Modulation and Digital Cable with Comcast

I have Comcast Digital Cable for my home. The box is a Motorola DCT-2000. I also have a Channel Plus 5445 (4-channel modulator)

I am able to inject 4 channels into my home signal but I have a few questions / problems.

Depending on what channel I inject to ( 77, 79, 81, 83), I lose some of my digital channels. The channel displays something similar to , "signal unavailable should be available soon". If I disconnect my modulator the channels appear fine. If I choose different channels to inject to, I lose different digital channels.

How can I find which channels are available for creating modulated channels? Also, I'm unclear on Low Pass Filters The ChannelPlus Low Pass Filter LPF-750 passes CATV channels 2 to 116 Would this work? Does Comcast use any channels higher than 116?

thanks, tom

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That's because digital channels are compressed and occupy a range of frequencies just above your analog frequencies. Analog CATV channels occupy frequencies starting around 55.25Mhz with a bandwidth of 6Mhz per channel. Digital channels, on the other hand, roughly occupy 1Mhz per channel, depending on the carrier. I think most CATV providers put their digital channels somewhere starting above the 450 or 500 Mhz range. In our case (time warner), it's above channel 74 which is 523Mhz. (Don't let Channel 95 to 99 fool you. They are actually located around channel 7 in the freq domain). Channel 116 would be roughly 750Mhz. If you are losing digital channels when injecting at 77 which is ~540Mhz, its probably safe to assume that Comcast is similar to Time Warner, and starts around 530Mhz. If so,

750 to 530 is 220Mhz or 220 Digital channels give or take a few. Given that most systems have 500 or more digital channels, I would guess that the LPF would kill many of your digital channels.

Not what you wanted to hear, but this has been a common problem for those with digital cable and modulators. I've always recommended running two RG6's to each TV to solve this. One RG6 would contain your analog channels, LPF and modulated channels. The second RG6 would carry your unaltered CATV feed straight to your Digital STB and use component/s-video/composite out of the STB to your TV. Then you just use your TV remote to select between tuner or video 1 (eg).

Another alternative is to move your Digital STB to the modulator and modulate it, but then you are turning your nice digital channels into a composite grade video signal.

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unaltered CATV


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It should. According to CP's website, it will pass channel 116 which is 745 Mhz, and not channel 117 which is 751 Mhz. Also according to CP's website, it allows for injection of modulation at channel 121 to 125. 121 is at

775Mhz, so the hysterisis of the band cutoff is apx 25 Mhz which means unpredictable results in that area. As long as your modulator can inject at channel 121 you should be good. As a test, what happens now if you inject at channel 121? Do you lose digital channels above your HBO channels? Can you see a noisy version of your modulated channel? I'm not familiar with the DCT2000, but you need to make sure that it will tune in that channel as an analog feed, and not expect there to be a digital channel to decode in that range.
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I would be very hesitant to use a low pass filter on any signal that feeds a digital cable box. The issue is that while truncating the digital signal may work now, channel changes in the future may cause problems. Digital cable sort of like satellite TV over a wire and with the "dynamic" nature of channel line ups you cannot know when things might change.

Two options:

  1. Use a notch filter. Here is the ChannelPlus model:
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    notch filters are available for any frequency range from some vendors like ChannelVision:
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    items 3205/61-68, 3205/71-78 and 3102-Custom The idea is to remove analog channels that you can get along without while leaving a "hole" for your modulated channels. Unfortunately for you the stock filters only allow inserting 3 channels.
  2. Modulate the digital box(es) and keep your distribution system all analog. Use a low pass filter in the wiring closet to remove the digital signal prior to inserting the local modulated signals.
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Lewis Gardner

Connect a lowpass filter in line with the output of the modulator. Be sure the filter cust-off above the highest output of the modulator.

I've had many installations with different modulators where harmonics from the modulator have trashed digital channels and now filter the modulator output on all installations.

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I've never once seen that happen. Perhaps you need to use a better line of modulators. Try ChannelPlus next time.

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