Broadcast

Calibration (AM)

Breakaway Broadcast Processor is ideal for AM broadcasting.

Breakaway Broadcast Processor contains strict low and high pass filters as well as asymmetric clipping (up to 150% positive peaks). For the tightly processed waveform to pass through the transmitter accurately, any filtering and clipping in the transmitter must be completely disabled! This is imperative, as any further clipping of the waveform will contaminate the spectrum beyond legal limits.

 

Breakaway Broadcast Processor has built in calibration features to equalize both low and high frequency response of your transmitter.

You will need an oscilloscope. If you do not yet own one, a used Tektronix 465 can be had for under $100 on eBay.

Connect the vertical input of the oscilloscope to the transmitter's sampling loop (or another convenient RF source)

Connect the sync (trig) input of the scope to an available Breakaway Broadcast Processor audio output (for example the Right channel).

Low Frequency Tilt Correction

 

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.

 

am square tilt 

If the waveform looks like this, you will need to adjust the Tilt control in Breakaway Broadcast Processor.

 

am square perfect 

Correct tilt adjustment.

High Frequency Response 

The second common problem is High Frequency Response. Breakaway Broadcast Processor can compensate for this with the Parametric EQ (PEQ). Select the 1000hz Squarewave.

Admittedly, the waveform is not particularly square. Perfect squarewaves require infinite bandwidth. Breakaway Broadcast Processor is a digital processor, and as such bandwidth is limited.

However, one can still read useful information from the corners of the waveform, as marked by the arrows. 

 

am 1k square ideal 

Correct HF response is indicated by the sharpest possible corner without ringing / overshoot.

 am 1k square boost 

If the waveform looks like this, high frequency boost is occuring. This is potentially dangerous, as the lower edge overshoot could reach -100% and completely shut off the carrier! Adjust the PEQ to achieve the sharpest corner without overshoot.

 

am 1k square rolloff 

If the waveform is rounded, high frequency loss is occuring. Compensate by adjusting the PEQ to achieve the sharpest corner without overshoot.

Transmitter linearity 

The sawtooth waveforms in Breakaway Broadcast Processor allow you to verify transmitter linearity. Three flavours of sawtooth are available: 60hz with detent, 60hz without detent, and 400hz without detent.

The detent is a plateau in the waveform at 0% modulation. When asymmetry is set to 0%, this detent will be in the middle of the wave. If asymmetry is set to 50%, the sawtooth wave will grow upwards, and the detent will  be 1/3 of the way up.

Because Breakaway Broadcast Processor is designed to work with standard sound cards, a flat edge is added to the bottom of the sawtooth wave when asymmetry is use. This maintains equal positive and negative energy despite the difference in peak level, preventing the sound card from de-centering the waveform. 

The important thing when looking at the sawtooth wave is to verify that the lines are completely straight. If the lines show curvature, the transmitter is not linear, and will cause audible distortion when processing for loudness.

All the following images display ideal transmitter behaviour.

 

am saw symm   

am saw detent symm

am saw asymm  

am saw detent asymm

 

 If your transmitter matches the images above, Breakaway Broadcast Processor will be able to make your on-air signal sound its absolute best.