More Modulation -- Less Processing
12 June 89                  Copyright(c)1989 Modulation Sciences


INTRODUCTION

     With ModMinder an FM station can increase loudness, reduce
processing, or do some of both.

     Surprise -- ModMinder is NOT a new processor, but rather a
rational way of measuring modulation.  ModMinder provides a unique
systems approach to peak modulation measurement.  It can use any
stereo modulation monitor for its front end - only a source of
composite baseband is needed.  It can even be driven by a good quality
FM tuner, within the limits of the tuner's bandwidth.


HISTORY

     The principle of operation of ModMinder can be found in the
history of the FCC treatment of FM modulation.  Before the
deregulation of modulation monitors in 1983, the Rules said that in
order to get Type Approval, a modulation monitor had to peak flash if
10 consecutive cycles of 10 kHz exceeded 100 percent.  This pegged the
response time of the peak flasher at 1000 microseconds (1
millisecond).  In the Automatic Transmitters System (ATS) Rules of the
same era, the maximum number of overmodulation events permitted per
minute was ten.  In order to make the one minute count concept valid,
it was also specified that after each event of overmodulation, no new
occurrences would be counted for five milliseconds.  This period is
often called the "standoff time." While the ATS modulation Rules were
never formally applied to non-ATS operation, they were generally
regarded as a statement of policy that applied to any FM operation.

     A fascinating aspect of virtually all modulation monitors made
both before and AFTER the deregulation of 1983 was that they had peak
flash indicators that operated in ONE cycle of 10 kHz or less!  This
translates into a response time of 100 microseconds or less.  Such
fast peak flashers seem to have stemmed from two considerations:  1]
it is easier to design a fast peak detector than a slow one, and 2] it
was probably felt that faster was more conservative.  The FCC Type
Approval process before 1983 was slow, cumbersome and expensive.  It
was likely thought that making the peak flasher response time ten
times faster would facilitate approval.

     Deregulation opened the door for slowing down the response time
of the peak flasher even more.  Unfortuately, no hint was given as to
how much the flasher could be slowed down.  Thus, even after
deregulation, most modulation monitor manufacturers continued to "play
it safe" and provide peak flasher response times that were much faster
than needed.

     Another significant holdover from the era of regulated modulation
monitors was a requirement for an overall accuracy of only +/- 5
percent.  That meant, for an indication of 100 percent, the actual
modulation peaks could be anywhere from 95 to 105 percent.  Everyone
accepted such sloppy performance because it was all the FCC required.
After deregulation it was maintained from force of habit, and because
it saved money in manufacturing.  However, bear in mind that at 100%
moduation, an accuracy window of ten percent (+/- 5%) is about 1 dB
wide.  Don't confuse resolution with accuracy.  Just because a peak
flasher trip point is set with a thumb wheel switch with 1% steps does
not mean that the accuracy of the instrument is 1%.


MODMINDER SOLUTION

     The concept of the ModMinder evolved from a careful examination
of assumptions underlying the measurement of modulation in FM systems.
This was prompted by problems with the measurement of modulation in
BTSC stereo television.  With many television stereo generator/monitor
combinations, overmodulation indication was almost continuous.  Stereo
television differs from FM stereo radio in that the modulation must be
set exactly to the reference level of the dbx L-R noise reduction
encoder.  It cannot be adjusted arbitrarily without greatly reducing
the system's stereo separation.  Because modulation level could no
longer be "tweaked" to fit the modulation monitor (as had always been
the case in FM radio), differences in modulation monitor
characteristics became much more apparent in stereo television than
they were in FM.

     In the course of trying to resolve television aural modulation
problems, several FM radio modulation monitors were obtained and their
peak flasher performance characterized.  We discovered just how fast
most peak flashers responded.  Once the television aural peak flasher
was slowed down to just meet the FCC spec of 10 cycles of 10 kHz, most
of the television overmodulation problems vanished.  We became curious
as to what effect the almost one millisecond response time would have
on typical FM stereo modulation.

