Guess I'll start here.
One of my pet peves are folks who say "there's x number of dollars worth of parts in this so it should sell for x number of dollars". Seriously? How many modulation monitors do you think they sell in a year? Do you think parts is the only cost of business? They have to generate enough income to pay for their building, be it rent or a mortgage. Taxes. Unemployment insurance. Social Security. Any benefits to workers. You know, those people who are building the equipment, answering the phones, doing repairs and warranty work. How about advertising? Their electric bill. Insurance on the building and equipment. Heat in the winter, A/C in the summer. There's a hell of a lot more cost in the product than the parts. You must get really angry when you stop and buy a bottle of water for $1.29 that contains such a small monetary value of water it can't even be calculated! It's less than .00011 of a cent! What a rip off!
Modulation monitors haven't been a required piece of equipment in any commercial AM or FM station in well over 30 years, maybe even 40. Back when the FCC did all that deregulating things like modulation monitor rules, directional monitor point rules, transmitter logs, and all that went out the door and were replaced with basically "You need to check your stuff as often as you deem necessary to insure complliance". In modern times stations are set up by their engineer who could be using something as cheap as an oscilloscope from 40 years ago to set the modulation, then the processors are set so that overmodulation is just not possible. At both the full power commercial AM and FM I'm chief engineer for, no matter how much audio you pump out to the transmitter it won't overmodulate. It will distort and sound awful because you're overdriving various equipment between the sound source and the transmitter, but it simply cannot and will not overmodulate.
Back when deregulation first hit, the station I was working for immediately removed the Belar mod monitors from the FM studio simply because they were blocking the street view window. It greatly improved natural light and opened up the room nicely.
Transmitter logs aren't required either. I have participated in mock inspections through the state broadcast association every three years for more than 30 years. An actual retired inspector is sent out and does a full FCC station inspection, except without the ability to fine you, etc. Any deficiencies can then be rectified. Once you pass they gaurantee you will not be inspected for three years unless there's a valid complaint that needs to be checked out.
At our last inspection he didn't even want to see the transmitter logs. He said it was "nice to see I had a system in place". They want to know the EAS works, the tower lights are being checked and that you have a system in place to know if the transmitters are operating out of compliance. Ours have a remote system that calls a list of people until a human is reached who acknowledges the problem.
Years ago I used to have to go out and to directional monitor point field strength readings every month. That got deregulated to whatever you feel is necessary to be sure you're legal. I do it about once a quarter now. In 29 years at this station we've never been over spec.
There is a lot more to a modern modulation monitor that most think. As I engineer for several stations, and I test a lot of Part 15 gear, I own my own. Both the latest from Inovonics. For FM I use the 531N which I can connect direct when at a site, but since most of the stations I deal with (including the three where I'm the chief engineer) I can monitor off the air from my shop. See the spec info for this at:
http://www.inovonicsbroadcast.com/model/53101
For AM I use the 525N which is also perfect for monitoring my Part 15 station right off the air. See the info at:
http://www.inovonicsbroadcast.com/model/52501
I've been using these as a reference for stations where vintage analog units are still in use to see if their old gear is even close to spec. I have them on my home network and can connect to them to check modulation at seeral stations via a computer in the studio or my iPhone at a transmitter site. I actually have two sites on my routine that have no internet access.
Having a mod meter at the station and/or the transmitter is quite handy. Over the years I've installed Belar units that I've rebuilt and calibrated at both the AM and FM's where I'm employed, just for my own convenience.
But, not required. No one is being forced to purchase expensive mod monitors. About the only folks who really need one are the engineers setting up the transmitters, and heck, apparently they could just go ahead and build one for $200!
For those interested, check out the manufacturers web sites and see what goes into these units and what they can do. Start at BSWUSA to read the catalog info and foloow the links to manufacturer websites, manuals, spec sheets, etc. Many offer good theory information in how these work, and the many things they can do these days.
Most modern mod meters work off the air, and are frequency agile and self contained (no separate meters, or RF amps necessary) and of course will also work from a direct sample at a transmitter.
And as much as I don't want to tell ya'll this, your VU meter plan really has nothing to do with modulation. You can easily land on a station that's loud due to aggressive processing even with low modulation. And versa-visa. And of course different radios are going to have different AGC that is making the audio levels sound the same on all stations. Granted, this might give you some indication to use as a guide. Best bet would be to listen and increase modulation (audio level) until you hear distortion, then back off a bit and note the VU meter peaks and use that as a reference.
Don't get me started on how it's possible for a theater to have the guts to charge $8 for popcorn that cost 5 cents!
TIB
For many years now i use a Pira P75 Analyzer.
It is very accurate with no drift over time, and it uses dsp to analize IF baseband.
So it has no analogue demodulation of the IF.
It shows pilot level, as well as rds level and phase, related to the pilot.
Coupled to a pc, it also shows a histogram of the band etc.
I wish there was an AM version !
Paul.
I use an O-Scope. Cheap and relativly accurate. Starting using scopes when I discovered applications reading the Radio Amateur's Handbook. There are a few YouTube videos showing how to do this. If one looks around, they can most likely find an old used scope for the price of a cassette deck.
Never used one. Seriously, anyway. I may have to do this. Thanks!
Doug
A scope is the only true standard to trust, it will see every peak, and spec-out the transmitter's linearity too.
A good friend just gave me a McKay Dymek
AM-8. This is an AM only analog tuner
with a very nice long green lit analog
dial. This unit is from the 1970s and was in
an equipment rack at a broadcast station.
It was there to monitor the "dominant" AM station in
that broadcast area, I think. It has narrow and wide
filters. This unit went through many modifications in
the last couple of decades. Actually, it used to be a
receiver, but the audio amp had been removed. My
friend saved this unit from an uncertain future. After
fixing it up, he gave it to me.
The AM-8 is rare, and I could find no info
about it on line. However, McKay Dymek
also came out with the AM-5 and AM-6 models, and
photographs of those were easy for me to find. Those
models look just like the AM-8.
The AM-8 also has a nice analog signal strength meter
that is illiminated and marked with S units.
It is not a modulation monitor. But for monitoring
my AMT-3000, or anything else between 530 and
1610 kHz, it is a fun thing to have. It's connected to
my stereo system right now and sounds very good.
Best Wishes To All
Brooce, Hartford CT