How to quickly realize high-quality TWS earplug design with dedicated tweeter and bass unit
Time:2022-11-10
Views:1580
Author: Steven Keeping
In the early days of audio streaming, the wireless data rate was limited, and users accepted the loss of fidelity in exchange for the convenience of putting thousands of digital music into their pockets. However, with the introduction of wireless technologies that support higher wireless throughput and enhanced compression algorithms, consumers become more picky. This means that designers now need to provide true wireless stereo (TWS) earplugs to meet consumer expectations. TWS earplugs promise to more accurately reproduce the sound of the entire audio spectrum, especially at higher frequencies, which are often lost in older designs.
But sound quality is only one aspect of modern wireless audio reproduction. In the highly competitive market, earphone developers must pay close attention to the needs of consumers, and use the found demand points to provide differentiated terminal products in an effective and economic way as far as possible. For example, consumers hope to effectively carry out active noise reduction (ANC) and alleviate the blocking effect, so as to better enjoy the hearing experience. For older listeners, the demand for automatic compensation of high frequency natural hearing loss (hearing personalization) is also growing.
To meet these requirements, it is necessary to modify the design and separate the bass unit and tweeter. This is beyond the scope of skills of many development teams, leading to extended time to market, and may lead to dismissal or loss of career development opportunities.
This paper summarizes the development of commercial wireless audio technology and its impact on the hardware and software design of earplugs. The article then introduces the reference design of TWS earplugs, and shows how designers can use it to quickly bring earphone solutions to the market, so as to achieve differentiated functions, and accurately reproduce the powerful bass and extraordinary treble captured by modern audio compression software.
The development of digital sound
In the real world, sound is an analog signal, but our recording and playback equipment mainly processes digital signals. The sound is digitized through an analog-to-digital converter (ADC), which is driven by an encoding/decoding ("codec") algorithm that controls the Hertz (Hz) sampling rate and bit depth (bits). Sampling captures the amplitude of an analog waveform of sound at a specific time interval.
The sampling rate needs to be chosen. The lower rate results in less data to be processed and lower resolution. Bit depth refers to the number of information bits in each sample; Similarly, there needs to be a trade-off between the number of bits and the quality of the sound. Common bit depths are 16, 24, and 32 bits (Figure 1).
Figure 1: Analog sound digitized by sampling at a given frequency and bit rate. Increasing the sampling rate and bit depth can ensure that the digitized information is closer to the analog signal and improve the reproduction quality. (Image source: Knowles)
Sampling rate × Bit depth × The number of channels determines the bit rate in bits per second (bps). In order to obtain acceptable sound quality, the bit rate is usually greater than 192 kilobits per second (kbps). For example, CD quality depends on a sample rate of 44.1 kilohertz (kHz) and a bit depth of 16 bits. Therefore, stereo reproduction is obtained with a bit rate of 1.411 megabits per second (Mbps).
Traditional codecs usually use compression technology to discard the information that has been determined not to excessively affect the audience‘s knowledge of the decoded audio. The aim is to reduce the bit rate as much as possible without unduly affecting the sound quality. This codec is called "lossy" because the decoder can never reproduce the original signal because it does not have all the original information. Usually higher (treble) frequencies are eliminated by lossy codecs.
Due to the progress of low power consumption and short distance radio, wireless links can support greater throughput without affecting battery life. For example, the recently released wireless streaming media form based on low-power Bluetooth, low-power Bluetooth audio, can now provide much higher audio quality than classic Bluetooth audio, and reduce power consumption.
The engineers also increased the efficiency of their codecs. These newer "lossless" codecs, combined with higher throughput wireless connections, enable higher wireless audio (Table 1). Audio services from Apple, Amazon and Spotify now provide high-quality, lossless audio streaming. However, designers should note that the coding bit rate of the lossless codec is often higher than the bit rate that the wireless link can reliably support. For example, Sony‘s LDAC codec can encode at a bit rate of 6.1 Mbps (32 x 96 x 2), but the bit rate of wireless links is limited to 990 kbps.
Table 1: Comparison of "lossless" codecs (Sony, Savitech, and Qualcomm) with CD quality and lossy codecs (Qualcomm and the Bluetooth Technology Consortium (SBC)). Note that the maximum bit rate of the lossless codec is limited by the capabilities of the Bluetooth wireless link. (Image source: Knowles)
ANC and personalized sound
Consumers‘ expectations for TWS earplugs exceed the range of sound quality. High end products must also provide ANC and other functions. ANC is very popular because it provides users with high-quality hearing experience in the case of high background noise, such as in the aircraft cabin. The ANC uses a microphone built into the earplug to operate. Before users are aware of low-frequency noise, they can extract low-frequency noise and eliminate it. That is to say, a noise of 180 ° relative to the original noise is generated in the earplug ˚ Inverted secondary sound to eliminate noise.
Another key improvement that wireless earplugs now offer is personalized sound. Users with congenital hearing impairment or hearing impairment as they grow older may find it particularly difficult to hear higher frequencies (Figure 2). There are smartphone applications and other tools that allow users to increase specific frequencies to compensate for hearing loss, but they are often crude and ineffective. But now, high-quality products further improve this by conducting detailed hearing tests on users and using algorithms to set the hearing level in the entire frequency range. This creates earplugs with perfectly adjusted output and compensates for hearing defects.
