FM Systems



ADM-1

TV AUDIO DEVIATION METER

INSTRUCTION BOOK

IB612801

TABLE OF CONTENTS

1.0 GENERAL DESCRIPTION

1.2 OPERATION, PEAK-TO-PEAK MEASUREMENT

1.3 OPERATION, RMS MEASUREMENTS

1.4 HOW TO EQUALIZE THE ENTIRE HEAD-END

1.5 STABILIZING TV AUDIO VOLUME

2.0 SPECIFICATIONS

3.0 FUNCTIONAL DESCRIPTION

4.0 RELATIONSHIP BETWEEN PEAK AND RMS READINGS

4.1 WHAT PEAK READINGS REVEAL

4.2 WHAT RMS READINGS REVEAL

5.0 MAINTENANCE

6.0 ALIGNMENT & CALIBRATION

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1.0 GENERAL DESCRIPTION

The ADM-1 measures Peak Deviation and RMS Audio Loudness of TV Audio signals in CATV and SMATV system. One of the difficult problems faced by Cable Head-End Engineers is trying to adjust TV audio deviation so that all channels sound equally loud during the loudest passages. Comparing loudness of various channels by ear is very difficult because the channels being compared are not likely to be modulating 100% at the same time. Trying to adjust all channels to equal loudness is next to impossible because the ear is not very level sensitive and cannot compare the loudness heard on one channel some time ago with another channel currently being "audited". VU meters, if provided, usually are affixed to each channel modulator, and are only useful for rough settings and do not read the actual FM deviation at all.

The ADM-1 measures both the actual peak deviation of the sound carrier modulated by live program and also the relative loudness of the audio modulation present on each television channel. The ADM-1 not only measures these parameters with digital accuracy, but also automatically holds the highest reading for later observation. This insures that the 100% modulation level is measured, not some intermediate lower level.

The ADM-1 read-out is a digital meter capable of measuring up to +/- 199.9 KHz on the peak deviation scale and +/- 10 dB from 0.0 dB in steps of .1 dB. These readings are stored until released by the reset switch. After the measuring interval, four readings are available.

A. Positive Peak Deviation (+P)

B. Negative Peak Deviation (-P)

C. Peak-to-Peak Deviation (P-P)

D. RMS Loudness (RMS)

All four readings are stored in the meter and each may be selected for observation on the digital meter. Activating "RESET" returns all storage to ZERO, permitting a new measurement to be made.

The ADM-1 contains a precision +/- 25 KHz TV audio modulator (operating on exactly 4.5 MHz), along with a TV video carrier (un-modulated) on Channel 3. Channel 2 or 4 are also available on specific order.

A regular TV monitor is used to tune in the channel, or a RF set-top converter does the tuning for the TV set, depending on the cable system operations plan. In this way the ADM-1 insures that the cable system is measured exactly as it is used.

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The ADM-1 is connected to the TV monitor (and set-top converter, if used). The audio output of the TV monitor is connected to the audio input of the ADM-1 so that any desired channel may be tuned in. The TV monitor is then calibrated to read exactly 50 KHz peak-to-peak deviation when the ADM-1 precision deviation calibrator is turned on. Now that the TV monitor is calibrated, tuning to any other channel stores the actual program audio being transmitted in the registers.

1.2 OPERATION, PEAK-TO-PEAK MEASUREMENT

A regular TV monitor is used to tune in the channel, or an RF set-top converter that does the tuning for the TV set. Connect the ADM-1 to the TV monitor as shown in diagram BD6128-12. The A/B switch is not necessary, but in fixed installations will facilitate calibration. Next, turn power on by pressing the "POWER" button on the front panel.

Set the A/B switch so that a direct connection is made from the "TV CHANNEL OUTPUT" of the ADM-1, to the TV monitor RF input. Tune the TV monitor to the specified output channel (CH2, CH3, or CH4), and turn on the "+/-25 KHz REFERENCE". A 400 Hz tone should be audible and the screen should be black.

Connect the audio output of the TV monitor to the ADM-1 "AUDIO INPUT". For TV monitors with no external audio output connections See Note 1 for obtaining an audio output. Adjust the audio output level of the TV monitor so the level will not overdrive the ADM-1. Set the level to obtain adequate range of the "Calibration" knob of the ADM-1.

