e2e 3.0 energy efficient inter-aural audio transmission

Transcription

1 e2e 3.0 energy efficient inter-aural audio transmission Thomas Lotter, Ph.D., Ulrich Schätzle, Dipl.-Ing., Thomas Fischer, Ph.D. Siemens Audiology, 2014.

2 Abstract The Siemens e2e 3.0 enables a series of new true binaural signal processing algorithms by providing a bidirectional digital inter-aural audio link. Data transmission rate of the new e2e wireless 3.0 technology is times higher than its predecessor system, while current consumption stays almost the same. In this paper, design aspects of the e2e 3.0 system are explained which contribute to the functionality upgrade, while keeping power consumption and size of the hearing instrument virtually identical. History of e2e In 2004, Siemens e2e wireless was the first system, which allowed two hearing instruments to exchange digital information and function harmoniously. This enabled synchronization of digital signal processing features, such as directional microphone technology and digital noise reduction algorithms as well as the synchronization of hearing instrument onboard controls. In 2008, e2e wireless 2.0 introduced audio streaming from accessories to hearing instruments, which was used to improve the signal to noise ratio (SNR) for TV applications and to provide better audibility when using the phone by providing a binaural signal. The philosophy of e2e wireless technologies centers on designing the communication system dedicated to the essential requirements of hearing aids ultra low power and very small size. By doing so, relevant user benefits can be provided without having to compromise on the size of the product, or battery life, which would create acceptance or usability problems. This design approach also then provides the e2e benefits to be available throughout the complete hearing instrument portfolio, from a Super-Power instrument down to a very small completely-in-the-canal (CIC). Introducing e2e wireless 3.0 In 2014, e2e wireless 3.0 enables a series of new signal processing algorithms, which provide signal processing that allow for hearing impaired people to have better speech recognition in demanding listening environments than people with normal hearing [1]. The e2e 3.0 system is capable of transmitting dual-microphone bidirectional audio data from ear to ear, creating a virtual 8 microphone network. To achieve this sophisticated communication, the effective inter-aural data rate of e2e 3.0 is raised by a factor of compared to e2e 2.0. The e2e 3.0 achieves this without size or battery drain drawbacks due to the use of a dedicated design for hearing instruments. This includes the choice of the frequency band, the design of the analog and digital transmission system as well as the system integration into the hearing instrument Figure 1: e2e wireless 3.0 creates a virtual 8 microphone network (2 physical and 2 virtual microphones on each side).

3 The choice of the e2e 3.0 transmission frequency The benefits of Near Field Magnetic Induction (NFMI) include a frequency band from 3 15 MHz, which easily propagates through and around the human head and body. Ease of propagation makes it suitable for ear-to-ear communication between two hearing aids. Radio frequency bands (RF) such as 900 MHz and 2.4 GHz are well suited for long range transmission (the wireless signal propagates proportional to the inverse of the distance for RF while for NFMI it propagates proportional to the transmission distance cubed). However for inter-aural true audio exchange, the transmission over the RF channel has to be considered inefficient [2] since the human body consists out of around 60% water, and water absorbs effectively the RF energy, especially in the 2.4 GHz frequency region in a process called dielectric heating. (e.g. microwaves operate at a frequency of 2.45 GHz). Figure 2: NFMI vs. RF for the use of e2e high data rate transmission. Within the NFMI frequency range, the band between and 3.4 MHz is chosen for e2e 3.0 since this is recommended by the International Telecommunication Union (ITU) for world-wide use in hearing aids. Consequently, one can expect less magnetic noise by other transmission systems in that band. The e2e 3.0 transmission system The e2e 3.0 inter-aural transmission system targets to create virtual microphones, i.e. each local ear side is provided with full audio information about the other remote ear side. Essentially, this requires a high-quality and high-rate bi-directional data transmission, since the quality of the transmitted audio signal must be comparable to when picking up sound with a physical, real microphone. In each of the two hearing instruments, the system as shown in Figure 3 is realizing the creation of virtual microphone signals and the transmission of the local microphone signals to the remote side.

