RnR Engineering

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Section 73.1570 of the FCC Rules permits a maximum positive peak modulation of 125%.  Achieving this consistently in practice has often proved elusive.  Many station engineers report that, at times during the course of a broadcast day, the modulation is observed as robustly kicking above 100% and at other times is observed to be flat, barely hitting 80 to 85%.   And further, this phenomenon is often source specific and is problematical for stations with talk formats importing audio from a variety of external sources.  This article will show how the asymmetrical nature of audio causes this to occur.

 

BASICS FIRST

 

Program audio is complex, consisting of hundreds of sine waves ranging from about 50 Hz to 8000 kHz.  The basic properties of a sine wave are shown graphically in Figure 1.

 

 

 

Figure 1.  Sine Wave

 

Note that one complete cycle of the sine wave occurs in 360^○ of rotation.  The time for one cycle is related to the frequency of the wave.  For example, a 1 kHz sine wave has a time, or period, for one cycle of 0.001 sec, or 1 msec.

A sine wave has a second important property called symmetry.  Note that the amplitude of the positive peak at 90^○ is the same as the amplitude of the negative peak at 270^○.  We say the sine wave is symmetrical.

 

 

MODULATION WITH SINE WAVE

 

Let's modulate a radio carrier with a sine wave and see what that looks like.  Figure 2 shows the result.

 

 

Figure 2.  Modulation with Sine Wave

 

Note that negative peak B just touches the -100% point (any greater than -100% is overmodulation) just as positive peak A touches the +100% point.  And also note that the peak amplitude of peak A is twice the amplitude of the 0% or no modulation point.

 

ASYMMETRICAL SINE WAVES

 

If we purposely distort a sine wave, we can create an asymmetrical wave that is sinusoidal.  We will use this distorted asymmetrical sine wave to illustrate the problems in achieving high positive peak modulation.  Figure 3 shows an asymmetrical sine wave.

 

Figure 3.  Asymmetrical Sine Wave

 

In Figure 3, A is the positive peak and B is the negative peak.  Note the amplitude of peak B is only 80% of peak A. If we flip, or invert the phase of this wave, peak B becomes positive and peak A becomes negative, as shown in Figure 4.  Note in Figure 4, peak B is still 80% of the amplitude of peak A.

 

 

Figure 4.  Inverted Asymmetrical Sine Wave

 

 

Next, we will modulate a radio carrier with these two asymmetrical sine waves to see the results as shown in Figures 5 and 6.

 

 

MODULATION WITH ASYMMETRICAL SINE WAVES

 

 

Figure 5.  Modulation with Asymmetrical Sine Wave

 

Figure 5 shows a carrier wave modulated by the asymmetrical wave in Figure 3.  Due to the difference in amplitude between peak A and peak B, peak B just hits -100% as peak B hits +120%.

 

Now let’s modulate the carrier with the inverted asymmetrical sine wave shown in Figure 4.  The results are in Figure 6.

 

 

 

Figure 6.  Modulation with Inverted Asymmetrical Sine Wave

 

In Figure 6, peak A is now the negative peak, and peak B is now the positive peak.  Due to the difference in amplitude between peak A and peak B, peak A just hits -100% as peak B hits +80%. Comparing Figure 5 with Figure 6, the positive peak modulation difference is 40% (120% in Figure 5 vs. 80% in Figure 6).

 

What have we learned?  Just this.  If your modulating source is asymmetrical, the phase of the modulating source has a critical effect on your positive peak modulation.  Flip the source one way, and you get high positive peaks.  Flip the source the other way to invert it, and your positive peaks are flat (<100%).

 

ASYMMETRICAL AUDIO

 

Typical program audio, particularly voice, is strongly asymmetrical.  This is due to the combination of the hundreds of different sine waves comprising program audio.  Figure 7 is a sample of the human voice taken from a program source.

 

Figure 7.  Asymmetrical Audio

 

In Figure 7, you can see a strong asymmetrical characteristic.  Peak A hardly resembles Peak B and further, the amplitude of Peak B is only about 80% of Peak A.  We call this 2 dB of asymmetry.  The actual amount of asymmetry in program audio depends on the nature of the source.

 

MODULATION WITH ASYMMETRICAL AUDIO

 

Let's modulate our radio carrier with the asymmetrical audio waveform in Figure 8 to see the results.

 

Figure 8.  Modulation with Asymmetrical Audio

 

Note in Figure 8 that negative peak B just touches the -100% point just as positive peak A touches the +120% point.  This is due to the difference in amplitude peak A and peak B in the program audio source.

 

Now, let's flip or invert the phase of the asymmetrical program source and use that to modulate our radio carrier.

 

 

Figure 9.  Modulation with Inverted Asymmetrical Audio

 

In Figure 9, peak A is now the negative peak, and peak B is now the positive peak.  Due to the difference in amplitude between peak A and peak B, peak A just hits -100% as peak B hits +80%. Comparing Figure 8 with Figure 9, the positive peak modulation difference is 40% (120% in Figure 8 vs. 80% in Figure 9).  This is consistent with what we observed with asymmetrical sine wave modulation.

 

Once again we see that if your modulating source is asymmetrical, the phase of the modulating source has a critical effect on your positive peak modulation.  Flip the source one way, and you get high positive peaks.  Flip the source the other way to invert it, and your positive peaks are flat (<100%).

 

In a normal broadcast day, audio from various external sources can cause the phase of the program audio to flip back and forth between normal and inverted.  This will cause peak positive modulation to flip between greater than +100% to less than +100%, with a resultant affect on station loudness.

 

THE SOLUTION

 

The solution to this would be some kind of device that continually monitors the phase of the incoming program audio to sense either normal or inverted phase.  If it detects inverted phase, it flips the output to maintain the stronger audio peak in the positive modulation direction.

 

Back in the '70s, UREI made a device called the BL-40 Modulimiter.  Built in it was circuitry to do this normal/inverted phase sampling and output switching.  Also, CBS Laboratories made a version of their Volumax that had this circuitry built in.  Sadly, both units are hard to find these days, and tend to be pricy.

 

RnR Engineering manufacturers a device called the Posi-Phase which accomplishes the program audio sampling and output phase switching.  Within the unit, special sampling circuits track the asymmetrical peaks in your program audio and correct output phase as needed to keep the stronger peaks modulating above 100%.

 

The main points of the Posi-Phase are:

 

●       Housed in compact 3"X4"X2" die cast box

●       Balanced input and output

●       Input and output levels set for +4db (other levels available, if desired)

●       Convenient bypass switch for testing or troubleshooting

●       Dual color LED indicates normal or reversed output phase

●       User adjustable sensitivity control for fine tuning

●       Powered by 12 VAC wall transformer

●       RFI proof

 

RnR's Posi-Phase box was designed and built by engineers that understand the importance of loudness and quality.  Order Posi-Phase now.  You will be amazed with the results.

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