Set the "MODE" knob on the ADM-1 to the "P-P" setting and momentarily push the "RESET" switch. Observe the digital display, if it displays a measurement less than 50.0, slowly turn the "CALIBRATION" knob clockwise until a reading of 50.0 is achieved. If the display is higher than 50.0 turn the "CALIBRATION" knob counter clockwise, then momentarily push the "RESET" switch. Repeat the previous procedure until the meter displays a measurement of less than 50.0. Then turn the "CALIBRATION" knob clockwise until the meter displays exactly 50.0. The ADM-1 is now calibrated, make no further adjustments either to the calibration knob on the ADM-1 or the audio output level of the TV monitor.

IMPORTANT: A momentary push of the "RESET" switch must accompany any adjustments made during calibration and mode selections made to the ADM-1.

Set the A/B switch so the RF set-top converter is now directly connected to the TV monitor RF input. Tune the converter to the channel to be measured. Momentarily press the "RESET" switch and observe the digital display meter.

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As the program audio, on the channel being measured, increases in volume, the meter will continually record the actual P-P RF deviation and hold the highest peaks reached. Repeat the previous procedure for all TV channels to be measured. Note that the P-P deviation measurement does not necessarily correlate to the audio loudness perceived. Relative loudness is best measured using the "RMS MODE".

Note 1:

Several manufacturers of TV monitors provide no external audio output on the monitor. In order to obtain an audio signal from these monitors, it will be necessary to remove the cabinet and tap (through a parallel connection) the audio signal to the speaker.

1.3 USING THE RMS MODE

The RMS mode is used to set all of your modulators to the same volume level. First select a channel with a volume that you want all you other channels to sound like. Tune your cable converter to that channel.

Set the "MODE" selection knob on the ADM-1 to the "RMS" position and push the "RESET" switch. If the number is above +0, then turn the "CALIBRATION" knob counter clockwise and push the "RESET" switch momentarily until the number is below -0. (Note: Allow a few seconds for the RMS circuitry to sample sufficient audio program to produce a stable RMS measurement which is displayed on the digital display meter). Then turn the calibration knob clockwise until you reach 0.0 on the meter.

Now you can turn your set-top converter to all of your channels and set your modulators to the same number 0.0 Please remember to push the "RESET" switch before each new measurement on each channel.

Once you have calibrated the ADM-1 to your TV set, adjusting the volume on your set or turning the "CALIBRATION" knob will cause a loss of calibration. If this happens simply repeat the calibration steps.

EQUALIZING THE ENTIRE HEAD-END

Many cable systems switch ad-insert audio among many satellite and local origination channels. Setting audio levels correctly can be a massive problem. The ADM-1 can reduce this problem to a series of easy steps as follows:

First determine the audio level that is to be common to all sources. This standard operating level should be what ever level all of the TV modulators can accommodate.

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For example, we assume that all TV modulators can operate at 0.0 dBm, so this is chosen for all program sources.

Next calibrate the ADM-1 or "RMS" to read 0.0 dBm on the meter, when 0.0 dBm is present at its input. Since the audio switching system probably operates "balanced" meaning that both wires have audio on them, and since the ADM-1 has an unbalanced input, connect a balancing transformer between the ADM-1 and the equipment to be measured.

Connect each program source in turn to the ADM-1, and adjust the audio level of each source to the same level (0.0 dBm). this would equalize all sources to the same volume, but they may not remain that way since ad-insert tapes and even successive programs on the same satellite transponders may differ in volume.

STABILIZING TV AUDIO VOLUME

The ALM672 "AUDIO LEVEL MASTER" at the input of every TV audio modulator will insure constant volume, even with program source switching and ad-insertion. It maintains a constant output level (0 dBm, +4 dBm or +8 dBm internally adjustable) over a 30 dB input range.

With the ALM672 in place, even widely varying audio level sources will modulate the TV channel at a constant volume. While the ADM-1 is needed to be able to set the channels all to the same level, only the ALM672 can insure that these levels are held at the correct volume day after day.

