Energy Conservation in AM Transm_savings

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					Energy Conservation in AM Broadcast
Transmitters Using Carrier Control
Algorithms

Tim Hardy                 April 21, 2009
Overview



Historical Notes and References
Technical Description
Waveforms
Performance
Final Thoughts
Historical Perspective

Earliest References date to the 1930’s to a system known as “HAPUG”.
This system was named after its inventors, Harbich, Pungs and Gerth.
This system never made it beyond the experimental stage.

The first modern reference that I have found:

Reference 1: Energy Conservation and Reception Quality for Dynamic
Amplitude Modulation, Institute of Radio Technology Report Number
22/80, G. Petke and J. Mielke, 7 August 1980.

Dynamic Amplitude Modulation (DAM) was then commercialized by
Telefunken (then AEG Telefunken, today Transradio) during the 1980s.
Historical Perspective

ABB of Switzerland developed a very similar technology that they refer to
as Dynamic Carrier Control (DCC). First on air tests were done on a
600kW transmitter in Vienna, Austria in 1983.

Reference 2: Dynamic Carrier Control, DCC, a Valuable Method to Save
Input Power of Medium Wave Transmitters, Dr. Wolfram Schminke and
Hans-Ulrich Boksberger, IEEE Transactions on Broadcasting, Vol. 35,
No.2, June 1989

In the UK at the BBC Research and Development group, a different
approach was developed referred to as Amplitude Modulation Companding
(AMC).

Reference 3: Amplitude modulation radio broadcasting: application of
companding techniques to the radiated signal. BBC Research Department
Report No. BBC RD 1985/13. W.I. Manson, 1985
AM Carrier Power

                                            Baseband AM Signal
The AM carrier does not     2.5

carry any information yet    2
contains more than 2/3
of the transmitted power.   1.5

How can the transmitted      1
waveform be modified to
                            0.5
reduce power without
reducing received quality    0
                                  0   0.2    0.4              0.6   0.8   1
in simple AM receivers?                            Time (s)
 DAM Gain Function

                                                                                     Voltage
Carrier (only) is                                                   DAM
                                                                                     Power
decreased the most at
moderate modulation                             100                                         100
                                                 90                                         90
levels.

                            Carrier % Voltage
                                                 80                                         80




                                                                                                  Carrier % Power
                                                 70                                         70
Received loudness is                             60                                         60
increased when carrier                           50                                         50
                                                 40                                         40
is reduced.                                      30                                         30
                                                 20                                         20
The carrier is increased                         10                                         10
at higher modulation                              0                                         0
                                                      0   25   50         75   100       125
levels so that distortion
                                                               Modulation %
does not occur.
AMC Gain Function

                                                                                              Voltage
Carrier and modulation                                            AMC (3 dB)
                                                                                              Power
together are decreased
with increasing                                    100                                              100
                                                    90                                              90
modulation index.

                            Carrier & Sideband %




                                                                                                          Carrier & Sideband
                                                    80                                              80
                                                    70                                              70
There is little impact on                           60                                              60




                                                                                                                 Power
                                    Voltage
received loudness.                                  50                                              50
                                                    40                                              40
The carrier is increased                            30                                              30
                                                    20                                              20
to full power during                                10                                              10
quiet periods when                                   0                                              0
noise is most easily                                     0   25   50       75     100   125      150

perceived.                                                             Modulation %
Time Domain Considerations
Typical settings for the audio peak detection of the audio are:
     – Attack time: ~1 ms
     – Decay time: ~200 ms


A Decay time of 200 ms is determined by the need for the receiver AGC to
respond more quickly to closely track the changing carrier. Typical receiver
AGC circuits are reported to be in the range of 20 to 60 ms.

With DAM the fast attack time is important so that the carrier may be recovered
as quickly as the increase in audio levels so as to prevent pinch off distortion.

With AMC the action of the receiver AGC is to increase the audio gain when
the carrier is reduced so that no net change is observed. However noise and
interference is also boosted by the same amount. This increase in noise floor
should be masked by the increased loudness during these stages.
DAM Waveforms

                            AM Signal

       2

       1

       0
            0   0.2    0.4        0.6         0.8   1
                      Compressor Level
      1.5
       1
      0.5
       0
            0   0.2   0.4        0.6          0.8   1
                        DAM Output

       2                                                Average
                                                        Power
       1
                                                        Reduction
       0                                                40%
            0   0.2   0.4               0.6   0.8   1
                            Time (s)
AMC (3 dB) Waveforms

                                  AM Signal

       2

       1

       0
            0   0.2          0.4         0.6          0.8   1
                   Peak Detector and Compressor Levels
      1.5
       1
      0.5
       0
            0   0.2         0.4        0.6            0.8   1
                            AMC 3dB Output
       2
                                                                Average
       1                                                        Power
                                                                Reduction
       0                                                        23%
            0   0.2         0.4               0.6     0.8   1
                                  Time (s)
DAM Block Diagram



