Comcast Internet download frequencies

[AntAltMike]

Does anyone know what they are? I have to diplex broadcast HDTV (UHF channels 14-51, 27-51 or 34-51) with Comcast internet. I have the filters needed to neatly wipe out spectrum in each band where necessary. I have no problem accommodating the sub-band upstream signals, but I haven't been able to talk to anyone at Comcast who has the information that I need. I have previously observed unusual digital waveforms in Comcast systems between channels 4 and 5 (72Mhz to 76Mhz) and in the FM band (between 88 and 108Mhz). Does anyone know what frequencies I will have to clear to assure downstream communication with the Comcast modem?

[Scott Greczkowski]

Those low 72Mhz to 76Mhz are the upstream data connections. The downlink frequencies very by area.

[Jim S.]

Does the modem have a diagnostic mode available? Maybe you could get the downlink frequency from there.

Alternately, using a QAM DVB card in your computer, capture some data from each frequency that's active but not giving unscrambled video and examine it until you find one that looks like Internet traffic.

[AntAltMike]

Those low 72Mhz to 76Mhz are the upstream data connections.

I want to make sure I am using the terminology correctly. I am interested in the signals that travel in the same direction as the cable television programming. I am calling that "downstream" because it flows that way, but I suppose that someone else could say it comes from "upstream" Data going from the web system to the user is, in my terminology, downstreamed data. Signals from 72 to 76Mhz and from 88 to 108 Mhz have to be going downstream, at least, as I use the term, because they are detectable on a line with no modem connected and they would be blocked by the line extender amplifiers if anyone tried to send data in those bands upstream.

Is there anyone known to work for Comcast who is a participating member here who might have access to that information, but who might regard this as a dead forum and not visit it regualrly?

[Scott Greczkowski]

Its different for each area, where I am i am at 600Mhz for downstream (receiving) however in the next town over its at 800mhz.

[codee]

Its different for each area, where I am i am at 600Mhz for downstream (receiving) however in the next town over its at 800mhz.

Scott is correct, it varies from system to system. In my area, we use 729 for the HSI Carrier. I'd be surprised if anybody at customer service would know that though. You could ask a tech, and they would easily know, as we take signal levels on 3 video + the HSI frequency every call at the tap. Most modems will show you the downstream frequency also. Try going to 192.168.100.1 and looking through the menus for something along these lines

"
RF Parameters
Downstream
Freq/Power: 729.000 MHz 1 dBmV
Signal to Noise Ratio: 37 dB
Modulation: QAM256

--------------------------------------------------------------------------------
Upstream
Freq/Power: 37.008 MHz 36 dBmV
Channel Type: DOCSIS 1.x (TDMA)

Symbol Rate: 2560 kSym/sec

Modulation: QAM16

Status
System Uptime: 26 days 7h:25m:40s
Computers Detected: 1
CM Status: Telephony-AC Power Iso OFF Reg Complete
WAN Isolation: OFF-Access to WAN is allowed
Time and Date: THU SEP 21 18:39:05 2006
"

The Downstream
Freq/Power: 729.000 MHz 1 dBmV
Shows that my area is using 729 for the HSI carrier. Motorola modems should be the same way, I'm currently using a Touchstone though so I can't direct you to the exact page for moto. Feel free to PM me if you need more help with this. :-)

[lesdotcom]

I have a similiar situation trying to merge some local modulators with Comcast digital cable. Does anyone know where to find the digital channel frequency assignments? The analog assignments are pretty well known but when I install my filter I lose most of my digital channels. Are the digital assignments standardized?

Thanks,
Les

[AntAltMike]

I have a similiar situation trying to merge some local modulators with Comcast digital cable. Does anyone know where to find the digital channel frequency assignments? The analog assignments are pretty well known but when I install my filter I lose most of my digital channels. Are the digital assignments standardized?

Thanks,
Les

Bad news. Cable companies tend to fully utilize the spectrum immediately above the analog channels, so if your highest analog channel is, say, channel 75, then your first digital channel will usually be channel 77 or 78.

UHF channels 14- 43 are at about the same frequencies as cable channels 65- 94. UHF channels 44-69 are at about the same frequencies as cable channels 100-125. Realisticaly, no matter where you modulate a house channel, it will land on a cable channel if your cable system is an 860Mhz system, but if it is a 750Mhz,system, then you can modulate it to UHF channels from 63 and up, or to cable channels from 119 and up without incurring a conflict

[lesdotcom]

My local channel doesn't necessarily have to land on empty space just not on a channel I watch frequently. I am looking for the channel assignments so I can pick and choose which channel range to remove and insert my own.

[AntAltMike]

It is not easy to manage. You don't have the means to cleanly delete digital cable channels to clear the space for analog insertion, and even if you could do that, the digital channels can get "remapped" by the cable company to other transmission frequencies without the viewer ever realizing that, the same way that DirecTV moves its programming from one transponder to another without changing its virtual channel numbers.

There are companies that make affordable elimination filters that wipe ouf half a dozen adjacent cable channels. I think Channel Plus and Channel Viusion make them. Typically, there is one model that wipes out maybe cable channels 65 to 70, another for around 71 to 75 and maybe another for about 76 to 80. You can modulate your hose channels to alternate, avaialble channels within those bands.