By Jennifer Groth, MA, and Torben Groth, BScEE
Open any kitchen utensil drawer and languishing in the back is likely a collection of wine aerators, garlic peelers, cherry pitters, and other gadgets that promised to make life easier for the home cook but turned out not to be all that useful. The hearing aid industry also has had its share of junk drawer items that did not prove their utility, and quietly disappeared from the market.
Utility as it applies to hearing instruments means that they are useful or helpful to the consumer. It has been established that the number of listening environments in which hearing instruments are found to be useful correlates with satisfaction. High levels of satisfaction with hearing instruments are achieved when they are useful in many different listening environments.1
If, as Thomas Edison reasoned, sale is proof of utility and success, one hearing instrument feature that seems to be of particular utility is digital wireless capability. According to Hearing Industry Association statistics from the first quarter of 2013, 70% of hearing aids sold in the United States have digital wireless capabilities (Figure 1).2
|Figure 1. Wireless versus non-wireless hearing aid market share divided into three product classes: traditional behind-the-ear (BTE Trad), in-the-ear (ITE), and receiver-in-the-canal or receiver-in-the-ear (RIC). Wireless hearing aids now constitute 70% of the total hearing instrument market, with wireless RIC/RITEs being the most-used device type.|
The most straightforward application of digital wireless technology in hearing instruments is connectivity to external devices and sound sources. Used in this way, digital wireless technology is a natural progression from well-established analog wireless functionality in hearing instruments. Telecoils and induction loop systems, and FM systems, are examples of how analog wireless capability made it practicable to selectively amplify desired sounds and connect users more directly to sound sources via their hearing instruments.
But just as digital hearing instruments opened new possibilities for sound processing, digital wireless technologies also offer new user benefits. While existing analog wireless technologies in hearing instruments can convey sound from a remote source to a hearing instrument, each has drawbacks that might be addressed with digital technology. Using a digital wireless transmission system can generally improve the signal-to-noise ratio (SNR) compared to analog and, depending on the particular technology, be less susceptible to interference. Digital wireless features in hearing instruments can thus extend already established benefits of analog wireless.
Digital wireless systems for hearing instruments can be based on different principles. Increasingly, differences among systems are described in terms of how many “middlemen” are required to connect the end user’s hearing instruments to a sound source. While this does capture a main advantage or disadvantage of the systems—namely, how many extra devices besides the hearing instruments the wearer will need to purchase, set up, and use—there are other dissimilarities that impact what they can do.
The ReSound digital wireless system is unique for hearing instruments in that it is based on radio frequency (RF) transmission in the unlicensed 2.4 GHz band. The choice to develop this technology represented a leapfrog over the state-of-the-art at the time, as other digital wireless systems were based on a combination of near-field magnetic induction (NFMI) and Bluetooth. NFMI uses induction coils in the hearing instruments to pick up signals emanated from a NFMI streaming device. The NFMI streaming device receives signals via Bluetooth from the sound source.
Wireless systems that combine NFMI and Bluetooth require the user to wear the NFMI streaming device to connect to sound sources, while the 2.4 GHz RF system does not. The 2.4 GHz RF system uses “far-field” transmission where an antenna generates and transmits a propagating electromagnetic wave. The power density of the RF signal attenuates slowly over distance, allowing a useful range for the ReSound 2.4 GHz system on the order of 23 to 30 feet (7 to 10 meters).
Apart from the long range of transmission that eliminates the need for a body-worn streaming device between the sound source and hearing aids, the choice of 2.4 GHz as the transmission frequency has other advantages. These include high data transmission capacity, ability to stream in stereo, small antenna requirements, and worldwide acceptance. For similar reasons, other consumer electronics with digital wireless functionality also operate in the 2.4 GHz band. Thus, this technology positions the hearing instrument for the next step in connectivity, which is direct compatibility with other digital devices such as smartphones, tablets, or digital audio players.
The Next Generation of Wireless Accessories
Consumer electronics often seem to be obsolete the moment you have purchased them, as new versions of products come out with astounding frequency. The lifecycle for hearing instruments and related products is much longer—typically several years. One advantage of this longer cycle is that more input from clinicians and users can be taken into account in renewing products.
This is particularly evident for the second generation (2G) versions of the 2.4 GHz wireless accessories. In addition to feedback and ideas from consumers, changes to these accessories also reflect new functionality in ReSound hearing instruments, as well as changes in the connections to modern TVs.
