MULTICHANNEL VOICE RECORDER

Abstract: A data compression system for combining a plurality of audio channels whilst retaining the intelligibility of the respective channels. Said system enabling the recordal of speech from one or more individuals within a predetermined area, such as the bridge of a ship, utilising a plurality of well-spaced microphones. Each of the respective audio outputs from each of the respective microphones is weighted in accordance with a predetermined weighting factor which is derived from the short term and long term amplitude characteristics of the audio signal. The weighting function modifies the response of individual audio signals such that the recovered digitally encoded signal has an optimised compression ratio and signal to noise ratio. Furthermore, parameters obtained from the weighting process retain spatial information which would otherwise be lost by simple mixing.

Description: MULTICHANNEL VOICE RECORDER This invention relates to a data compression system for combining a plurality of audio channels whilst retaining the intelligibility of the respective channels. Said system enabling the recordal of speech from one or more individuals within a predetermined area utilising a plurality of spaced-apart microphones. Said system efficiently captures audio signals, as part of, for example, a broadcast system or data-recorder. Data-recorders have found application in a number of fields, for example, flight data-recorders fitted to aircraft, which are popularly referred to as black box recorders. Data-recorders fall broadly into two categories: those which are used to record flight parameters including the aircraft instrument readings, surface position and system status, termed flight data recorders, and cockpit voice recorders. Such data-recorders have to be constructed as to allow the stored data to be retrieved easily in the event of a crash. Because of their construction and the need for qualification approval in terms of shock damage, fire resistance and water immersion, these recorders are expensive. In an aircraft, such recorders continually write data over previously recorded data, so that only generally the last 30 minutes of information is retained. In the event of a crash, the recording is halted, thereby storing a record of data indicative to the conditions immediately prior to the crash. In recent years, it has also become compulsory for maritime vessels to carry a similar data-recorder. Earlier generations of recorders utilised magnetic tape as the storage medium, although recently these have been superseded by recorders utilising solid state memory, such as typically EEPROM, flash EEPROM, static or dynamic RAM. However, for maritime applications, where the timescale of a crash or other event is somewhat longer, typically a 12 hour time window will be recorded. As with a flight recorder, parameters such as the surface position, and other instrument readings are recorded, including the voices of the crew stationed on the bridge of the ship. However, this is achieved using just a single data-recorder. Clearly, as the bridge of a large ferry or tanker can be in excess of 30 metres long by 5 metres wide, a single microphone would be insufficient. Indeed, the International Maritime Organisation requires conversations to be recorded within 2 metres of at least seven well-spaced workstations, each containing a microphone. It is desirable that 16 bit (CD) dynamic range recording is required to maintain intelligibility, since the output of a microphone is clearly dependent upon the amplitude of the speech, the distance of the talker from the microphone and the magnitude of the background noise. Unfortunately, this creates problems in terms of the storage requirements for solid state memory. To illustrate, the International Maritime Organisation specifies that the recordal of speech requires a bandwidth of 150 to 6000 Hz, which implies a sampling frequency of 14 to 16 KHz. If no data compression is used, the seven encoded channels from each microphone would require 7 x 16000 x 16 bits per second, which is approximately 9.7 gigabytes for the required 12 hours of recording. Since, the present cost of flash memory is about £2000 per gigabyte this would represent a sum of around £20,000 to store the audio signals alone. Furthermore, it has been found experimentally that significant improvements in recording clarity can be achieved if at least eight microphones are utilised which, detrimentally, increase the storage requirements to over 11.1 gigabytes for the required 12 hours. It has been proposed that the outputs of, say, four microphones could be simply summed, which would reduce the storage requirements by a factor of four. However, this creates four problems: firstly, information is lost about the location of the talker. Secondly, for most of the time there will be only one microphone that is close enough to the talker to provide a clear speech derived output, whilst the other three will pick up noise. Thirdly, the intensity of speech in air follows an inverse square law with distance; however this only generally applies for the first few metres. After that, multi-path effects reduce the law so that the microphones that are distant from the talker receive some speech-like sounds from attenuated replicas transmitted and reflected around the acoustically noisy bridge that are themselves summations of many audio signals having measurably different path lengths. Finally, distant microphones will receive speech that is delayed relative to the microphone that is the closest. Unfortunately, it is the last two of these problems that create the greatest difficulty. As the speed of sound in air is only around 300 metres per second, path differences of a few metres are significant when compared to the overall length of, for example, a syllable. Adding clear speech to delayed unclear speech, will of course, be detrimental to the overall intelligibility of the stored signal. It is the object of the present invention to provide a data compression system for combining a plurality of audio channels whilst retaining the intelligibility of the respective channels. Said system enabling the recordal of speech from one or more individuals within a predetermined area utilising a plurality of well-spaced microphones. The corresponding data compression ratio is around 16:1 and the requirements for storage are reduced to 0.7 gigabytes for the required 12 hours of storage, a significant improvement. According to the present invention there is provided a data compression system for combining a plurality of audio channels whilst retaining the intelligibility of the respective channels relative to one another and minimising the relative storage