Well, guys, my moment of truth has arrived. Here are my measurements of digital and analog signals using both a digital and an analog signal level meter (SLM).
Because my wife and my friends were VERY generous to me at Christmas, I am now able to compare readings from my Sadelco 719E with readings from a DisplayMax 800.
I wanted to do this before the transition while I still had a good selection of digital and analog signals coming from Norfolk. I picked 6 signals, 3 digital and 3 analog. The virtual number indicates digital. The measurements, including the ones for CH13, were made using a CM4221 antenna. I used a splitter to make simultaneous readings on both meters because OTA signals don't stay at the same level.
Channel 719E 800
13 -9.5 -11.4
16(15.1) -0.5 +2.9
27 +6.8 +4.5
31(10.1) -0.5 +1.2
43 +9.5 +7.6
50(27.1) -2.8 -2.3
And as a chart:
Quote: Originally Posted by AntAltMike
...you can use an inexpensive analog signal meter and add 8dB to whatever it says the signal level is. I often use a Leader LF941 analog signal meter with a digital display when working on digital antenna reception because it weighs less than my other meters. You can get an analog (NTSC) signal meter...with a digital readout for $200 or less. You can even do the job with a clunker with a rotary dial and a needle meter for maybe $10 to $30.
On the other hand, if someone wants to use a meter to aid in developing quality antenna reception in a poor reception environment with multipath, the signal strength measurement, which just becomes an extrapolation of the signal level at and around 1.25 MHz above the lower channel band edge, does not show the signal quality the way that a spectrum analyzer can. If you are getting nailed by multipath, the waveform will usually have a big notch in it, whereas if it is pure, it will be a level plateau.
The chart gives me the corrections needed to make correct readings of digital signals with my analog signal SLM, but I'm really not too concerned about corrections because I mostly use my SLM for comparative readings to aim antennas, compare antennas, and find the "hot spot" location for an antenna. These comparisons rely on the the linearity of my meter which is easy to check by inserting a 20dB attenuator to see if the reading drops by 20dB.
The corrections are the result of differences in the calibration of the SLMs, the tolerance allowed for each SLM, and the fact that analog signals are measured with a peak value of the video and sound carriers and digital signals are measured with an average value across most of the 6MHz channel. Sadelco says in the manual that two 800 meters can differ by as much as 2dB and still be in spec.
My 719E is reading too high on digital and
analog signals because it is off calibration on both. If it were on calibration the red line would be lower all across the chart, its readings of analog signals would be the same as the 800 readings, and its readings of digital signals would be a lot lower. This would make it necessary to add a positive correction to the 719E reading for digital signals, which is what it is supposed to be. It was just a matter of luck that both meters were fairly close together.
As I previously mentioned, the engineers at Potomac Instruments seem to think it's OK to measure digital signals with an analog signal meter if the proper corrections are applied: http://www.pi-usa.com/pdf/dtva.pdf
Highdefjeff seems to think it's OK to use an analog signal meter to measure digital signals (He is talking about digital signals coming from a satellite, but the digital signals coming from a terrestrial transmitting tower are similar). To quote him:
Here I am in the digital world suggesting that you use an "older" analog type meter. Sounds wrong, doesn't it? It's not. Let me explain "why?" starting with a little background.
What If I asked you "Why are you using a digital meter to read an analog signal?" Many of you would say I'm full of "it", or I couldn't be talking about satellite signal. But, I am talking about satellite signal, the signal at the "front end" that you measure is analog.
To anyone who thinks that an analog SLM couldn't possibly be of any use for digital signals I say: Try it for yourself first.
As far as I'm concerned, this test proves that an analog SLM can be very useful when dealing with digital signals, which is what I set out to prove at the beginning of this thread. Or, as my math teacher used to say, QED.