     A modulation monitor with a one millisecond response time was
prototyped and tested using off-air signals.  The prototype monitor
and a conventional modulation monitor were connected to a high quality
FM tuner.  A Scala log-periodic antenna provided a signal with minimum
multipath and a low VSWR.  The system was calibrated to 100% using the
Bessel null technique.  By tuning to various stations it was possible
to determine how great an advantage ModMinder would provide over a
conventional modulation monitor.  The results were impressive.  On
some stations the ModMinder indicated as much as 5 dB below the
standard modulation monitor.  This was because the ModMinder was
ignoring the brief (less than one millisecond) spikes that have no
impact on occupied bandwidth.

     It is interesting to consider that much of the complexity of
modern processing comes from the need to control brief transients
(less than one millisecond).  A way to look at ModMinder is as a
device that allows reducing the apparent ratio of peak to average
modulation.  This improvement can either be used to increase the
number of peaks let through (reduce the amount of processing) or
increase the average modulation (make it louder) or a mix of each.  A
moderately heavily processed station such as WNSR showed an
improvement in loudness of almost 2 dB by merely adjusting total
modulation using the ModMinder - no changes in the adjustment of the
processing were made.  For a lightly processed classical station such
as WNCN, the difference was almost 5 dB.  Very heavily processed
stations, such as WPLJ or Z-100, showed less than 1 dB of change.


CONCEPT

     Conventional monitors provide little information of value about
modulation by real program material.  The peak flasher, which has
always been the mainstay of dynamic measurements, is misleading
because of its excessively fast response time.  And no one has ever
trusted the moving-pointer meter for anything but sine waves because
its ballistics are so different from those of the peak flasher.

     ModMinder provides, in addition to its unique peak flasher, a
readout of the highest peak attained during the previous second.  This
sure beats trying to use a meter to set modulation.  It reduces the
task of setting modulation from a slow painstaking effort to a brief
simple one.  Another set of digital displays show the number of
overmodulation occurrences taking place in a rolling one minute
interval.  By "rolling minute" we mean a window one minute wide moving
along in time continuously.  That is NOT a counter that accumulates
overmodulation counts for one minute then resets.  A rolling minute
counter adds each overmodulation event to its total, while at the same
time it subtracts each event as it becomes one minute old.  In
addition, a "smart" silence-sense alarm is included.  This provides a
reliable indication of loss of modulation.

     A unique feature of ModMinder, not found in other monitors, is
that all of its indicators agree!  Whether digital displays or analog
meters, all the modulation readings are the same even when displaying
complex program modulation because they are all derived from the same
ballistics.

     ModMinder is more accurate than other monitors.  The measuring
circuitry of most monitors is spec'd to +/- 5 percent, an accuracy no
greater than that demanded by the pre-1983 Rules.  ModMinder delivers
better than +/- 1 percent accuracy over a wide temperature range.  The
digital displays provide a resolution of 0.5 percent.

     Another element in the "real world" accuracy of modulation
measurement is where the monitor is located.  Measurements made
off-air are notoriously inaccurate, often indicating much higher peaks
than when measured at the transmitter.  The obvious solution is to
measure the modulation at the transmitter and bring the readings back
to the studio via some kind of remote control.  The problem is that
few modulation monitors are set up to fully interface with a remote
control.  ModMinder is designed for remote operation in two modes.
The first meets the standards to the pre deregulation FCC Rules:  The
peak flasher closes a relay when its user-set threshold is exceeded.
The silence-sense operates a relay.  A maximum number of
overmodulation occurrences per minute can be set and a relay will
close when that count is exceeded.  The one second peak-hold indicator
drives a digital/analog converter, producing a 0 to 2.5 volt signal,
scaled to peak modulation.  Because the reading is only updated once
each second, even the slowest sampling remote control can accurately
follow the modulation.  A "fast" DC voltage for driving a conventional
moving-pointer meter is provided.  This is useful for a local meter or
a remote control if the sampling rate is fast enough.  The
electronically generated ballistics have a unique characterisic that
makes it very easy to determine peak modulation as well as dynamic
range.

