DTV Reception Signal Quality
Signal Quality is a factor with DTV reception!!
I have spent literally the last 4 months doing field strength tests in our market with some very interesting discoveries about the DTV signal itself. It seems signal quality is a BIG factor that can determine ease of reception of DTV signals, and signal quality is something that the Station Engineers actually control to a certain extent.
Not all reception problems are directly related to an end users antenna, low signal levels, or reception equipment in all cases. A transport stream reader can tell a lot about a signals quality, and that may be the key to a lot of the reception issues that viewers are seeing.
Many stations still do not have the equipment needed to monitor the quality of their signals due to the high costs of the equipment itself. High error vector magnitude, bit error rates, modulation error rates and high SNR ratios are all killers of reliable reception for some DTV signals. Signal quality can vary drastically between different broadcasters in any given market.
If these parameters are near the bottom of the FCC minimum legal specifications, reception will suffer greatly, especially at longer distances from the transmitter ( these parameters along with actual RF power levels help determine where the digital “Cliff” is located ). It also seems that antenna aiming is a critical factor in reliable reception. I went to a cable head end a few weeks ago who was having problems receiving our signal from one of our transmitters.
Our transmitter is located on a 4,000 foot mountain, and the Cable Head End is located on a 4,300 foot mountain with a direct line of sight of only about 23 miles, and they were having occasional unexplained reception issues at random times.
After learning that the RF signal levels were in the great range, at least 20 db above the receivers minimum, the transport stream analyzer was used to aim his antenna, and we improved the critical numbers of his receive antenna by re-aiming it based on the transport stream analyzer numbers and his reception improved drastically.
We are only talking about a few degrees off of optimum aiming, and the analyzer numbers improved greatly. So far, he has had no further issues with reception of our channel. This is an indicator of how critical antenna aiming can be at some more difficult reception locations.
One other issue that contributes to this particular situation is that our transmit antenna has a deep null in the exact direction of the cable head end in question that was intended to protect an analog channel that is now off the air.
Our Broadcast antenna is also side mounted on the tower directly opposite of the receive site, And the combination of the deep null, and the side mounted Broadcast antenna (Which can cause an effect I call tower signal scatter ) can all lead to unreliable reception.
One note to add is that this head end site does have a lot of VHF and UHF 2 way radio and Cell phone antennas, and we may have only nulled out the offending signal by re-aiming the antenna, but I can imagine some viewers who could be experiencing these same issues based on how close they may be to an offending RF source, and a lot of DTV receivers are weak on filtering of RF signals at the front end of the receiver.
If anyone on this forum has any transport stream analyzers available to them, it would be an interesting experiment to show how the signal quality can be effected by antenna aiming itself. I have found that critical signal parameters can be improved by exact aiming of the antenna, and this even applies to signals that are already in an acceptable quality range.
The above scenario with signal nulls, side mounted antennas, and receive sites that have a lot of RF present can all contribute to reception issues for viewers who have no possible way to gauge the quality of any given DTV signal.
The point I am trying to make is that the transition is not complete for many stations. A lot of stations are in the same situation in regards to the transition itself, and a lot of the reception issues viewers are now seeing may be due to circumstances such as this one, and if that is the case then all of the antennas and amps you can buy will never solve these problems. How is the average viewer supposed to know about or deal with all of these possible problems that are yet to be solved in some markets?
If you are having problems with a particular station when most all of the rest of the signals in your area are OK, you may want to call that stations engineering department and inquire about any transmitter site issues or other problems they may be having that can affect reception before you try and solve them yourself by buying amps and antennas that may not solve the problem in question, which could be at the Broadcast end of the DTV signal chain.
There are many broadcasters who are not finished with the DTV transition, and something you do now to solve a problem with a particular station, may not be needed if or when they decide to address their own transmitter site issues, which most of them will do when the cash becomes available. Most Broadcasters are still strapped with loan payments for the new antennas and transmitters, HD equipment for master control etc. so funds to correct these issues may not be immediately available.
See the wave form pictures for examples of good and poor quality DTV signals. When the data levels are not linear across the entire 6mHz channel, the signal quality suffers. If the data levels become as high as the pilot carrier due to a non linear tube or transmitter amplifier, or if it becomes very low and closer to the noise floor, signal quality suffers.
The pilot carrier is the only reference that receivers use to “Find and lock onto” the presence of a DTV signal, and if the data levels are high and not linear, the DTV receiver is sometimes not able to “See” the pilot carrier, or it appears intermittent to the receiver, thus causing drop outs. The DTV transition is not complete for all broadcasters as of now!!