Macroware Technology
Cable TV Reception
posted Wednesday, 31 January 2007
The cable TV reception in my house is not very good. A block diagram of how I have it set up now is shown as follows.
The amplifier that I have now is a unidirectional amplifier (10 dB amplification, 50-900 MHz frequency range). One problem this presents is that the cable box cannot send back to the cable company so the on demand and pay per view services do not work.
I started to investigate how to improve this setup. My worst problems with reception are on the low channels, especially channel 6 for CBS. I have a bit of ghosting on this channel. I was reading on Time Warner's FAQ that this can be caused by interference from local channels. CBS is broadcast over the air on channel 5 in my area. Without the amplifier turned on the TV signal looks very fuzzy. With the amplifier turned on the analog channels look a little fuzzy and some of the digital channels don't even tune in at all sometimes. I'm thinking that more amplification would help improve the picture more. Ideally I'd like to amplify before the first splitter cuts the signal in half, but then it would have to be bidirectional to support the high speed internet.
The first question is what frequency range the amplifier needs to be valid for. Here is a link to a list of frequencies for analog TV. My cable service only has analog channels up to channel 100. Channels 100 and above are digital. So according to the frequency chart, I need an amplifier that goes up to 648 MHz for the analog channels. What bandwidth do the digital channels and services use? Each analog NTSC TV channel takes 6 MHz of bandwidth. There are also some gaps in bandwidth allocation for the 100 channels that go up to 648 MHz. Digital channels take less bandwidth. I couldn't find anything from Time Warner or elsewhere on the internet to say how the digital channels are allocated. As a side note, HDTV channels will also fit within the 6 MHz bandwidth. The Time Warner technicians installed 1 GHz splitters, so I'm guessing the bandwidth of an amplifier should be 1 GHz.
The next question is how much amplification I need. My topology has the first splitter which is a -3 dB loss on the signal. The 8 way splitter would have to be at least -9 dB loss on each split output, but nothing is printed on my splitter for exact number. The other loss mechanism would be the cable. RG-6 coax cable specifications can be found at this link from the RF Cafe web site. The spec says you can expect 7.5 dB per 100 ft. of loss at 400 MHz. Since the loss is probably dominated by skin effect, the total loss per 100 ft. is proportional to the square root of frequency. Loss at 1 GHz could be calculated as follows:
7.5 dB * sqrt(1000 MHz / 400 MHz) = 11.86 dB
The longest runs are probably about 100 ft. in my house, so round it off and say 12 dB of loss in the cable.
Total loss from input line to television is: 3 dB + 9 dB + 12 dB = 24 dB.
With my current setup with the 10 dB amplifier, I have an actual loss from input line to television of 14 dB. It looks like there is room to go with a bigger amplifier in my setup.
So I'm looking for a bidirectional amplifier with a gain of 24 dB and a bandwidth out to 1 GHz. The home stores seem to have pretty limited selection for video amplifiers (especially bidirectional). In looking at Radioshack.com, it seems like there is an additional spec that I hadn't thought about, and that is the reverse frequency range that the amplifier supports. The unit I saw had a reverse range of 5-40 MHz. Is this sufficient for cable modem and television use? I also need to identify the best topology - the amplifier I saw from Radioshack was only 8 dB gain but gave 4 outputs. There's probably a lot of different options out there. So I clearly need to do more research.
The amplifier that I have now is a unidirectional amplifier (10 dB amplification, 50-900 MHz frequency range). One problem this presents is that the cable box cannot send back to the cable company so the on demand and pay per view services do not work.
I started to investigate how to improve this setup. My worst problems with reception are on the low channels, especially channel 6 for CBS. I have a bit of ghosting on this channel. I was reading on Time Warner's FAQ that this can be caused by interference from local channels. CBS is broadcast over the air on channel 5 in my area. Without the amplifier turned on the TV signal looks very fuzzy. With the amplifier turned on the analog channels look a little fuzzy and some of the digital channels don't even tune in at all sometimes. I'm thinking that more amplification would help improve the picture more. Ideally I'd like to amplify before the first splitter cuts the signal in half, but then it would have to be bidirectional to support the high speed internet.
The first question is what frequency range the amplifier needs to be valid for. Here is a link to a list of frequencies for analog TV. My cable service only has analog channels up to channel 100. Channels 100 and above are digital. So according to the frequency chart, I need an amplifier that goes up to 648 MHz for the analog channels. What bandwidth do the digital channels and services use? Each analog NTSC TV channel takes 6 MHz of bandwidth. There are also some gaps in bandwidth allocation for the 100 channels that go up to 648 MHz. Digital channels take less bandwidth. I couldn't find anything from Time Warner or elsewhere on the internet to say how the digital channels are allocated. As a side note, HDTV channels will also fit within the 6 MHz bandwidth. The Time Warner technicians installed 1 GHz splitters, so I'm guessing the bandwidth of an amplifier should be 1 GHz.
The next question is how much amplification I need. My topology has the first splitter which is a -3 dB loss on the signal. The 8 way splitter would have to be at least -9 dB loss on each split output, but nothing is printed on my splitter for exact number. The other loss mechanism would be the cable. RG-6 coax cable specifications can be found at this link from the RF Cafe web site. The spec says you can expect 7.5 dB per 100 ft. of loss at 400 MHz. Since the loss is probably dominated by skin effect, the total loss per 100 ft. is proportional to the square root of frequency. Loss at 1 GHz could be calculated as follows:
7.5 dB * sqrt(1000 MHz / 400 MHz) = 11.86 dB
The longest runs are probably about 100 ft. in my house, so round it off and say 12 dB of loss in the cable.
Total loss from input line to television is: 3 dB + 9 dB + 12 dB = 24 dB.
With my current setup with the 10 dB amplifier, I have an actual loss from input line to television of 14 dB. It looks like there is room to go with a bigger amplifier in my setup.
So I'm looking for a bidirectional amplifier with a gain of 24 dB and a bandwidth out to 1 GHz. The home stores seem to have pretty limited selection for video amplifiers (especially bidirectional). In looking at Radioshack.com, it seems like there is an additional spec that I hadn't thought about, and that is the reverse frequency range that the amplifier supports. The unit I saw had a reverse range of 5-40 MHz. Is this sufficient for cable modem and television use? I also need to identify the best topology - the amplifier I saw from Radioshack was only 8 dB gain but gave 4 outputs. There's probably a lot of different options out there. So I clearly need to do more research.
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