, tomorrow they may reside in another place. This is because Comcast may reorganize the system whenever a new channel is added or removed.
 
L

lesdotcom

Thread Starter
Member
May 11, 2006
5
0
Thanks Mike!

The diagnostic info should be very helpful. If I remember correctly I have a 6412. Just out of curiosity where do the factors in the formula come from?

Les
 
M

Mike_CA

Active SatelliteGuys Member
Sep 27, 2006
17
0
lesdotcom said:
Thanks Mike!

The diagnostic info should be very helpful. If I remember correctly I have a 6412. Just out of curiosity where do the factors in the formula come from?

Les
Channel 2 or band 2 is at 54 MHz, channel 3 is at 60 MHz, channel 4 is at 66 MHz, etc. and the bands are 6 MHz wide. So:

(Frequency - Starting Band for channels ) / bandwidth size + Starting channel Number = the band or physical channel being used

I could have used a simpler formula such as the following but I didn't want to confuse you.

(Frequency - 42) / 6 = band or physical channel being used

but this assumes that there is a channel 0 at 42 MHz and a channel 1 at 46 MHz which do not exist.

A little more information. When you see a digital channel number via OTA or a QAM tuner such as 7.1, 7.2, etc. it is seldom on the channel 7 band. The logical channel number is in the transmission packet and is PSIPed (aliased) to indicate the logical channel number. When a scan is done for digital channels, a map is saved in non volatile memory between the physical channels and the logical channels. So the OTA digital channels for your OTA channel 7 may be coming from physical channel 53. Some digital channels on a cable system are PSIPed or aliased for digital tuners (most major national network channels but sometimes they forget to alias them) while others are not (most non national network channels).

Also you will notice that if you are on a 550 MHz cable system, there is very little room to add more digital channels. If there are 70 analog channels on the system, only a total of 17 bands will be available for all the digital channels. If there are currently 140 SD digital channels transmitting at 3.5 mb/s, that will use about 14 bands. Then there will only be 3 bands remaining for HD channels or about 6 to 8 HD channels.

Since about 15% of Comcast cable systems are 550 MHz systems, this has held back providing more HD channels. Comcast doesn't want to provide too many HD channels to 750 MHz and 860 MHz customers since they charge the same price for all customers and 550 MHz customers would probably get very mad if they are left furthur behind. As an example, the SF Bay which has a mixture of systems do not get UHD and only recently got TNT-HD (but INHD2 was dropped when we got TNT-HD)which have been available in other areas. Currently in the bay area 550 MHz customers only get 2 of the 4 HD movie channels, no TNT-HD, no Discovery HD, no ESPN2 HD, and no On Demand that 750 MHz and 860 MHz customers get.

I'm ready to eliminate analog channels and broadcast all digital!!
 
Last edited:
M

Mike_CA

Active SatelliteGuys Member
Sep 27, 2006
17
0
Oops, I may have given you a bit of incorrect information in my first post. I said that Video On Demand (VOD) and Internet use frequencies below 54 MHz. It appears that most implementations have downstream VOD and/or internet at frequencies above 54 Mhz. However, upstream data is usually sent at frequencies below 54 Mhz.
 
codee

codee

SatelliteGuys Family
Jun 3, 2005
97
0
Twin Cities, MN
Mike_CA said:
Once you go digital, it would be extremely hard to put any type of filter on the line.

The cable system is either a 550 Mhz, 750 MHz, or 860 MHz system. Usually everything below 54 MHz is phone, Internet, and On Demand. Analog channel 2 is at 54 MHz and every analog channel after that is 6 MHz apart up to channel 100. If there are empty analog channel numbers, those frequency bands may be used by digital channels. Only one analog channel can fit into a 6 MHz band but if it is used by digital channels, a band can supports multiple channels up to a total of 38.6 mb/s on a 256QAM system. Typically digital SD channels use between 2.5 mb/s to 3.5 mb/s so approxiamtely 10-16 digital SD channels can fit into one 6 MHz band. HD channels are anywhere from 10 mb/s to 19 mb/s so either 2 or three HD channels can fit into a 6 MHz band. Also HD and digital SD channels can be intermixed into one 6 MHz band.

Above channel 100 are the remainder of the digital channels. The STB aquires a map from the headend to map the digital channels to the displayable channel on the STB. Since there are various size cable systems and mapping can be done by the STB, there isn't any fixed channels when it comes to digital. It would be nearly impossible to have fixed channel numbers like analog due to the large number of digital channels which would probably cause conflicts if they were fixed.

If you have a Motorola 6200, 64xx, 34xx, or possibly a standard digital STB (I can't tell you how to do it though with a standard STB) you can determine where each of the digital channels lies with the cable system. Perform the following to determine the band for the channel.

  • Select the channel that you want to determine which band that channel resides in.
    Power down the STB/DVR and hit Select/OK within 2 seconds.
    A diagnostic screen will appear.
    Down arrow to d06 Current Channel Status.
    Click Select/OK.
    The Inband Frequency for the channel will be displayed.
    Use this formula to determine the band. (Inband frequency - 54)/6+2.
As an example and Inband Frequency of 717 will produce the following.

(717-54)/6+2 = 112 (the band or physical channel number).

You can possibly determine if you are on a 550 Mhz, 750 MH or 860 MHz cable system. If you not see bands or physical channels above 84 you are on a 550 MHz system. If you see channels above 117 your are on an 860 MHz system but if see bands only below 117 but also above 84 you are on a 750 MHz system.

Even if you we able to determine where your channels resided today, tomorrow they may reside in another place. This is because Comcast may reorganize the system whenever a new channel is added or removed.

I have just the thing you are looking for....unfortunately I cant fidn it right now lol. But, I do have a chart showing what every frequency is used for, and what channels are assigned to what frequency.

"Usually everything below 54 MHz is phone, Internet, and On Demand"
Not at all. The downstream for these usually reside in the high end of the spectrum. Internet for example is 729Mhz in my system (which incidently is channel 113). More importantly though, 5-42Mhz is the return path only. Nothing is sent downstream in these frequencies. The lowest freqs are used for upstream due to the fact that they attenuate at a lower level then the higher frequencies. This way the customer equipment doesnt have to trasnmit at a very high level to communicate with the headend.

You would prob be safe filtering between the returnpath (42) and the start of analog 2 (54). But nothing is certain as small things can change from system to system. How much do you need to filter out? I wouldn't really recommend doing this at all, as frequencis in use can change as often as needed and what is fine one day, may cause problems the next.
 

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