USBM Contract No. H0346045 TECHNICAL SERVICES FOR MINE COMMlTNI CATIONS RESEARCH APPLICABILITY OF STATE-OF-THE-ARli7 VOICE BANDWIDTH COMPRESSION TECHNIQUES FOR WIRELESS MINE COMMUNICATION Richard He Spencer -- Task Leader Warren G. Bender ARTHUR D. LITTLE, INC. Cambridge, Massachusetts 02140 C-77229 The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies or recommendations of the Interior ~epartment's Bureau of Mines or of the U.S. Government. USBM CONTRACT REPORT -- TASK A (Contract No. H0346045) TASK ORDER NO. 1 July 1975 UNITED STATES DEPARTMliSNT OF THE INTERIOR BUREAU OF MINES Arthur D Little, fnc. This report was prepared by Arthur D. Little, Inc., Cambridge, Massa- chusetts under USBM Contract No. H0346045. The contract was initiated under the Coal Mine Health and Safety Program, It was administered under the technical direction of the Pittsburgh Mining and Safety Research Center with Mr. Howard E. Parkinson acting as the technical project officer. Mr. Michael W. College was the contract administrator for the Bureau of Mines. This report is a summary of the work recently completed as part of this contract during the period May 1974 to July 1975. This report was sub- mitted by the authors in July 1975. No inventions or patents were developed and no applications for inven- tions or patents are pending. Arthur D Little, lnc TABLE OF CONTENTS Page LIST OF TABLES vi LIST OF FIGUWS vii I. EXECUTIVE SUMMARY A. INTRODUCTION B. FINDINGS 1. Real-Time Techniques 2. Non-Real-Time Techniques C. CONCLUSIONS D. RECOMMENDATIONS 11. THE NATURE OF VOICE SIGNALS 111. METHODS OF VOICE BANDWIDTH COMPRESSION A. MODIFICATION OF SPEECH SIGNALS 1. Band Limiting 2. Time Assignment Speech Interpolation 3. Temporally Interrupted Speech 4. Clipping 5. Frequency Division (Stretched-out Voice) B. ANALYSIS-SNYTHESIS TECHNIQUES - VOCODERS 1. Spectrum Channel Vocoder 2. Voice-Excited Vocoders 3. Cepstrum Vocoder 4. Harmonic Compression 5. Autocorrelation Vocoder 6. Phoneme Vocoder 7. Adaptive predictive Coding 8. Pattern Matching Vocoder 9. Formant Vocoder iii Arthur D Little, Inc TABLE OF CONTENTS (Continued) Page IV. FUNDAMENTAL LIMITS TO BANDWIDTH COMPRESSION V. DISCUSSION OF BANDWIDTH COMPRESSION TECHNIQUES A, ADAPTIVE PREDICTIVE: CODING B0 SPECTRUM CHANNEL VOCODER C, FORMANT VOCODERS D, BASEBAND VOICE E.,. BAND LIMITING Fo TWORALLY INTERRUPTED SPEECH Go FREQUENCY DIVISION (STRETCHED-OUT VOICE) Ho HARMONIC COMPRESSION I. CLIPPING J. PHONW CODER K. PATTERN CODING Lo COMPANDING VI. COMPARISON OF SYSTEMS A. METHODS OF COMPARISON B. DISCUSSION OF COMPARISON PLOT VII. TIME-DELAYED SPEECH VIII. REFERENCES APPENDIX I NARROW BAND VOICE USING FREQUENCY SLICING APPENDIX 11 ALTERNATIVES TO VOICE BANDWIDTH COMPRESSION SYSTEMS A. LIMITED SYMBOL NARROW-BAND CODE SYSTEM B. MORSE CODE-TYPE SYSTEMS iv Arthur D Little, lnc. LIST OF TABLES Table No. Page 1 Comparison Matrix 30 1-1 Twenty Frequency Bands of Equal Contribution to 52 Speech Intelligibility Arthur D Little, lnc TABLE OF CONTENTS (Continued) C. AN ALPWIBET SYSTEM WITH KEYBOARD D. FLAG SIGNAL SYSTEMS E. PANTOGRAPH SYSTEM F. THE ELECTRONIC PAD APPENDIX 111 WHISPERED SPEECH APPENDIX IV THE NATURE OF TRANSMISSION CKANMELS 63 A. TRANSMISSION OF ANALOG SIGNALS ON DIGITAL, 63 CHANNELS - PCM Bo TRANSMISSION OF DIGITAL SIGNALS ON ANALOG 67 C-LS - MODEMS C. BPS PER HERTZ IN DIGITAL TRANSNISSION ON 68 AN ANALOG C D. HERTZ PER BPS IN ANALOG TRANSMISSION ON 69 DIGITAL CHANNELS Eo CHANNEL CAPACITY RELATIONSHIPS 73 APPENDIX V LINCOMPEX 77 BIBLIOGRAPHY 81 Arthur D Little. Inc LIST OF FIGURES (Continued) Figure No. Page IV-6 channel' Capacity As Function of Bandwidth for 76 Fixed Signal Power in Presence of Flat Noise V-1 Simplified Block Diagram of Lincompex System 78 Arthur D Little, Inc LIST OF FIGURES Figure No. Page Energy Density Spectrum for Male Speech 8 Energy and Articulation for Speech Signals with 10 Low-Pass Filter Energy and Articulation for Speech Signals with 11 High-Pass Filter Band Limiting Speech Signal 14 Spectrum Channel Vocoder Voice-Excited Vocoder Harmonic Compressor 21 Autocorrelation Vocoder Formant Vocoder Articulation Index as a Function of Signal to 39 Noise Ratio Comparison of Compression Techniques 41 Formats for Reconstruction of Stretched-Out Messages at 6 to 1 Stretch-Out Ratio Relation Between A1 and Various Measures of 53 Speech Intelligibility Articulation Index for Various Frequency Sliced 55 Systems as Function of Signal Power Handheld Keyboard and Display 58 PCM Quantizing Noise 66 ~its/~econd/HertzVs Signall~oise 70 Differential Quantizing at 56kbps. l6kbps DCDM Performance Relative Power Required as Function of Bandwidth 75 for Fixed Channel Capacity in Presence of Flat Noise vii Arthur D Little, Inc I, EXECUTIVE STJMMARY A. INTRODUCTION Since the passage of the Coal Mine Health and Safety Act of 1969, the Bureau of Mines has made many advances in mine communications. These efforts have resulted in the following developments and demonstrations: Surface-to-surface electromagnetic transmission of baseband voice signals through overburdens