Breakaway Broadcast Processor contains a line-up tone generator capable of generating several useful tones. Whether you're using the Left/Right outputs with an outboard stereo generator, or Breakaway Broadcast Processor's MPX output directly into an exciter, the procedure is the same.
Connect the oscilloscope signal and ground to Breakaway Broadcast Processor's output, in parallel with the load (exciter or stereo generator).
The oscilloscope must be DC coupled. AC coupling will yield disastrously incorrect results.
Processed audio contains many waveforms resembling squarewaves. Most sound cards are not DC straight, and will thus cause squarewaves to tilt inwards, in turn causing as must as 20% overmodulation with no increase in loudness! The good news is that this can be compensated for.
Set Breakaway Broadcast Processor to output 60hz Square. You can find the Test Tone generator by pressing the Settings button in the Breakaway Broadcast Processor main window.
Set the oscilloscope to DC coupling. This is imperative -- an AC coupled scope will severly distort the waveform.
The signal should ideally look like this:
Nice, sharp edges, and free of tilt. All lines are parallel. If it looks like this, you're lucky -- your sound card needs no tilt compensation!
If the squarewave is tilting inwards like this, turn up the tilt control in Breakaway Broadcast Processor to make it as square as possible.
If the squarewave looks like this, you have overcorrected. Turn down the tilt control.
If the squarewave is this bad, either your wiring is bad, or your sound card is unsuitable for Breakaway Broadcast Processor. Please see the Recommended Hardware page for a list of recommended sound cards.
HF frequency response
Some sound cards have slight high frequency rolloff. This is not as critical as tilt -- slight HF rolloff will only slightly reduce stereo separation on a composite signal, but since we have the oscilloscoped connected already, let's make it perfect.
Select the Quick Sweep from the Test Tone menu. The Quick Sweep waveform is a quick spike followed by a very rapid sweep - 1000 - 56000 Hz for MPX, 1000 - 18000 Hz for L/R. The sweep is very quick, repeating roughly 50 times per second, thus essentially turning the oscilloscope into a spectrum analyzer!
Adjust the oscilloscope trig level until the sweep waveform is stationary, and set sweep time as appropriate.
This is a perfect QuickSweep. Frequency response is 1000 to 56000hz is perfectly flat. This will yield excellent loudness and stereo separation on air.
This QuickSweep shows HF rolloff. Adjust the PEQ in Breakaway Broadcast Processor to make the sweep perfectly flat.
This is a BAD QuickSweep. The missing high frequencies indicate the sound card is locked to 96 KHz (instead of 192 KHz), either because of a setting, or because it is incapable of 192 KHz sampling rates. This is unusable for MPX output and would yield a severe lack of stereo separation, and poor peak control.
This QuickSweep is even worse. This sound card is locked to 48 KHz sampling rate. The stereo pilot will still get through (and turn on the stereo light on the FM tuner) but there will be NO stereo separation, since the entire stereo subcarrier is filtered out.
When you have achieved perfect tilt and frequency response, the only thing remaining is to set the modulation level. This can be done either by using a modulation monitor, spectrum analyzer, or the modulation meter on your exciter, although the latter may not always be trustworthy. Spectrum Analyzer (Bessel Null) is the most accurate way -- if you happen to have access to one, read on.
(Also known as Bessel Null)
If you have a spectrum analyzer capable of displaying your FM carrier frequency, you can use the built in Bessel Null tone generator in Breakaway Broadcast Processor. This is the single most accurate way to adjust deviation; This is how mod monitors themselves are calibrated.
Connect an appropriate antenna to the input of your analyzer, and adjust as follows:
- Center frequency (CF) = f0 (carrier frequency of the transmitter)
- RBW 10 kHz or lower (IF filter)
- VBW 10 kHz or lower (video filter)
- Span: 340 kHz
- Input attenuation is dependent on input level.
- Settings for digital signal processor (DSP) analyzers will be different but should provide equivalent results.
Turn the Ref Level in Breakaway Broadcast Processor all the way down. Select the 31187.6 Hz test tone. This tone is called a Bessel Null Tone. At this exact frequency, the carrier will null (disappear) when your transmitter is at exactly +/- 75 KHz deviation, leaving only sidebands.
Adjust Ref level (in Breakaway Broadcast Processor), or Input Gain (on your exciter) as necessary to achieve the best possible null.
The spectrum analyzer readout should look roughly equal to the following images. Notice that it's the single center carrier that must null, not any of the sidebands.
Calibration may take a few minutes, or even require a token investment in hardware, but it guarantees the difference between a mediocre station and a station that really jumps out of the speakers.