The full family of wireless accessories and how they relate to the hearing instruments and external devices is shown in Figure 2. The updated second generation accessories include the Unite Phone Clip+, the Unite TV Streamer 2, and the Unite Remote Control 2. Furthermore, Phone Clip+ users now have extended remote control functionality via a smartphone app.
|Figure 2. The hearing instruments are the center of the ReSound 2.4 GHz wireless system. Wireless accessories provide connectivity to external devices without the need for a body-worn streamer. (Click to enlarge.)|
Better Quality Cell Phone Conversations
Telephone use is a continuing challenge for hearing instrument wearers, particularly those with more severe losses. Wireless streaming of the phone signal to both ears has been shown to be the most advantageous way for these hearing instrument users to hear on the phone,3 and the ReSound Unite Phone Clip provided this functionality. But phone calls are two-way communication. It’s not much help if you can hear the person you are calling but they can’t hear you well. Bothersome noise on the far end of the call was an issue for the Phone Clip that is commonly known to many owners of cell phone headsets where ambient noise can easily be picked up and transmitted to the person on the other end of the call. The updated Phone Clip+ incorporates a specially developed noise reduction algorithm that provides an additional 3 dB of noise reduction to the outgoing signal to ensure the best possible quality for the far-end caller.
New to the Phone Clip+ is the use of two microphones. To fully utilize the directional benefits from the two microphones necessitates correct positioning of the Phone Clip+ when worn. To this end, a swivel clip system has been introduced, allowing greater flexibility in placement of the device. Correct positioning for acoustic purposes will not only optimize the 2-microphone performance, it will also optimize the antenna performance due to its orientation. This leads to an improved and stable RF connection between the Phone Clip+ and hearing instruments, as well as between the Phone Clip+ and cell phone.
The above-mentioned noise reduction method relies on two microphones, which the original Phone Clip did not have. Well-known adaptive filtering techniques are used to subtract the noise from the output of the front microphone.4 This noise reduction algorithm makes the assumption that the front microphone picks up both the signal of interest, which in this case is the voice of the hearing aid user, and the interfering background noise. A further assumption is that the rear microphone only picks up the interfering noise. Then, adaptive filtering is applied to make the noise from the rear microphone equivalent to the noise from the front microphone. This filtered noise is subtracted from the front microphone signal, leaving a clear signal of interest.
Obviously, the assumption that the rear microphone only picks up noise is not true. The way this issue is solved is by estimating direction of arrival of sounds, similar to what directional hearing aids are able to do. Using this information, the filter applied to the rear microphone noise is only updated in the frequency bands where sound is estimated to come from the rear. As a consequence, the filter can “tune in” to noise sources from the rear and remove them, but leave signal sources from the front intact.
To quantify the improvement resulting from the noise reduction, two benchmarking trials were carried out. In the first trial, normal-hearing test participants evaluated the quality of speech that was recorded via the Phone Clip versus the Phone Clip+. The device was hung by a lanyard on a Head and Torso Simulator (HATS), and speech was played through the mouth of the HATS. The voices of male and female speakers were used in quiet and in noise. Cocktail party noise was played from 3 ceiling-mounted speakers to simulate the noisy environment. The Phone Clip/Phone Clip+ devices were paired to a Bluetooth connector that fed the signal to a PC, where the sound picked up and transmitted by the Phone Clip/Phone Clip+ was recorded. Then normal-hearing listeners compared recordings made via the two devices, and were asked to judge which was better in terms of speech quality and listening effort for each sample.
The listeners showed a significant preference for recordings made via the Phone Clip+ in quiet. However, the results in the noisy condition were even more striking. As shown in Figure 3, recordings made with the Phone Clip+ were judged as better for speech quality 78% of the time, and better for ease of listening 99% of the time.
In the second benchmarking trial, hearing instrument users fit with ReSound Verso hearing instruments compared the Phone Clip and Phone Clip+ in daily use. Half of these 10 participants started with the Phone Clip and switched to the Phone Clip+, while the other half followed the opposite procedure. Subjective ratings of sound quality and speech intelligibility on the near end (what the hearing instrument user heard) did not differ significantly for the Phone Clip and Phone Clip+. And both were rated significantly better than using their cell phones by holding them up to their hearing aids with an acoustic phone program. These results support the benefit of the concept as well as the performance of both the Phone Clip and Phone Clip+ for the hearing instrument user.
|Figure 5. ReSound Unite Phone Clip+.|
To evaluate far-end call quality, participants also were asked to estimate the percentage of those who received calls from them and complained of noise or difficulty hearing/understanding. These results are presented in Figure 4. While the real-world results with the Phone Clip were positive, the results with the Phone Clip+ were exceedingly so.
Other enhancements that Phone Clip+ users will appreciate include remote control functionality to adjust volume and change programs on the hearing instruments, music streaming to the hearing instruments in stereo from Bluetooth devices, and various hardware and software modifications to improve ease of use.
Finally, communication via the Phone Clip+ allows wearers to use their iPhones and smartphones as a remote control for the hearing instruments. The ReSound Control app can be downloaded at no charge to devices with AndroidTM and iOS operating systems.
