Knowles EF-26810-000

In recent years, the widespread use of mobile phones has improved the quality of life for many people in our fast-paced world, but has diminished it for some hearing aid users. When people use certain mobile phones employing digital transmission near hearing aids, an interfering "buzz" may be heard due to EMI.

Wireless technologies such as GSM, PCS, and DECT all use TDMA as their multiplexing scheme. This RF signal can be demodulated by hearing aids, producing an unwanted "buzz." Products using a spread-spectrum transmission mode known as CDMA are now emerging. The power density at any given frequency is very low, and rectification issues are fewer. Fortunately, next-generation wireless products appear to be adopting CDMA and other spread-spectrum technologies.

A brief discussion and comparison of the various systems helps explain why some are more likely to cause problems than others. Digital systems appear more prone to interference because rectification and demodulation of the RF amplitude-varying waveform produces products within the audio range. If demodulated, the actual envelope variation frequency of the signal is too high to be of concern. However, since mobile phones transmit data in "frames," the signal manifests as high-speed pulse bursts.

The actual data rate of GSM is 270.833 kbps, which on its own is too high a frequency to cause any demodulation problems. Each GSM frame lasts 4.615 ms and consists of 8 time slots of 576.92 μs each. The mobile phone transmits 270.833 kbps pulses only within these time slots. Since these time slots repeat every 4.615 ms, they have a spectral component at 216.68 Hz — clearly within the audible frequency range. There are other less critical low-frequency spectral components caused by multiframes (a GSM multiframe consists of 26 frames with a duration of 120 ms, giving a spectral component of 8.3 Hz, which is clearly inaudible).

To highlight the issue, GSM is one of the systems with the highest portable transmitter output power, with a maximum of 1 W and an average of 125 mW.

By comparison, the IS-54 specification for USDC has a data rate of 48.6 kbps. The frame duration is 40 ms (25 Hz), consisting of 6 time slots of 6.66 ms each (150 Hz). Its harmonics may still present potential demodulation problems.

The maximum output power of a USDC mobile phone is 600 mW, with an average output power of 200 mW, which tends to further reduce the likelihood of interference compared to GSM.

Among the other three major mobile protocols — DECT (frame duration 10 ms, spectral rate 100 Hz, power 250 mW), PHS (frame duration 5 ms, spectral rate 200 Hz, power 80 mW), and PACS (frame duration 2.0 ms–2.5 ms, spectral rate 400–500 Hz, power 100 mW) — PACS appears to be the most likely to cause problems, with the fortunate exception of its lower power level.

Although there are many "standard" wireless communication protocols, GSM is currently the transmission method most likely to cause problems in hearing aids.

The interference of certain TDMA mobile phone transmissions with hearing aid circuits is a particularly challenging problem for several reasons. The radiating antenna is in very close proximity to the high-gain hearing aid circuit — an unusual situation that produces large RF field gradients across the sensitive hearing aid circuitry. The hearing aid also operates in the near field of the antenna, so no prior assumptions can be made about the orientation of the electric field (E) and magnetic field (H). If such assumptions could be made, it would be sufficient to mechanically redesign the device to ensure that the pickup points are located in the field-null regions.

RF energy can enter the hearing aid through any one of five basic components: the microphone, volume control/trimmer, IC, wiring, or receiver.

Depending on its intensity and location within the hearing aid system, RF energy affects hearing aid circuits in two distinct ways. If the RF signal amplitude is large, it may reduce the dynamic range of the intended audio signal within the circuit. This manifests as apparent distortion or clipping of the audio signal at levels well below the normal clipping amplitude. This can occur at any point in the circuit, but is most likely at the output stage, where the intended signal has its greatest amplitude.

In theory, any type of mobile phone — analog or digital — could cause this type of interference. Testing indicates that neither analog nor digital phones currently cause severe interference of this kind. However, this potential interference source must be continually recognized and monitored to ensure it does not become a problem.

The second mechanism involves the reception and rectification of RF energy by semiconductor circuits. TDMA digital modulation schemes are particularly problematic in this regard because the RF carrier is switched on and off in pulses. As previously noted, the primary TDMA modulation scheme (GSM) uses a pulse rate of 217 Hz. This produces a demodulated interference spectrum at 217 Hz and all of its harmonics. These frequencies are readily audible across much of the audio frequency range and cause the hearing aid to emit an audible "buzz." Suppressing this type of interference requires keeping RF energy away from any nonlinear circuit elements where demodulation may occur, including the hearing aid circuitry.