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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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