wajo
11-15-08, 01:59 PM
A Digital Signal Strength Problem You Might Run Into
This is an FYI on just ONE possibility to explore if you have some problems with digital channels, like loss of tuning, pixellation, macroblocking, freeze frame, etc.
ONE possible and surprising cause of DIGITAL channel problems is the "Cliff Effect" or "Digital Cliff" which is a complete loss of DIGITAL signal at the ends of its ideal signal strength… too weak or too strong a signal can make DIGITAL channels just "fall off the cliff" as opposed to analog channels that get snowy or whatever at the ends of their signal strength.
This Wiki entry illustrates this effect (http://en.wikipedia.org/wiki/Cliff_effect) and says "The picture may break up (pixellation), lock on a freeze frame, or go blank."
The Cliff Effect is exactly the OPPOSITE of all our previous experience with ANALOG channels, where we were always trying to get the STRONGEST signal.
If you have DVDRs or TVs with digital tuners that have no problem with digital channels, that doesn't mean things are OK for this DVDR... it has an AMPLIFIED coax circuit that could be over-driving an already strong digital signal... and your signal-strength meter won't tell you that.
From what I've read, DIGITAL tuners can be over-driven with a strong signal and "splatter," so the "broadcast" DIGITAL signal needs to be ~10% of the analog strength... and the possibility of a Cliff Effect is NOT indicated in the "meters" some of our equipment has... in fact, one person said a very strong signal can register normally while it's splattering.
Here's a good discussion of the digital-signal-too-STRONG issue and the role of signal strength indicators in our DVDRs. (http://www.avsforum.com/avs-vb/archive/index.php/t-800078.html)
Here's a post on my experience with a digital signal that was too WEAK for my TV to tune just ONE digital channel group in my basic (analog) cable feed... all others tuned OK! (http://www.avsforum.com/avs-vb/showpost.php?p=15011387&postcount=2135)
IMO, some DVDR users may experience this effect NOW cuz stations are transitioning to the digital age and trying to balance their analog/digital signal strengths. Hopefully, with an all-digital broadcast system after Feb 2009, this problem will go away with the OTA signal... not sure about the cableco signal.
"Hang-By-A-Thread" Attenuation Test
A quick-n-easy prelim. test for TOO-STRONG a signal is to unscrew the incoming coax until it's hanging by only one or two threads... pull on the cable to make sure the center wire is withdrawn some and not "fully-seated" anymore. This usually degrades the signal just enough to see if there's any difference.
You *might* have to do a new channel scan with the coax "hanging by a thread," then check for stability of the digital channels... LEAVE THE COAX HANGING THRU ALL YOUR TESTING CUZ IF YOU SCREW IT BACK IN, IT WILL BE AT FULL POWER ONCE AGAIN AND YOU WON'T HAVE PROVED ANYTHING!.
See this AVS post for a successful "coax-on-a-thread" test by "flamike," who had "interference" on certain channels. (http://www.avsforum.com/avs-vb/showthread.php?p=14671133#post14671133)
How to Attenuate Your Digital Signal
IF you determine your digital signal is too strong, then you can use one or more splitters or buy one or more RF attenuators (they're "cheap") for a permanent installation
If the quick-n-easy prelim. test suggests attenutation is needed, you can do the following:
Remove any upstream amplifiers. This might be the solution... no further action required?
If no upstream amplifier, install a 2-way splitter on the incoming coax with one output to your DVDR and one to your TV. Note how many -dB each output of the splitter has, e.g., if balanced -3.7dB or unbalanced -3.7dB and -7.4dB, and try both outputs to see if a specific dB-weakening thru the splitter helps. You could also go directly to Step 3 with attenuators... they're pretty cheap.
Install one or more signal attenuators of appropriate strength on the incoming coax to the DVDR. (Flamike's attenuator source is here (http://www.homenetworksupply.com/products.aspx?search=attenuator)). To help decide which one(s) to buy, here's how flamike used two of the three he bought (http://www.avsforum.com/avs-vb/showthread.php?p=14745880#post14745880)... however, each system will be diff. and his was a rather complex one that might not apply to anyone else?
Below are a couple of other options for attenuation... don't have any experience with these, but they sound very interesting:
An inline single-strength attenuator: "The Model TC-200A Tilt Compensator is an in-line filter designed to create an attenuation loss equal and opposite to cable loss." $10.95. (http://www.summitsource.com/channel-plus-tc200a-tilt-compensator-tc200a-1-pack-fconnector-high-frequency-attenuator-inline-filter-female-to-male-coupler-with-female-to-female-barrel-adapter-part-tc200a-p-7480.html?osCsid=8ea5ccebca747be5a4f05b56e7cba5fd)
A Variable attenuator: Winegard's TA-8700 is a 75 Ohm variable attenuator designed to cover the frequency range of 5 to 1000 MHz. The TA-8700 provides continuously variable attenuation from 1-17 dB. The unit is AC-DC passive to allow remote amplifiers to be powered thru it. Connectors are standard F-type." $19.95. (http://www.summitsource.com/winegard-ta8700-variable-attenuator-018-db-offair-antenna-ta8700-variable-uhfvhf-attenuator-pad-75-ohm-acdc-passive-1-amp-24-volts-max-aerial-signal-strength-filter-part-ta8700-p-7719.html?osCsid=8ea5ccebca747be5a4f05b56e7cba5fd)
This is an FYI on just ONE possibility to explore if you have some problems with digital channels, like loss of tuning, pixellation, macroblocking, freeze frame, etc.
