HDTV Forum

Situation:
I have a great big Wingard OTA antenna in the attic feeding through a ASK4485T 4X8+OTA Multiswitch. The need for the Multiswitch for my DirecTV was eliminated when I added (my first) 5lnb on the garage roof for the primary HiDef set in family room. When upgrading the 3lnb on the house (2 story) roof we eliminated the multiswitch in the attic as that dish is only feeding the 2 DirectTV DVRs on the second floor. I continued to use the ASK unit for the OTA distribution ( first and second floor sets ) but get symptoms of weak signals, especially on the main set down stairs and the second HD (Master Bedroom) set... the family room setup splits the OTA signal out to the HR20-700, VCR, and TV and the Master Bedroom also splits to HR20-700, VCR and TV. The TV's do fair but the ATSC tuners in the HR20-700's don't do so well. I used to record/watch dependably on the ATSC tunners in my previous DVR (HR10-250) but he DirecTV units seem to have much weaker ATSC tuners.
I have not been able to find OTA signal amplification specs for the ASK4485T so not sure if it is the best box to use for the splitting/amplification of the OTA signal nor what would be a higher amplification.
Possibilities:
I am considering replacing the ASK4485T with a CM-3045 (4db per each of 8 outputs), or adding a separate amplifier at the receiver end of the coax for each of the 2 HR20-700 locations before the 3 way splitter. Am open to other thoughts as well as comments/opinions on these possibilities.

First, normally a preamp at the receiver is a waste of time and money. If you have a noisy signal it'll only give you a strong, noisy signal. IF OTOH when running the cable to the set with no splitters and the signal is good, *then" an amplified splitter should help if the signal shows up weak when split out to multiple sets. Unless there are some strong stations that could/would cause overload the best place for the amp is at the antenna. Amplified splitters are good only if they have a good signal at the input. Like the amp at the receiver, if you feed them a noisy signal you just get a lot of noisy signals out.

As regards OTA amplification and "noise", there is a difference between analog (old NTSC) and digital systems. When talking of noise in analog TV, we usually mean broadband (technically "thermal") noise, which manifests itself in the familiar "snow" on the screen. The quality of the signal is measured in dB as "signal-to-noise ratio" or s/n. In a black-and-white picture the noise at -46dB was "perceptible", but not "objectionable". In color such snow was more annoying and a s/n of -54dB was aimed for.

All amplifiers and tuners unavoidably introduce some noise of their own, commonly expressed as the "noise figure" of the device. This can vary from some 3dB for amplifiers to as high as 12dB for some tuners. The s/n is also degraded by losses in long cable runs, hence the advantage of a low noise figure preamplifier at the antenna end.

The recovery of a digital signal is more efficient, so a tuner can accept a much lower s/n ratio of the order of -20dB to -30dB. But that is valid only for broadband noise. There is another noise, namely impulse noise, more accurately signal interference. On an analog screen boadband noise (more or less uniform "snow") is readily distinguishable from impulse noise (flashes, patterns, waves) but in a digital system they produce the same symptoms - dropouts, pixelation etc.

And while marginal s/n can be improved by amplification, especially at the antenna, impulse noise or interference often, depending on the source, cannot. The cause must be identified and remedied.

Incidentally, modern tuners often do not possess the lowest noise figures that the state-of-art can
provide and there may be difference in this respect between manufactures and indeed different sets.

LH- When I switched from the HR10 to the HR20 I think I noticed a decline in OTA performance. But if so it was small and I am not certain there is a significant difference. Noise issues are discussed at www.hdtvprimer.com/ANTENNAS/basics.html .

In your situation I would retire the ASK unit, get a Channel Master 7777 amplifier, put it at the antenna, attach the cable to one TV, and observe it for a time. If the reception is not adequate then you need to move the antenna outdoors. Next split the signal and send it to all your other TVs. If there is any degradation in reception then you need to add a distribution amplifier. -Ken

I think the comment on DVR tuner sensitivity was well put. I've never set up the test equipment to measure the ultimate sensitivity or noise figure of a DVR, but I think most of these things are designed to expect input from a cable, or satellite system. That means they probably don't have near the ultimate, state-of-the-art sensitivity or the dynamic range of a good TV set's tuner.