2.0 SPECIFICATIONS

INPUT SPECIFICATION

Base-band Audio (from TV set) -10 dBm to +10 dBm

Calibration Range -10 dBm to =10 dBm

CALIBRATOR

Output Channel Ch-2, Ch-3, Ch-4 (specify)

Output Sound Carrier Deviation +/- 25 KHz

Deviation Accuracy Better than +/- 0.1%

MEASUREMENT

Maximum Peak Deviation +/-199.9 KHz

Resolution of Peak Deviation 0.1 KHz

Accuracy (at +/- 25 KHz) +/- 1%

Type of Peak Reading Pos, Neg, P-P (Hold)

RMS Deviation Range +/- 10 dB (Ref)

Resolution of RMS Deviation 0.1 dB

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3.0 FUNCTIONAL DESCRIPTION

The ADM-1 can be considered to consist of six main sections as follows:

A. Input Signal Conditioning Circuits

B. Peak Reading Sample and Hold Circuits

C. RMS Reading Sample and Hold Circuits

D. Selection and Metering Circuits

E. Calibration System

F. Power Supply and Regulators

The Input Signal Conditioner consists of U1 and associated circuitry. Potentiometer VR1 is the "CALIBRATION" control on the front panel. This control is adjusted to read 50 KHz peak-to-peak deviation when calibrating the ADM-1 prior to taking readings. U1 sets the basic gain of the system, removes 15.734 Khz, high frequency noise, stereo L-R products, Second Audio Programs (SAP), and other signals not being measured. This enables stereo signals to be directly compared to mono signals.

The Peak Reading Sample and Hold system individually measures negative peak deviation and positive peak deviation. the positive peak deviation circuit consists of U4, U5 and associated components while the negative peak deviation circuit consists of U2, U3 and associated components.

The RMS Reading Sample and Hold circuit consists of U6, U7, and associated components.

Selection and Metering Functions are performed by "MODE" switch SW2 and by digital meter M1. The "MODE" switch selects Positive Peak (+P), Peak-to-Peak (P-P), Negative Peak (-P), and RMS Readings to be displayed on the digital front panel meter.

The Calibration System consists of U9, U10. U11, and associated components. The sound carrier deviation is a precise 50 KHz peak-to-Peak. The Deviation Balance control centers the deviation so that +/-25 KHz is produced. The Peak-to-Peak deviation is always exactly 50 KHz and will not change with time or environment.

The power supply consists of T1, U12, U13, and associated components. Four regulated voltages are produced, +12, +5, -12, and -5 volts. Fuse F1 is on the circuit board.

- DANGER - HIGH VOLTAGE -

REMOVING THE TIP COVER WILL EXPOSE 115 VAC WIRING.

SOME TRACES ON THE CIRCUIT BOARD CARRY AC POWER.

REMOVE AC POWER CORD FROM SOCKET BEFORE OPENING CASE.

ADM-1.ISB PAGE 6 OF 9

4.0 RELATIONSHIP BETWEEN PEAK AND RMS READING

The relationship between Peak and RMS (P/RMS) is very complex in audio program signal and is consciously manipulated to further particular ends by the station operators. The P/RMS ratio is very high in unprocessed sound, reaching as high as 18 dB (or more). Digital disks can record such P/RMS ratios, but more other recording, broadcast, and transmission systems require that such wide P/RMS be reduced. this is done by a wide variety of "Compressors", "Limiters", and "Audio Processors". this compression of P/RMS is most noticeable on FM stations, which vie with each other to be the "loudest" station. Since the FCC requires that the FM stations limit peak deviation to +/-75 KHz, the increase in loudness of one station over another is dine by reducing the P/RMS ratio with audio processors.

Since the Cable Operator receives TV audio signals from many sources, and each source "processes" the audio to varying degrees, he can be sure that the P/RMS ratio will not be the same from each station. While TV stations regulate their peak deviation carefully, to avoid a ticket from the FCC, they may very well process their audio very differently and thus have different sound volumes. The TV signals received form a satellite vary both in peak deviation and degree of audio compression and are much more difficult to adjust for equal volume in a cable system.

The day-to-day and program-to-program variation in volume on any given satellite channel may be made into a constant volume with the AUDIO LEVEL MASTER (FM SYSTEM, INC. model ALM672). The ALM672 is wired between the satellite receiver and the TV modulator audio input. The ALM672 reduces input volume variations to a minimum, then and ADM-1 is used to set the modulator output deviation to a constant volume with respect to the other channels.

A sine-wave tone exhibits a P/RMS of 3 dB, while a lightly processed audio signal averages about 13 dB. The difference between a test tone (3 dB) and program audio (13 dB) is 10 dB. This is why Average Program Level (APL) and peak Program Level (PPL) is normally considered to be 10 dB. But remember, this is only true of lightly processed (compressed) audio. For heavily compressed audio (typical of rock music), the P/RMS ratio may go as low as 3 dB or even less (making the station sound 7 dB or more louder for the some peak-to-peak deviation).

