How to improve the performance and efficiency of personal sound reinforcement products (PSAP)
Time:2022-11-16
Views:1547
Author: Bonnie Baker
Personal Amplification Products (PSAP) provide a low-cost way to address the need for minimal hearing amplification for motion and hearing loss. Although these intelligent adjustable ear wearables are becoming more and more popular, they continue to challenge designers to improve performance while minimizing cost and power consumption.
These challenges come from the need to reduce the environmental leakage and bone conduction signals with problems in the auditory canal, as well as the delay caused by the electronic devices of the hearing equipment. These electronic devices include microphones, speakers, DSP and codecs. Combining the gain and delay signals of electronic equipment with the environment and bone conducted audio will produce a comb effect that needs to be understood. Only in this way can it be effectively alleviated to achieve a cost-effective energy-saving design.
This paper introduces the structure, operation, typical design requirements and key technical concepts of PSAP, such as comb effect. Then, it introduces the low-power, high-performance audio codec (which can be used to solve comb effect) of Analog Devices/Maxim Integrated for PSAP, and shows its application method.
Operation and design requirements of PSAP
As we grow older, it is often more difficult to hear radio, television or conversation clearly. Sometimes the background noise will interfere with the hearing of the restaurant or social gathering discussion. So far, the solution to hearing problems has been to rely on expensive hearing aids, which are classified as medical devices and regulated. Regardless of the degree of hearing loss of individual users, these devices are much more expensive than unregulated PSAP ear wearable devices.
The rechargeable PSAP is used for entertainment or low-level hearing enhancement. It has a customizable low-level amplification function to help users hear more clearly by reducing or increasing medium and high frequencies. This amplifier usually has amplifier reset and noise reduction circuits to reduce feedback and background noise (Figure 1).
Figure 1: PSAP like PFY C350+has customizable, low-level amplification functions that can improve clarity. (Image source: Health Products for You (HPFY) website)
The frequency range of each device depends on the main application, such as voice and music. For voice applications, the operating frequency range is from 20 Hz to 8 kHz, while the music range is the maximum audible 20 kHz. Most PSAP devices have a battery power supply and PC software that allow custom amplification across the entire frequency range. These devices are also designed to provide excellent sound quality and speech clarity for the sounds around users, from their phones, and audio streams.
A typical audio PSAP system includes an audio codec and a DSP core. This PSAP audio system has a simplified view of an audio codec and a microphone input connected to an analog to digital converter (ADC). The audio codec decodes the digital output of ADC to prepare for digital transmission to the Bluetooth SoC/DSP core (Figure 2).
Figure 2: Typical audio system for PSAP includes microphone, ADC, decimator, Bluetooth/DSP core, interpolator, digital to analog converter (DAC), amplifier and loudspeaker. (Image source: Maxim Integrated, modified by Bonnie Baker)
The Bluetooth SoC/DSP core further attenuates the signal to prepare for the DSP block. The DSP block processes the signal, interpolates it, and sends the digital signal back to the audio codec. The audio codec converts the digital signal back to the analog signal to drive the speaker output.
This activated PSAP has two types of sound reaching the user‘s eardrum. S1 is the superposition of residual voice environment leakage (S1A) and bone conduction (S1B). For S1, the ear wearable device covers the opening of the ear to prevent sound from reaching the inside and escaping from the ear canal (Figure 3).
Figure 3: Three sound sources reaching the eardrum in PSAP: environmental leakage (S1A), bone conduction (S1B) and processed environmental sound (S2A). (Image source: Maxim Integrated, modified by Bonnie Baker)
The PSAP microphone captures the ambient sound (S2), which is processed by the DSP, and the output signal (S2A) is sent to the ear canal through the audio transducer. Importantly, the design of the audio processing chain creates a delay. These three sounds converge on the user‘s eardrum, creating a PSAP experience.
PSAP comb effect
To achieve the PSAP experience, the audio system requires that all sounds be added before entering the eardrum. The time for S1A and S1B to reach the user‘s eardrum is the same, but as shown in the figure, the propagation of S2 signal in the audio system causes a slight delay. If the delay and gain are not sufficiently adjusted, the echo effect will appear when the signal sources are added together (Figure 4).
Figure 4: Signal model of superposition of three sounds: S1A, S1B and S2. (Image source: Bonnie Baker)
The variables in Figure 4 are delay and gain (G). S1 signal enters eardrum directly. By adding S1 sound in the environment to the electronic S2 path, the gain function in S2 will generate a delay. The addition of S1 and S2 may generate echo, but this can be reduced by manipulating the delay time and gain size.
Figure 5 shows the signal response when the delay is equal to 0.4 ms and 3 ms, G is equal to 0 dB, 15 dB and 30 dB.
Figure 5: The superimposed frequency response of two sounds based on the signal model. The delay changes from 0.4 ms to 3 ms, and the gain changes are 0 dB, 15 dB, and 30 dB. (Image source: Maxim Integrated, modified by Bonnie Baker)
The normalized frequency response in Figure 5 illustrates the delay and gain effects on the eardrum. There is a distortion, or comb effect, with multiple notches when G equals 0 decibels. The comb effect may degrade sound quality through reverberation or echo. In Figure 5A, the 3 ms delay creates more gaps at lower frequencies.
As the gain in Figure 5B increases, the importance of the comb effect decreases. The gain change from 0 dB to 15 dB forms a ripple of~3 dB at a gain of 15 dB. In Figure 5C, with a gain of 30dB, the response of the two delays is almost flat.
How to reduce comb effect
As mentioned above, increasing the gain and reducing the delay can reduce the comb effect in traditional PSAP systems, thereby reducing their reverberation or echo. The advanced PSAP equipment will replace the delay/gain component with an additional low delay digital filter to perform the anti noise function (Figure 6).
Figure 6: In an advanced PSAP system, four sounds reach the eardrum: S1A, S1B, S2A and S2B. (Image source: Maxim Integrated, modified by Bonnie Baker)
In Figure 6, the MAX98050 low-power, high-performance audio codec generates anti noise (S2B), which interacts with the original passive ambient sound to form a new sound. The MAX98050 has noise cancellation and voice/environment enhancement functions, which rely on a low-power, low latency digital filter to ensure that S2B reduces low-frequency noise.
Figure 7 shows a simplified block diagram based on the MAX98050 PSAP solution.
Figure 7: The MAX98050 codec forms the PSAP signal interface to change gain and reduce noise and delay. (Image source: Bonnie Baker)
The simulation based on the block diagram in Figure 7 illustrates the comb effect of the MAX98050 system and the influence of gain and delay time on noise (Figure 8).
.Figure 8: The figure in Figure 7 is simulated to show the comb effect of MAX98050 and the influence of gain and delay time on noise. (Image source: Maxim Integrated)
Figure 8 shows that Maxim‘s anti noise scheme emphasizes the gain difference between S1 and S2. In addition to simulation, measurement based on real shape and real-time evaluation system also verified the proposed anti noise solution.
Note that reducing the latency of the audio system requires relatively high ADC and DAC sampling rates. These changes increase the calculation load and reduce the power efficiency. In general, it will cause audio performance degradation.
epilogue
PSAP brings obvious and low-cost benefits to anyone who wants to improve their listening ability. For designers, they will continue to face the challenge of improving efficiency and performance, and need to deal with comb effect more effectively. To sum up, using the Maxim Integrated low-power, dynamic MAX98050 codec, designers can reduce the comb effect of PSAP, improve audio and power performance, and bring flexible system design for the next generation of PSAP.
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