ONE possible and surprising cause of DIGITAL channel problems is the "Cliff Effect" or "Digital Cliff" which is a complete loss of DIGITAL signal at the ends of its ideal signal strength… too weak or too strong a signal can make DIGITAL channels just "fall off the cliff" as opposed to analog channels that get snowy or whatever at the ends of their signal strength.
This Wiki entry illustrates this effect (http://en.wikipedia.org/wiki/Cliff_effect) and says "The picture may break up (pixellation), lock on a freeze frame, or go blank."
The Cliff Effect is exactly the OPPOSITE of all our previous experience with ANALOG channels, where we were always trying to get the STRONGEST signal.
If you have DVDRs or TVs with digital tuners that have no problem with digital channels, that doesn't mean things are OK for this DVDR... it has an AMPLIFIED coax circuit that could be over-driving an already strong digital signal... and your signal-strength meter won't tell you that.
From what I've read, DIGITAL tuners can be over-driven with a strong signal and "splatter," so the "broadcast" DIGITAL signal needs to be ~10% of the analog strength... and the possibility of a Cliff Effect is NOT indicated in the "meters" some of our equipment has... in fact, one person said a very strong signal can register normally while it's splattering.
Here's a good discussion of the digital-signal-too-STRONG issue and the role of signal strength indicators in our DVDRs. (http://www.avsforum.com/avs-vb/archive/index.php/t-800078.html)
Here's a post on my experience with a digital signal that was too WEAK for my TV to tune just ONE digital channel group in my basic (analog) cable feed... all others tuned OK! (http://www.avsforum.com/avs-vb/showpost.php?p=15011387&postcount=2135)
IMO, some DVDR users may experience this effect NOW cuz stations are transitioning to the digital age and trying to balance their analog/digital signal strengths. Hopefully, with an all-digital broadcast system after Feb 2009, this problem will go away with the OTA signal... not sure about the cableco signal.
"Hang-By-A-Thread" Attenuation Test
A quick-n-easy prelim. test for TOO-STRONG a signal is to unscrew the incoming coax until it's hanging by only one or two threads... pull on the cable to make sure the center wire is withdrawn some and not "fully-seated" anymore. This usually degrades the signal just enough to see if there's any difference.
You *might* have to do a new channel scan with the coax "hanging by a thread," then check for stability of the digital channels... LEAVE THE COAX HANGING THRU ALL YOUR TESTING CUZ IF YOU SCREW IT BACK IN, IT WILL BE AT FULL POWER ONCE AGAIN AND YOU WON'T HAVE PROVED ANYTHING!.
See this AVS post for a successful "coax-on-a-thread" test by "flamike," who had "interference" on certain channels. (http://www.avsforum.com/avs-vb/showthread.php?p=14671133#post14671133)
How to Attenuate Your Digital Signal
IF you determine your digital signal is too strong, then you can use one or more splitters or buy one or more RF attenuators (they're "cheap") for a permanent installation
If the quick-n-easy prelim. test suggests attenutation is needed, you can do the following:
Remove any upstream amplifiers. This might be the solution... no further action required?
If no upstream amplifier, install a 2-way splitter on the incoming coax with one output to your DVDR and one to your TV. Note how many -dB each output of the splitter has, e.g., if balanced -3.7dB or unbalanced -3.7dB and -7.4dB, and try both outputs to see if a specific dB-weakening thru the splitter helps. You could also go directly to Step 3 with attenuators... they're pretty cheap.
Install one or more signal attenuators of appropriate strength on the incoming coax to the DVDR. (Flamike's attenuator source is here (http://www.homenetworksupply.com/products.aspx?search=attenuator)). To help decide which one(s) to buy, here's how flamike used two of the three he bought (http://www.avsforum.com/avs-vb/showthread.php?p=14745880#post14745880)... however, each system will be diff. and his was a rather complex one that might not apply to anyone else?
Below are a couple of other options for attenuation... don't have any experience with these, but they sound very interesting:
An inline single-strength attenuator: "The Model TC-200A Tilt Compensator is an in-line filter designed to create an attenuation loss equal and opposite to cable loss." $10.95. (http://www.summitsource.com/channel-plus-tc200a-tilt-compensator-tc200a-1-pack-fconnector-high-frequency-attenuator-inline-filter-female-to-male-coupler-with-female-to-female-barrel-adapter-part-tc200a-p-7480.html?osCsid=8ea5ccebca747be5a4f05b56e7cba5fd)
A Variable attenuator: Winegard's TA-8700 is a 75 Ohm variable attenuator designed to cover the frequency range of 5 to 1000 MHz. The TA-8700 provides continuously variable attenuation from 1-17 dB. The unit is AC-DC passive to allow remote amplifiers to be powered thru it. Connectors are standard F-type." $19.95. (http://www.summitsource.com/winegard-ta8700-variable-attenuator-018-db-offair-antenna-ta8700-variable-uhfvhf-attenuator-pad-75-ohm-acdc-passive-1-amp-24-volts-max-aerial-signal-strength-filter-part-ta8700-p-7719.html?osCsid=8ea5ccebca747be5a4f05b56e7cba5fd)