as deep as 600 feet. This transmission was achieved with equipment from the Interim Mine Rescue and Survival System Program, and demonstrated that baseband voice communications could be obtained through these overburdens in the absence of mine-generated noise; that is, when the mine power system was shut off, However, during mine operational conditions, the mine-generated noise proved to be so large that effective baseband voice communi- cations to the desired depths could not be effected. r The development of beacon transmitters for locating trapped miners. This work was again the outgrowth of the Interim Mine Rescue and Survival System. Extensions of this work have led to reliable detection and location of trapped miners to depths of 1000 feet. The system, which makes use of a loop antenna deployed where the trapped miner is lo- cated, uses low-duty-cycle, tone-burst transmission. Por- table tuned receivers and loop antennas are used on the sur- face to determine the location of the trapped miner, An extension of .this work has led to a call-alert paging . system concept, wherein a transmitter, similar to that used in the trapped miner system, is used to audibly and/or visually alert key mine personnel carrying pocket receivers that they are wanted at the nearest mine telephone. In addition, a roof-bolt paging system has been developed, wherein commercial paging and mine carrier phone equipment are used in conjunction with a long-wire "roof bolt" antenna to page individuals and relay to them short voice messages. 1 Arthur D Little, Inc Despite these advances in communication capabilities within mines, some major unmet needs which require further research and development still remain. One of these msjor needs is wtreless electromagnetic through-the-earth voice communication channels capable of being operated in the presence of both surface and subsurface noise environments. Cal- culations have been made by both Arthur Do Little, Inc. and Collins Radio C~mpany'~)relating to the power requirements for such a voice communication system, In both instances the power requirements under operational conditions for depths of 600 to 1000 feet have been quite substantial, in fact, prohibitive, for uplinks and downlinks between the surface and the subsurface. The power requirements range from a few kilowatts up to the 100-kilowatt range. Similar levels were also pre- dicted for side-link communication paths within the mine. These paths may run from a central fixed transmitter to roving miners carrying por- table units, or between two portable units. Here the problem is not so much reaching the roving miners from the fixed transmitter location, but the reverse talk-back problem of the roving miner to the central loca- tion, or from roving miner to roving miner where transmitter power is se- verely limited. The Collins Radio Company's calculations .for power re- quirements for the roving miner to roving miner indicate that portable battery-operated equipment appears barely adequate to meet a 600-foot range requirement imposed on such side-link communication systems.* * Recent in-mine EIM propagation measurements made by Collins do show an unexpected propagation enhancement for frequencies near 500 kilohertz. It is not known how consistent this enhancement will be over a range of coal mine environments, but should it be found to be consistent, then the problem of achieving adequate side-link communication range will be considerably alleviated. However, it appears that the mech- anism that provides this enhancement of side-link propagation will serve to make vertical propagation more difficult and thus the prob- lem of subsurface to surface communication may become worse under such conditions. Arthur D Little, lnc One of the means proposed for aiding in overcoming these severe power and range limitations has been the application of voice bandwidth compression techniques. We anticipated that such techniques would per- mit greater range with the same transmitter power or the same range with less transmitter power. The purpose of this study was to determine the applicability of state-of-the-art voice bandwidth compression techniques for mine wire- less voice communication systems as one means of reducing the power requirements of portable units. One reason why voice bandwidth compres- sion appeared attractive is that at VLF to MF frequencies the noise received by a portable receiver in a mine is largely dominated by exter- nal mine-generated noise and not by receiver-generated noise. Therefore, if the received noise power could be lessened by reducing the bandwidth required for the voice signal, without paying a corresponding penalty in processing power and/or required signal-to-noise ratio, the desired range could be either extended with the same transmitter power or obtained with less power. Hence, a 10:l reduction in receiver-bandwidth could be expected to yield a 10:l reduction in received noise and hence a 10:l improvement in signal-to-noise ratio for the same amount of transmitter power.