requirements, comprising: input means for receiving a plurality of audio signals; weighting means for weighting each of said audio signals in accordance with a predetermined weighting factor; summing means for receiving and combining said weighted audio signals; encoding means for digitally encoding the output of said summing means; and storage means for storing said digitally encoded summed signal. Preferably, said weighting means utilises amplitude weighting, the weighting function being derived as a function of the short term and long term amplitude of the respective input signals, the short term amplitude factor being determined primarily from the intelligible portion of the audio signal, whilst the long term amplitude factor is primarily derived from background noise levels, the ratio of the short term to long term amplitude providing the required weighting factor for the respective signal. The weighting functions of each respective signal being a function of the short and long term amplitude of all input signals. Further preferably, the short term and long term amplitude information is digitally encoded and stored with the weighted combined signal for use during playback. In terms of storage requirements, the weighting functions take up only minimal memory space. Upon playback of this multiplexed signal, the information they contain may be used to provide some of the spatial information which would be lost by simple mixing. The audio signals are preferably supplied by the outputs from a plurality of microphones. Also according to the present invention there is provided a data compression system for enabling the recordal of speech from one or more individuals within a predetermined area utilising a plurality of spaced-apart microphones, comprising: means for receiving respective audio outputs from each of said microphones and weighting each of said respective signals in accordance with a predetermined weighting factor; summation means for summing said weighted audio signals; encoding means for digitally encoding the summed weighted signals; multiplexer means for combining said digitally encoded signal with amplitude data received from each of said weighting means; and storage means for storing the output of said multiplexer. The storage means preferably comprises a solid state flash memory. Further preferably, means for generating the weighting function may be based upon a digital signal processing (DSP) approach. Preferably, said data-compression system enables the recordal of speech on the bridge of a ship as part of a maritime data-recorder, said recorder also storing radar information and other instrument and system readings, including, for example, position, speed and heading. The advantages of the present invention are that the said weighting function modifies the response of individual audio signals such that said recovered digitally encoded signal has an optimised compression ratio and signal to noise ratio. Furthermore, parameters obtained from the weighting process retain spatial information which would otherwise be lost by simple mixing. \t will be obvious to those skilled in the art that variations of the present invention are possible, and it is intended that the present invention may be used other than specifically described herein. Specifically, the present invention may also be utilised for the recordal of voices of personnel operating in a large predetermined space with more than one microphone, for example, the control room of a power station or other safety critical establishment. A specific non-limiting embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the present system. Figure 2 illustrates the weighting process for a single audio signal. Referring now to the drawings, the proposed scheme is illustrated in Figure 1. For illustration purposes, four audio inputs , β, χ and δ are taken from four microphones which are situated, usually, in the bridge of a ship. The four audio inputs α, β, χ and δ being weighted 10, 12, 14 and 16 and then summed 18 before being encoded digitally 20 and stored. For each audio input from the microphone, two corresponding weighting factors are generated, the first follows short term changes in amplitude that may result from speech, whilst the second is largely determined by the background noise level experienced by the microphone. The weighting being a function of the short and long term amplitudes of all four inputs. Figure 2 illustrates the weighting process for one of the four audio inputs, termed α. The other three signals are processed in the same manner. The analogue signal 26 is split into two paths, in one its RMS value is determined 28 with is then passed through a low pass filter 30 having a -3dB frequency of around 10 Hz. The output of this low pass filter a is one of the signals used to determine the weighting function 42, together with the corresponding ones for the other three signals b, c and d. The other 44 being non-linear functions 34 of the output of a second low pass filter 32, which has a -3dB cut-off of around 0.1 Hz, and correspondingly designated A, B, C and D. The non-linear signals a, b, c and d follow the short term changes in amplitude that might have resulted from speech whilst A, B, C and D are largely determined by the background noise levels experienced by each of the microphones. The weighting function for that channel is now determined as ratio of the short term amplitude to long term amplitude of all channels, and implemented using a digital signal processor 36. The weighting factor determined for the respective signal is now applied 40 to the delayed audio input 38 before the signal is summed with the remaining three other signals. The eight weighting function matrices generated from each of the four signals may also be multiplexed with the encoded weighted audio signal, so that information may be obtained as to the relative position of the talking individual. In order to avoid the problem in which the weighting functions A, B, C and D approach closely to zero and hence the weighting terms become excessively large, a lower bound is set to the value that the A, B, C and D terms can have, by virtue of the non-linear function applied to the second low pass filter. The delay 38 in Figure 2, denoted by a time τ, is substantially equal to that of the RMS generator 28 and the 10 Hz low pass filter 30. In order to comply with International Maritime Organisation provisions, the storage requirements in terms of the weighted summation of, for example, two sets of four audio channels is less than that required through simple mixing of, say, seven microphones in three channels. Of course, the recovered audio signal will also benefit from an enhanced clarity and an optimised signal to noise.

