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MpxTool Input calibration, step by step.
You will need:
- Calibrated sound card for test tone generation. See the Output Calibration page.
- Known good FM Exciter
- Tuner with MPX output
- Audio Interface capable of 192 KHz input
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Equipment used in this example:
SB Audigy 2 sound card, Output Tilt Correction in MpxTool. Test tones are available in the output mode dropdown (default is L/R 75us).
Broadcast Warehouse PLL+ 1W Exciter
Tivoli Model One tuner (modified for Composite Output)
E-MU 0404 USB audio interface for capture
Click on any image to enlarge.
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Step 1:
Play the 1000 Hz Sine wave through the exciter. Adjust the exciter for 100% modulation. If your exciter does not have a modulation level indicator, adjust the modulation for roughly the same level as commercial FM stations in your area. It does not need to be perfect yet.
Adjust Gain in MpxTool for 100% modulation readout.
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1000Hz Sine wave |
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Step 2:
Play the 60 Hz Square wave through the exciter. It will probably look something like the image to the right. Adjust the Input Tilt control to level the edges of the square wave.
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Severe Tilt
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Step 3:
Adjust the Input Arc control to compensate for the slight arc that still remains.
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Slight Arc
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Step 4:
Verify that the square wave looks similar to the one to the right.
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Near-perfect square
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Step 5:
Play the Quick Sweep. Any high frequency roll-off will manifest itself as the waveform tapering off to the right, as in this image. Adjust the the Parametric EQ to compensate for the high frequency roll-off, and to yield the flattest possible response.
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High frequency rolloff
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Step 6:
Verify that the sweep is flat, with no significant dips or peaks. It should look like the one to the right.
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Flat frequency response
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Step 7:
Play the tightly processed program material clip. This waveform is very heavily processed - the edges should be near perfectly flat. If it looks like the image to the right, your signal path is calibrated and ready for use.
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Less than 1% overshoot
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| For reference, this is what the tightly processed program material in our example setup looks like before calibration. Not only do we regularly see 103% modulation (3% overshoot), many peaks barely hit 95% in this example. |
Significant overshoot
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Step 8:
If you don't have a calibrated modulation modulator or exciter modulation level readout available, the next best thing is monitor a few stations and find the 100% level through statistical reasoning. If most stations in your area measure at the same level, that level is likely 100%.
Another easy way of finding the 100% level is to look at the pilot level. Most stations use 8.0, 9.0 or 10.0% injection, and by statistical reasoning you can figure out which is which, and adjust accordingly.
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