     A second, entirly independent, remote option is provided
digitally.  This supports a slave modulation monitor panel.  Using any
data link of 1200 bits/sec or faster, the entire front panel of
ModMinder plus a moving-pointer meter can be duplicated at any
location - across the street or across the country.  No calibration of
the remote system is required, since, unlike analog remote systems,
the digital data provided by ModMinder is absolute.  If a two-way
circuit such as a dial phone line is used, the control functions of
the front panel as well as its indications are duplicated.  If only a
one-way circuit is used, such as an SCA channel, the settings as they
were left at the master unit are used, but all the indications of the
front panel are available.

     The serial data stream used to drive the remote panel also
provides an interface between the modulation parameters of the station
and any computer.  This may be useful for long term logging of
modulation data, either of the host station or a competitor.  With the
addition of some voltage controlled amplifiers a PC might easily be
programmed as a custom audio processor.  The data stream is
asyncronous 8 bit ASCII transmitted at 1200 bits/sec via an RS-232C
interface.  Most of the data is instantaneous modulation readings.
About once each second the status of all ModMinder indicators and
threshold trip points are transmitted.


BUT IS IT LEGAL?

     ModMinder is legal.  In this age of deregulation, sometimes it's
difficult to decide what is and is not legal.  The FCC is no help, at
least not until you get cited, and even then it's not so clear.
Sometimes two different bureaus of the FCC will differ on what is
permitted under the Rules.  Usually the Field Operations Bureau
squabbles with the Mass Media Bureau.  Watching internecine warfare at
the Commission may be entertaining, but it does not help you in
interpreting the Rules.

     We despaired of getting an answer from the FCC, so we decided to
do what any smart broadcaster would do - ask an FCC lawyer.  What we
got was a clear YES.  Called an "Opinion of Counsel," it is a lawyer's
way of putting themselves on the line.  This kind of letter carries
weight with the FCC, the courts and other lawyers.  We would be glad
to provide you a copy of our attorney's letter and invite you to send
it to your FCC attorney for comment.

     ModMinder has two modes of operation:  FCC and Peak Weighted.
All the previous discussion has dealt with FCC mode.  FCC mode is the
one that complies with section 73.332(f)(3) (October 1982) of the
rules prior to the deregulation of 1983 and is the subject of the
Opinion of Counsel.  The other mode, Peak Weighted, is less specific.
Its response time may be adjusted by changing the value of a resistor.
A table giving the response time set by various standard resistors is
included in the manual.  The FCC Rules now in force do not specify any
response time;  in fact, the Report and Order that deregulated
modulation monitors anticipated new standards driven by improvements
in the technology.  Unfortunately, no guidelines were provided.  In
order to provide a rational basis for choosing a suitable response
time, Modulation Sciences will embark on a research program to
correlate various peak flash response times with occupied bandwidth.
The results of this research will be published IN FULL as it
progresses.  It is anticipated that the information obtained will
enable a station to make a rational selection of peak flash response
times.  Thus a ModMinder will help to further reduce the amount of
audio processing a station is forced to do to remain competitive.


ON AIR TESTS

     Tests conducted at WNSR were a spectacular success.  A moderately
heavily processed station, they use an Optimod 8100 with an XT
chassis.  The improvement in modulation by changing from their
conventional monitor to the ModMinder was 1.7 dB.

     WNSR also maintains a second air chain with an 8100 without the
XT chassis.  For these tests processing was set rather light.  We
found that when this very gently processed air chain was switched in
and set using ModMinder, NO LOSS IN LOUDNESS resulted as compared with
the much more heavily processed chain set up using the orignal
modulation monitor!


CONCLUSION

     ModMinder holds the promise of combining maximum loudness with
greatest quality.  For the first time it will be possible to eliminate
excessive processing without giving up any loudness.  Stations that
have suffered a competitive loss because they refused to "nail the
processing to the wall" can now maintain maximum loudness without
overprocessing.  And heavily processed stations now have an
alternative which will still further enhance their competitive
position without additional squashing and squeezing.