Claims: CLAIMS 1. A data compression system for combining a plurality of audio channels whilst retaining the intelligibility of the respective channels relative to one another and minimising the relative storage requirements, comprising: input means for receiving a plurality of audio signals; weighting means (10, 12, 14, 16, 36) for weighting each of said audio signals (26) in accordance with a predetermined weighting factor; summing means (18) for receiving and combining said weighted audio signals; encoding means (20) for digitally encoding the output of said summing means; and storage means for storing said digitally encoded summed signal. 2. A data compression system as claimed in Claim 1 , wherein each of said audio signals (26) is provided by the output of each respective microphone. 3. A data compression system as claimed in Claims 1 or 2, wherein said weighting means (10, 12, 14, 16, 36) is implemented using a digital signal processor. 4. A data compression system for enabling the recordal of speech from one or more individuals within a predetermined area utilising a plurality of spaced-apart microphones, comprising: means for receiving respective audio outputs (26) from each of said microphones and weighting (10, 12, 14, 16, 36) each of said respective signals in accordance with a predetermined weighting factor; summation means (18) for summing said weighted audio signals; encoding means (20) for digitally encoding the summed weighted signals; multiplexer means (22) for combining said digitally encoded signal with amplitude data received from each of said weighting means; and storage means for storing the output of said multiplexer. 5. A data compression system as claimed in any preceding claim, wherein said predetermined weighting factor is derived as a function of short term amplitude characteristics (42) and long term amplitude characteristics (44) of each of said audio signals (26). 6. A data compression system as claimed in any preceding claim, wherein said predetermined weighting factor is provided by the ratio of short term amplitude characteristics (42) to long term amplitude characteristics (44) of each of said audio signals (26). 7. A data compression system as claimed in Claims 5 or 6, wherein said short term amplitude characteristics (42) of each of said audio signals (26) is determined primarily from the intelligible speech portion of said audio input signal (26). 8. A data compression system as claimed in Claims 5 or 6, wherein said long term amplitude characteristics (44) of each of said audio signals (26) is determined primarily from the background noise level of said audio input signal (26). 9. A data compression system as claimed in Claims 5 or 6, wherein said short term amplitude characteristics (42) and long term amplitude characteristics (44) of each of said audio signals (26) is digitally encoded (20) and stored with the weighted signal to provide spatial information. 10. A data compression system as claimed in Claims 5 or 6, wherein said short term amplitude characteristics (42) of each of said audio signals (26) is determined by passing each of said audio signals (26) through a RMS generator (28) and a first filter means (30). 11. A data compression system as claimed in Claims 5 or 6, wherein said long term amplitude characteristics (44) of each of said audio signals (26) is determined by passing each of said audio signals (26) through a RMS generator (28), a first filter means (30), a second filter means (32) and a non-linear function generator (34). 12. A data compression system as claimed in Claims 10 or 11 , wherein said first filter means (30) further comprises a low pass filter arrangement having an upper pass band of around 10Hz. 13. A data compression system as claimed in Claim 11 , wherein said second filter means (32) further comprises a low pass filter arrangement having an upper pass band of around 0.1 Hz. 14. A data compression system as claimed in any preceding claim, wherein said encoding means (20) is provided by adaptive digital pulse code modulation. 15. A data compression system claimed in any preceding claim, wherein said storage means further comprises a solid state flash memory. 16. A data compression system as claimed in Claim 4, wherein means for receiving respective audio outputs (26) from each of said microphones and weighting (10, 12, 14, 16, 36) each of said respective signals is implemented using a digital signal processor. 17. A data compression system as claimed in any preceding claim, wherein said data compression system is part of maritime data-recorder or other broadcast system.

Inventors: MULLARKEY WILLIAM JOSEPH, MULLARKEY, WILLIAM, JOSEPH
Applicants: DB RES LTD, DB RESEARCH LIMITED
Application year: 2001
Main classification: H04R3
IPC codes: B63B49, G10L19, G10L21, G11B20