MpxTool can be used as a highly accurate off-air modulation monitor.

To use MpxTool for this task, you need a sound card capable of 192 KHz recording, and a tuner with MPX output.

You will also need a known good exciter to use while calibrating the system.

Professional tuners, such as the Fanfare FT-1AP, generally feature MPX outputs.
Good consumer tuners, such as the Tivoli Model One, can me modified for MPX output with relative ease.

Unfortunately, few analog devices have perfect frequency response. Deficiensies in the sound card and tuner result in tilt and overshoots, and must be compensated for.

MpxTool has advanced compensation features to be able to compensate for it, but they must be manually calibrated. This becomes somewhat of chicken-and-egg problem.

To be able to calibrate the tuner and capture sound card together, you must have a known good exciter,

to be able to put lineup tones on the air.

You will also need an oscilloscope to be able to verify (and/or calibrate) the sound card, to have a reliable source for these lineup tones.

Examples of known good exciters include:

Broadcast Wavehouse PLL+ 1W - excellent little exciter, incredible value for the price, and near perfect performance. If you have to buy an exciter just for this purpose, there is none other in this price range (under $200) that will get the job done.
Nautel M50 - Impeccable performance.
Broadcast Electronics FXi 60 - as long as you use the composite input and disconnect all other inputs, this exciter will perform flawlessly and put an impeccably clean signal on the air.   However, avoid the AES/EBU input - it has a non-defeatable digital low-pass filter which will cause severe overshoots on tightly processed signals.

Any other exciter which passes a 30 Hz squarewave without tilt, and has flat frequency response up to at least 56 KHz, can be used for calibration.

Please note that the exciter is only necessary during calibration. Once your setup is calibrated, it's ready to use indefinitely.

The lineup tones to put on the air are:

60 Hz filtered squarewave. Used to verify low frequency performance and compensate for tilt.	Built into MpxTool
Quick Sweep 1000-56000hz. The quick sweep repeats 50 times per second, essentially turning the oscilloscope into a spectrum analyzer. Used to compensate for high frequency rolloff.	Built into MpxTool
Tightly processed program material. If your setup passes this waveform with less than 1% overshoot, it's ready for duty.	Download 1.5MB FLAC

Click here for instructions.