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Belar AM Mod Monitor Measures Up
by Mario Hieb, P.E.
The AMMA-2, the new digital AM modulation monitor from Belar, may
look like an ordinary piece of broadcast equipment. It's one rack
unit tall and has the typical buttons and LEDs on the front panel.
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Product Capsule
Belar AMMA-2 AM Modulation Monitor
Thumbs Up:
Precise monitoring capability
Measurement of many parameters
Many configuration modes
Thumbs Down:
Interface software not yet available
Retail Price: $3,500
For information contact Belar in Pennsylvania at (610) 687-5550
or visit www.belar.com
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But don't let its looks deceive you; this unit can help increase
coverage and improve dial presence.
I had the opportunity to test the AMMA-2 during the remodeling of the transmitter facility of KLO, Ogden, Utah. A four-tower directional facility, KLO installed a new Harris DX-10 transmitter to improve on-air sound and improve fringe area coverage.
The new transmitter had substantially better modulation performance than the old MW-10; this was immediately apparent on the AMMA-2. With better audio performance, it was possible for me to process the audio more aggressively, increasing average modulation. Also, the AMMA-2 reads complex waveforms more accurately, which adds to the increase in modulation.
Both of these improvements yielded greater signal strength and better coverage for KLO.
Making it work
Here's how it was done.
AM radio differs from FM radio in many ways; one important difference
is the way modulation affects signal strength. The FCC allows peak
modulation of 125 percent and negative modulation of 100 percent.
Fig. 1 shows an unmodulated AM carrier. (Click
for Fig. 1 - Fig. 3) Fig. 2 shows a carrier modulated at 100
percent peaks, positive and negative. The peak amplitude of the
100 percent modulated carrier is twice the amplitude of the unmodulated
carrier. Because power is proportional to voltage squared, when
you double the voltage (as with 100 percent modulation), you quadruple
the power emitted by the transmitter.
Increasing negative modulation by 100 percent clips the waveform,
but increasing the positive modulation to the FCC legal limit of
125 percent will increase peak power by a factor of 5 compared to
the unmodulated carrier. Fig. 3 shows an AM carrier modulated to
125 percent positive peaks. Stations wishing to increase signal
coverage will process their audio so that average modulation is
as high as possible without having positive peaks that exceed 125
percent.
This is where the AMMA-2 comes in.
No overshoots
The AMMA-2 is a DSP-based AM modulation monitor/analyzer that precisely measures positive and negative peak modulation, peaks per minute, average peak modulation, modulation density and more. Because it uses digital signal processing, problems inherent in older modulation monitors, such as overshoots, don't exist in the AMMA-2.
According to Arno Meyer of Belar, these overshoots could relate to modulation under-readings of up to 10 percent. By using the AMMA-2, the user could measure modulation more accurately, realizing higher modulation, which yields greater power levels and better coverage.
The AMMA-2 also features user-defined parameters and settings for maximum flexibility. Two large Up/Down Menu keys are used to cycle the 16-character alphanumeric display to the desired menu; the Up/Down parameter keys then are used to select the desired setting. The unit configuration may be saved to the on-board non-volatile memory so the settings are retained in the event of the power loss.
A noise menu selection displays RMS-detected signal-to-noise ratio in decibels (flat or de-emphasized) on the front panel. A carrier cutoff indicator consisting of three front-panel LEDs continuously monitors the approach of negative modulation to carrier cutoff.
A unique "Normal Modulation" indicator shows when modulation is falling within user-defined parameters. A built-in Audio Sentry audio failure alarm alerts you to loss of modulation or drops in modulation level.
An RS-232/RS-422 port allows the AMMA-2 to be accessed via personal computer. The ASCII command set provides an easy way for users to write their own command and control software. Alternately, the AMMA-2 will be supported in the future by software, which provides unit control, graphing and logging capability for the AMMA-2 measurements.
A remote alarm connector consisting of eight pairs of relay contacts provides alarm status information for hand-wired applications. NRSC de-emphasis may be selected internally by moving a jumper plug.
In addition to monitoring standard AM modulation, AMMA-2 accurately monitors modulation dependent carrier level (MDCL) AM transmissions. During MDCL broadcasts, the RF carrier level becomes a dynamic component of the AM signal. The AMMA-2 tracks the carrier, capturing the highest and lowest values of the carrier level appropriate for the MDCL system in use. The optional remote meter panel lets the user display positive modulation, negative modulation and carrier level simultaneously on three large analog meters.
(Ed. Note: Belar also states that the AMMA-2 has two
FIR filters, 8 kHz and 5 kHz, that can be enabled so that digital
broadcasts will not interfere with the peak modulation readings
of the analog signal. The digital shows up as noise and can add
significantly to the peak modulation readings.)
What might Belar do to improve this product? Unfortunately, interface software is not yet available for the AMMA-2. This would allow the user to view and record several parameters at the same time and view them on a computer monitor.
But with the AMMA-2, the AM waveform can be measured in ways never before possible. Combined with the latest in audio processing and transmission gear, AM radio stations can increase coverage and improve dial presence and on-air sound.
Mario Hieb, P.E., is a Salt Lake City-based consulting broadcast engineer.
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