Audio processing is a fact of life and is under the control of the originating station, therefore the Cable Operator must deal with this situation. The RMS reading ADM-1 provides the tool to measure all of these variables so the correct modulator settings may be achieved effectively with a minimum of effort.

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4.1 WHAT PEAK READINGS REVEAL

The ADM-1 accumulates the highest, Positive Peaks (+P), Negative Peaks (-P), and Peak-to-Peak (P-P) deviation of the TV audio carrier. These measurements are recorded and stored so that each may be read out on the digital front panel meter. Operating the "RESET" switch reduces the storage registers to zero and starts a new accumulation. The accumulation interval is set by the operator, and may be seconds, minutes, or hours. Several minutes accumulation usually assures that at least one 100% modulation occurs.

The P-P measurement should be close to 50 KHz (the maximum deviation permitted under FCC rules). Actual program audio is usually not symmetrical, that is, the +P and -P may not each be exactly 1/2 the P-P. Note that two programs, each recording 50 KHz P-P, may not sound the same volume due to differing rates of compression. The P-P measurement indicates whether the signal is less than, equal to, or exceeds the FCC maximum deviation. Remember that while Broadcast Stations must limit their deviation below this level, the Cable Operator may exceed this limit to some extent (to maintain equal volumes on all stations) so long as not to interfere with the adjacent TV channel on the cable.

The -P and +P measurement can be used to establish (and consequently adjust non-linearly in the TV monitor and non-linearly in the TV channel modulator.

If the +P and -P do not read exactly the same deviation with the "CALIBRATION" signal, then the monitor discriminator is not correctly aligned. It would be well worth the effort to correctly adjust the TV monitor audio discriminator using the ADM-1 calibration signal so the +P exactly equals -P.

Once the TV monitor is known to be linear then the audio modulator section of the TV modulator may be tested for deviation linearity by applying a low distortion 400 Hz sine-wave to the modulator and measuring that deviation with the ADM-1. This test may be done at several audio levels. Any difference between the +P and -P reading indicates even order harmonic content. Note that deviations well above +/- 25 KHz can be expected to show unequal readings, but +/- 25 KHz should be less than 0.3 KHz to indicate 1% distortion or less. Difference in readings greater that 0.3 KHz indicated excessive TV audio modulator distortion.

4.2 WHAT RMS READINGS REVEAL

The ADM-1 accumulates the highest RMS deviation of the TV audio carrier. This measurement is recorded and stored along with the +P, P-P, and -P measurements for observation on the digital front panel meter. The RMS reading is also set to minimum reading by the "RESET" switch. The minimum reading is -10 dB.

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The "CALIBRATION" control is set to produce a reading of 0.0 dB when the reference channel is connected. Other channels are then recorded as either more or less than the reference channel. Off-air channel is usually used as the reference channel, and all satellite channels are adjusted to that loudness with RMS reading.

The RMS measurement corresponds very closely to perceived loudness because the area under the music wave-form determines the volume perceived by the ear, not the peak voltage. Area under the wave-form is exactly the RMS value. Since RMS is a power function, the ADM-1 is calibrated in terms of dB in the RMS mode.

The RMS scale may be used to measure the frequency response of the TV modulator. The pre-emphasis network in the TV modulator requires that the modulation level for frequency response testing be a least 17 dB lower than 100% modulation. This corresponds to about 7 KHz P-P at 1 KHz. Use an audio level that modulates the TV (audio) modulator to 7 KHz P-P, then switch to the RMS scale and re-adjust the "CALIBRATION" control to read 0.0 dB. Now vary the modulation frequency over the audio range, actuating the "RESET" at each new frequency and record reading. The pre-emphasis in the TV audio modulator will be off-set by the De-emphasis in the TV monitor. Since the two networks may not exactly counter one-another, this frequency response test must allow for this source of error. In general, this source of error will not exceed 1.0 dB and would be noted as a gradual rise or decline in frequency response of all channels. A sharp departure from flat would indicate a defective TV modulator. If all TV modulators display a non-flat condition, then the TV monitor should be suspected.

5.0 MAINTENANCE

No routine maintenance is required other than the formal consideration due any fine test instrument, such as keeping it clean and free form mechanical damage.

6.0 ALIGNMENT AND CALIBRATION

Alignment and initial calibration are performed at the factory. If the unit displays inconsistent reading, or fails to reset to zero with the input disconnected, or reads higher than -9 dB ont the RMS scale after reset with no input, call the factory for further advice.

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