4 Figure 3: Block diagram of the e2e 3.0 transmission system, The virtual microphone concept demands a high quality / low latency audio encoder and decoder on the hearing aid since physical microphones and virtual microphones are processed together equally. The e2e 3.0 digital system controls the impact of environmental magnetic noise by utilizing a forward error correction technique, which adds redundancy on the transmission side allowing to correct errors on the receiving side. Digital modulation of the raw data to the carrier frequency is realized in a bandwidth efficient way via transmitting multiple bits within one transmission period. The low current consumption requirement demands relatively low transmit power, however a high receive sensitivity is necessary. Noise introduced within the hearing instrument itself by the digital electronic circuit can become a significant factor and must be limited in the internal design of the hearing instrument. This is accomplished by keeping magnetic noise sources like hearing instrument ICs relatively far away from the NFMI antenna like shown in Figure 4. Figure 4: Hearing instrument design minimizing internal noise to NFMI system. Power efficient system integration While the design of the analog and digital transmission system is essential to avoid increasing power consumption or size, the integration in the hearing aid system also plays a decisive role. At minimum, a hearing instrument chip system consists of an analog integrated circuit (IC) for input processing of the microphone or other input signals and a digital IC for audio signal processing. A separate radio communication IC can be interfaced

5 to the hearing aid system to outsource the task of wireless communication. However, this comes with the drawback of adding a base load current to the system in order to operate the extra IC and to administer the interfaces between the chips. A more efficient approach, therefore, is to integrate all the e2e functions into the existing hearing aid ICs. e2e 3.0 integrates the analog and digital part of the transmission system directly to the existing hearing aid ICs to avoid this drawback. Figure 5: Integration of the analog portion of e2e 3.0 (colored region) in comparison to the total analog hearing aid IC area. The recent advances in chip technology enable hearing aid signal processing in as many as 48 channel bands, and expand the capabilities of the ICs to fully add the wireless communication into the existing ICs. For example, the e2e 3.0 only occupies 7% of the complete IC area. Power consumption results It is well known that power consumption of hearing instruments varies in every-day life depending on the manufacturer s design, and also on what wireless functionalities are activated [3]. A further increase in power consumption can be expected when wireless streaming or inter-aural audio transmission are activated. Figure 6 compares the power consumption of seven wireless high-rate communication systems in hearing aids from six manufacturers. While all hearing instruments offer the ability to stream audio to the instruments, an inter-aural audio exchange is only offered from three manufacturers. One of the three manufacuturers provides unidirectional audio exchange only, while the other two including Siemens provide bidirectional audio exchange. We enabled inter-aural audio data transfer for those three, while the other four instruments did not have this feature and therefore audio streaming from a respective accessory was used for the measurement. Shown on the left, in the blue bars correspond to the instrument, which used NFMI, at different frequency bands between 3 MHz and 12 MHz, while the orange bars represent the instruments, which use RF technologies at 900 MHz or 2.4 GHz.

6 Figure 6: Additional current consumption auf audio transmission available on the market in August When the product offers audio transmission between the ears, the respective feature was activated. When e2e audio was not available streaming from an accessory of phone to the hearing instrument was activated. Observe that, with the exception of system 1, all NFMI systems consume only around ma. The high power consumption of NFMI system 1 is due to an added base load current of an additional IC for realizing the NFMI connectivity. The RF communication systems consume around 10 times more current that the NFMI systems. When activating RF, the current consumption of the hearing instrument (which is typically around 1 ma with connectivity deactivated) increases by a factor between 3 and 5. In summary, the e2e 3.0 functionality only marginally adds to the power consumption of the hearing aid itself. Therefore, the inter-aural audio exchange can be activated for full-time use to provide the respective user benefits without the need to exchange the battery more frequently. As mentioned, the inter-aural audio exchange facilitates binaural signal processing so that hearing impaired people can achieve even better speech recognition in demanding listening environments than people with normal hearing [1]. No RF system to date enables an inter-aural audio exchange. Also a more frequent battery exchange is needed when the RF wireless streaming functionality is activated frequently throughout the day. References: [1] Powers, T.; Froehlich, M.; Impact of binax directional technology on speech recognition, [2] Katayama, N.; Takizawa, K.; Aoyagi, T.; Takada, J.-I.; Huan-Bang Li; Kohno, R.;, "Channel model on various frequency bands for wearable Body Area Network," IEICE Trans. Commun. Vol. E92-B, No.2.,February 2009 [3] Strandbygaard Joergensen, H.; Baekgaard, L.; Bendtsen, B.; Battery Consumption in Wireless Hearing Aid Products Datasheet vs. Real-World Performance in June 2013