The ReSound Control app provides extended remote control functionality, relative to the Phone Clip+, with features such as individual left/right hearing instrument gain adjustment and individual muting of hearing instrument microphones. When in streaming mode—such as streaming the sound from a TV—the sound level balance between the streamed signal and the microphone signal can be adjusted continuously via the smartphone touch-sensitive display. Other features encompass a link status screen that keeps track of the wireless connection between devices and a program label editor for personal labeling of hearing instrument programs.
A Better Television Experience
Hearing instrument users’ and audiologists’ responses to the first version of the Unite TV Streamer were overwhelmingly positive. Sound quality, ease of use, transmission range, and robustness of connection emerged as strengths of this product. Because it is generally a good idea not to “mess with success,” the second generation of this wireless accessory retains these positive qualities.
Key feedback that led to improvements was related to keeping up with technology in the fast-moving consumer electronics world. Increasingly, televisions no longer have analog output, so a converter has been necessary for users with digital-only outputs to use the TV Streamer. Therefore, the Unite TV Streamer 2 has connectors for digital audio (coax) as well as optical (toslink) signals. To make room for these connections and to simplify installation for users, a standard 3.5 mm mini stereo jack replaces the right and left RCA phono connectors that served as input to the original TV Streamer. In fact, this change, added to the design of an integrated housing and stand solution and rearrangement of other electronic components, has actually enabled a smaller housing as well.
The TV Streamer 2 also includes a Dolby Digital™ decoder that automatically downmixes 5.1 type signals to a stereo signal. This typically becomes necessary when a modern TV only outputs a digital or an optical 5.1 signal intended for a surround-sound receiver. Owners of digital wireless hearing instruments from any manufacturer have until now had to separately purchase and install a Dolby Digital™ decoder if the output of their television did not have other format choices, such as PCM stereo. The TV Streamer 2 is currently the only accessory for digital wireless hearing instruments to incorporate Dolby Digital™ decoding.
Another issue with modern televisions and auxiliary equipment, such as surround-sound systems and set-top boxes, is that they can introduce a delay to the picture, leading to the sound and the picture being out of sync. The Unite TV Streamer 2 is the only device of its kind that allows the user to add a delay to the streamed audio signal so that it aligns with the moving picture.
Better Remote Control
Remote controls may be viewed by some consumers as a wonderful convenience and others as just another thing to put down somewhere and forget along with their car keys and sunglasses. Since a remote control is an auxiliary device to the hearing instruments, ease of operation and maintenance is of the highest priority. Clinicians who were in practice back in the 1990s might recall remote controls for hearing instruments that communicated via an infrared signal, similar to most television remote controls. Because infrared requires line-of-sight in order to transmit and receive signals, this meant that users had to point the remote control at the hearing instrument for it to work. This made using the remote control anything but discreet, and remote controls based on this technology did not last long in the commercial market.
The utility of remote controls has improved with digital wireless technology, and they have become an attractive accessory to users who prefer them. The updated Unite Remote Control 2 provides a larger display than its predecessor with well-defined and logical icons and a longer backlit time. It also answers users’ requests to be able to adjust the gain balance between streamed signals and hearing instrument microphones when in streaming mode, as well as the ability to mute the hearing instrument microphones at any point in time.
The biggest gripe users had with the first version of the Remote Control was that it relied on hard-to-find AAAA batteries for power. These are replaced in the Remote Control 2 with a rechargeable Li-Ion battery, allowing it to be recharged from a standard USB port (like on a laptop computer) or by using the universal ReSound wall-charger.
As with the Phone Clip+, attention also was given to improving the robustness of the wireless connection. The antenna has been relocated to the top of the device instead of around the sides. This improves the wireless link when users hold the Remote Control 2 by virtue of not shielding the RF signal emanating from the antenna with their hand. Updates to the mechanical layout encompass improved functionality and tactile feel of all buttons, the inclusion of a mechanical ON/OFF switch, and a slightly longer, but slimmer design.
Utility—making hearing instruments as useful as possible—is an ongoing quest, and digital wireless accessories help provide a way to do this. The connectivity to external devices provides a convenient way for users to receive sound directly to their hearing instruments in high audio quality without disturbing others and without the annoyance of feedback. Digital wireless hearing instrument systems based on RF technology provide further convenience, and a 2.4 GHz platform opens the possibility of direct communication with other electronic devices that transmit in this band. ReSound followed up the success of the first generation of Unite wireless accessories with a new incarnation of products where improvements were inspired by input from clinicians and users.
|Jennifer Groth, MA, is Director of Audiology Communications, and Torben Groth, BSc EE, MMT, is Senior International Product Manager at GN ReSound, Glenview, Ill. CORRESPONDENCE to HR or Jennifer Groth at: [email protected]|
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3. Picou E, Ricketts T. Efficacy of hearing aid based telephone strategies for listeners with moderate-to-severe hearing loss. Ear Hear. 2013;24:59-70.
4. Haykin S. Adaptive Filter Theory. 4th ed. Upper Saddle River, NJ: Prentice Hall; 2002.
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