AMM
AUDIO MODULATION METER
OPERATOR MANUAL
DESCRIPTION
The AMM Audio Modulation Meter measure maximum volume level of the TV aural carrier deviation by demodulating the 41.25 MHz I.F. (Intermediate Frequency) carrier and displaying the results in dB on an LCD digital display. The AMM will also measure peak power or volume level of a base-band audio input in dBm. At the bottom of the AMM there is an earphone jack for quality monitoring of the input signal.
The AMM is a small, hand-held, battery operated test instrument that enables TV audio loudness measurements to be made on any TV modulator that is equipped with I.F. test points. No special training or interpretation of display is required. The digital read-out insures consistant measurements.
HOW TO MEASURE TV AUDIO MODULATION LOUDNESS
Connect the "F" connector on the AMM to the audio I.F. test point or to the I.F. loop cable through a 20 dB directional tap so that the 41.25 MHz audio I.F. level is between 0 dBmv and +20 dBmv. Momentarily depress the POWER button and read the meter. The power will remain on for one minute or so, then automatically turn off to conserve the battery.
The AMM stores and displays the highest audio level received. The POWER button must be momentarily depressed each time a new reading is to be taken since levels lower than currently indicated will not read without resetting the meter with the POWER button. this storage mode causes the digital display to rapidly stabilize to the maximum modulation display.
A reference level is established for audio levels in the cable system because the loudness of off-air channels cannot be adjusted in I.F. coupled off-air processors.
Measure the audio volume of these off-air TV stations with the AMM by measuring at the I.F. coupling point of the TV channel converters. Average the readings taken on the off-air channels, and then use that dB level as a reference to set the remainder of the TV modulator channels. In this way, all channels will be of equal loudness. The AMM measures the true RMS value of the modulation, so measures the loudness as perceived by a listener.
Some program source levels are not as stable as a cable operator may want them to be. If audio volume tests reveal that a given channel volume varies excessively, the loudness of that channel can be completely stabilized by connecting a ALM673 AUDIO LEVEL MASTER into the channel. Contact FM SYSTEMS for information regarding control of audio levels.
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AUDIO QUALITY MONITOR
An earphone jack, located near the POWER switch, is provided so that the technician can listen to the quality of the modulated sound carrier. Noise, distortion and any other audio interference can be quickly identified by listening in on the sound as the modulations tests are being performed. Any ordinary hi-fi earphone can be used. Of course the sound is monaural, so the sound will only be heard in one ear. This is true even if the TV channel is being modulated with a BTSC stereo modulator because there is no BTSC decoder in the AMM. Stereo tests can be performed with the FM SYSTEMS CSPM-1 Cable Stereo Performance Meter.
AUDIO BASEBAND LEVEL MEASUREMENT
The AMM measures audio baseband power level at the "RCA" audio input connector. Removing the cable from the "F" connector squelches out the FM demodulator. Connect the audio circuit to be measured to the "RCA" audio connector. The input circuit is high-impedance (bridging) and so does not load down the audio level on the circuit being measured. The AMM can measure either balanced or unbalanced audio circuits at audio power levels ranging from +20 dBm to -60 dBm. The meter will read either test tones or program audio in true RMS, calibrated in dBm in a 600 ohm circuit. The AMM must be reset by momentarily depressinng the POWER button each time a new reading is desired.
HOW TO EQUIP A CABLE HEAD-END FOR MODULATION MEASUREMENT
Modulation measurements can be made with the AMM without interrupting television service to the cable subscribers by perparing the head-end with modulation test points on each television channel. The best way to do this is to install the MMC (Modulation Measurement Center) panels. Each MMC consists of a 1.75" panel with test points for six television modulators. The panel replaces an existing spacer panel, so it does not occupy additional rack space or obscure the necessary ventilation between TV modulators since the MMC only extends about two inches behind the front panel.
The MMC (Modulation Measurement Center) brings all modulation test points from the rear of the TV Modulators to the front panel where the modulation controls are located as well as enabling tests to be made without interrupting service.
Audio loudness modulation measurement is possible without installing the MMC panels, simply by connecting the 20 dB directional taps in the audio I.F. loop and making measurements at the rear of the rack. While this is not as convenient as measuring on the MMC panel, the AMM meter can certainly test audio modulation this way.
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BATTERIES
The AMM is equipped with two 9 volt alkaline batteries. When these batteries run down, replace both at the same time. As the end of battery life approaches, the "on" time shortens, then when the terminal voltage is too low, the AMM will not turn on. This prevents incorrect readings due to low battery voltage. Shortened "on" time should be taken as a warning that the batteries are getting ready to die out.
DO NOT ATTEMPT TO RECHARGE ALKALINE BATTERIES. THEY MAY LEAK CORROSIVE FLUIDS OR EXPLODE THEREBY DAMAGING THE CIRCUITRY AND VOIDING THE WARRANTY.
CHARGER
An optional charger and nicad batteries may be ordered. In this case, two 9 volt nickel-cadmium batteries must be installed in the battery compartment. An additional charge, lasting 24 hours must be applied before using the AMM. The wall mount charger operates from 115VAC mains and supplies 12 volts DC to the charging circuit within the AMM. If the nicad batteries must be replaced, change both batteries at one time. Do not mix old and new batteries.
Two types of 9 volt nicad batteries are available, the 8.4V (110milliampere-hour) and the 7.2V (80 milliampere-hour). USE ONLY THE 8.4 VOLT TYPE.
The charging current is low to prevent accidental overcharging, so the charger should be left on overnight. Use the meter until short "on" time indicates a low bettery condition prior to recharging, otherwise a moderate "discharge memory" will prevent use of the full capacity of nicad batteries.
CARE AND MAINTENANCE
This AMM is a precision measuring instrument and should be treated accordingly. while it can withstand ordinary everyday indoor use, it should not be left outside in the rain or otherwise mistreated. It is not waterproof. The batteries should be removed if it is placed into storage to prevent leakage of corrosive fluids from batteries as they discharge and age.
Replace non-chargeable batteries at least once a year even if ordinary use does not discharge the battery, because old batteries may leak and cause corrosion damage.
No routine maintenance or test procedures are required other than battery replacement. Attempts at field repair or adjustment will void the warranty.
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CARE AND MAINTENANCE (cont'd)
If the AMM fails to operate even after battery replacement, or does not read a known signal correctly, call the factory for a Return Authorization Number and return to the factory for repair.
MODULATION MEASUREMENT SYSTEM
The AMM is one of three test instruments that make up the Modulation Measurement System. The complete system consists of:
VVMThe Video Volt Meter measures the baseband video input to the TV modulator in volts peak-tp-peak or IRE units.
VMMThe Video Modulation Meter measures the depth of video modulation on the 41.75 MHz video I.F. carrier.
AMMThe Audio Modulation Meter measures the loudness of audio modulation on the 41.25 MHz sound I.F. carrier.
MMCModulation Measurement Center brings all three test points on each TV modulator to the front panel. Up to six TV modulators can be connected to one MMC panel.
Comtact FM SYSTEMS, INC. at (714) 979-3355 (in California), or at 1-800-235-6960 (nationwide WATS) to obtain further details on the Modulation Measurement System.
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