                                                                                                        Add Carrier
   Digital
   Audio
   Input
                                Matched
                                 Delay                                                                                                AMC
                                                                                                                                       Output


               Peak
              Detect




              Decay        Attack Filter                                  Gain Look
              = 200ms   1 ms Rise Time                                   Up Table
                                                                                                    Voltage
                                                                                   DAM
                                                                                                    Power

                                                               100                                         100
                                                                90                                         90
                                           Carrier % Voltage

                                                                80                                         80




                                                                                                                 Carrier % Power
                                                                70                                         70
                                                                60                                         60
                                                                50                                         50
                                                                40                                         40
                                                                30                                         30
                                                                20                                         20
                                                                10                                         10
                                                                 0                                         0
                                                                     0   25   50         75   100       125
                                                                              Modulation %
AMC Block Diagram


                                                                        1

                                                                                                                Compress
   Digital
   Audio
   Input
                         Matched
                          Delay                                                                                                              AMC
                                                                                                                                              Output

                                           Add Carrier
               Peak
              Detect




              Decay        Attack Filter                                     Gain Look
              = 200ms   1 ms Rise Time                                      Up Table
                                                                                                             Voltage
                                                                                 AMC (3 dB)
                                                                                                             Power

                                                                  100                                              100
                                                                   90                                              90
                                           Carrier & Sideband %




                                                                                                                         Carrier & Sideband
                                                                   80                                              80
                                                                   70                                              70
                                                                   60                                              60




                                                                                                                                Power
                                                   Voltage




                                                                   50                                              50
                                                                   40                                              40
                                                                   30                                              30
                                                                   20                                              20
                                                                   10                                              10
                                                                    0                                              0
                                                                        0   25   50       75     100   125      150
                                                                                      Modulation %
2008 Implementation

These algorithms are
quickly and efficiently
implemented on a modern
transmitter. The code is
written in C and in this
case compiled for the
Analog Devices fixed point
DSP.
With modern tools and
equipment engineering
time is only a few days.
Energy Savings
•   This technology is most relevant to high power stations ie. 50kW
•   State of the art AM transmitters are 90% efficient
•   Average power consumption is perhaps 73 kW. (70% average modulation)
•   8760 hours per year (24 hr station)
•   640,000 kWhr per year
•   Electrical rates range from 5 cents to 20 cents depending on the region

Assuming a 30% power reduction:
     – Savings are $19,200 per year at a 10 cent/kWhr rate
     – Savings are $28,800 per year at a 15 cent/kWhr rate


If converting an older 70% efficient transmitter:
     – Savings are $37,600 per year at a 10 cent/kWhr rate
     – Savings are $56,400 per year at a 15 cent/kWhr rate
AMC Perceived Quality

Reference 4: Implementation of Amplitude Modulation Companding in the
BBC MF National Networks, C.P. Bell and W.F. Williams, IEEE Transactions on
Broadcasting, Vol. 35, No. 2, June 1989

Key Notes:
     – Laboratory tests on subjective listening quality suggested the
        degradation was not significant.
     – Subjective testing with interference (co-channel) did not indicate a
        significant change in quality.
     – Comparisons with simple power reductions were done. A 1dB power
        decrease was imperceptible (with noise or interference) to 90% of
        listeners. This corresponded to 3 dB AMC with noise tests and 7dB
        AMC with interference.
     – In a field trial from Brookman’s Park, a 100kW site near London,
        Engineers made assessments at 29 locations in the daytime and a
        further ten locations at night. No impairments were observed.
Other Considerations

Processing: These systems do not perform as well with heavily compressed
audio levels. This is because high level peaks occur very regularly and quickly
so that the detected peak level always stays at nearly 100%. Voice program
tends to work well due to the pauses between words.

AM IBOC:
No work has been done to consider if the AM IBOC system could be modified
to operate with carrier control algorithms. It is not known if this system would
interfere with the normal operation of the IBOC receiver, although it seems
unlikely. Implementation with IBOC would also be more challenging.

Legal Status:
Due to restrictions in the FCC rules this system may not be legal. However due
to the extensive body of knowledge and the experience internationally,
operation with a Special Temporary Authority or a change in the rules could be
possible.
Final Summary

Carrier control algorithms were developed and tested in the 1980s and
continue to be deployed in high power AM transmitters around the world.
These systems are a specified requirement in most high power AM bids.

Research conducted in the 1980s revealed that significant reductions in
transmitted and consumed electrical power are possible with very minor
impairments in subjective quality measurements on the received signal.

With modern transmitters, the cost of implementation is very small or no cost in
the case of new systems. However the electrical energy savings are very
significant especially on high power transmitters. These energy savings are
complementary to the savings from modern high efficiency transmitters.
Thank You.

Questions?

				
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