Figure 2: As users grow older, they gradually lose the ability to listen to high frequencies. Personalized sound increases the volume of selected frequencies to compensate for the loss of hearing sensitivity. (Image source: Knowles)
The last technological development of modern earplugs is to reduce occlusion. Occlusion occurs when the earplug seals the outer part of the ear canal. This is a common problem with products designed to be relatively close to the ear. Earplugs effectively increase the acoustic "impedance" of the ear canal, which in turn increases the amplitude of sound pressure, especially when the ear is exposed to low frequency sounds produced by the user (examples include speaking, walking, and swallowing). The result is an echo like "roar" in the ear, which is boring and distracting.
Ear plug manufacturers have made efforts to reduce occlusion effects through mechanical design, such as adding a small opening between the ear plug and the ear canal to reduce acoustic impedance, and through software design, such as including occlusion reduction in the ANC program.
Advantages of separate bass and tweeter
Until recently, designing wireless headphones has been less challenging than designing full-size speakers connected to high-end, hot audio systems. Users accept low-quality earphones as the price of convenience, which makes it easier for designers to develop small-sized products at a reasonable cost. For example, it is common to use a full range driver instead of a separate woofer and tweeter to save space. The reproduction of higher frequencies may be sacrificed, but when these frequencies are not in the wireless audio stream, this is hardly a problem.
However, with the advent of high-throughput technologies such as lossless codecs and low-power Bluetooth audio, wireless audio can now provide a full range of bass and treble frequencies (Figure 3). Reproducing this audio requires more earplugs. In addition, consumers expect ANC, personalized voice, reduced blocking effect, and a wide range of applications, including music, television, video conferencing, and voice calls - all in a highly compact enclosure at a reasonable cost.
Figure 3: The lossless codec provides more high-frequency information, and can better reproduce the high pitched sound when playing music in properly designed earplugs. (Image source: Knowles)
Many of these requirements require design tradeoffs. For example, in order to provide effective ANC in noisy environments such as aircraft cabins, speaker drivers need to produce high and low bass output and low distortion. The semi open design to solve the blocking problem requires further bass output. At the same time, lossless audio playback requires that the speaker driver be able to handle the tweeter output of up to 20 kHz. In a very small overall dimension, it is almost impossible to use a dynamic speaker driver to meet these two requirements.
The solution is to divide the bass and treble frequencies into a dynamic bass unit and an independent balanced armature (BA) tweeter. BA tweeter unit is a special component, which was originally developed for hearing aid application, and is now increasingly used to improve the tweeter response of high-quality earplugs. In the BA tweeter, the electrical signal vibrates a tiny reed, balancing the two magnets in the compact housing. The movement of the reed is transferred to a very hard aluminum diaphragm to produce sound.
With the special bass unit and BA tweeter configuration, the bass unit can be designed to focus on providing powerful bass to support lossless reproduction, ANC and reduce blocking effects, while the output of the BA tweeter is optimized to clear and distinct tweeters. This reduces the need for equalization, which in turn saves power and increases dynamic headroom (Figure 4).
Figure 4: Dividing the speaker system into a dynamic bass unit (green) and a BA tweeter unit (blue) produces a flat frequency "mix" response (red) (picture source: Knowles)
There is another advantage to separate loudspeaker driver units: designers have more freedom in the arrangement of driver units. For example, the bass unit can be less directly aligned with the ear tip, so that the BA tweeter can be placed close to the ear opening to minimize the volume of air trapped between the tweeter and the ear tip, thereby limiting the occlusion effect (Figure 5).
Figure 5: Separate the woofer and tweeter in the earplug to make the tweeter face the front of the device, which helps limit the blocking effect. (Image source: Knowles)
In addition, the separation of the woofer and tweeter allows the designer to refine the frequency response. For example, they can shape the acoustic characteristics near the opening of the tweeter to refine the high-frequency response. Then, the designer can adjust the divider to mix the signals of the bass unit and the tweeter smoothly. Designers can also adjust the sensitivity of the tweeter by selecting a higher or lower coil impedance to achieve a better match with the bass unit. The final shaping of the overall frequency response of the earplug can be completed by supporting the fine tuning of digital signal processing (DSP).
In addition, because many Bluetooth ICs have dual outputs, the woofer and tweeter can be driven by separate amplifiers, giving greater flexibility in shaping the frequency response.
High quality wireless audio reference design
Engineers accustomed to using a single speaker driver in their wireless designs will face the challenge of the additional complexity of independent bass and tweeter units required to reproduce high-quality audio. However, the trend is clearly towards higher quality audio capabilities, so we must consider the dual driver design to achieve high-quality reproduction of lossless audio streams.
To help designers develop in this direction, Knowles, a BA tweeter manufacturer, introduced the TC-35,030-000 true wireless stereo earplug reference design. This reference design shortens the time to market of TWS earplugs by including many key advanced functions required by users, thus eliminating many common design challenges.
The reference design includes Knowles‘ own BA tweeter for good high-frequency sound, and a 10mm (mm) dynamic bass unit for solid bass. The device also includes a Micro Electro Mechanical System (MEMS) microphone for ANC and voice communication. The reference design provides 13 hours of play time or 8 hours of talk time through its built-in battery, and is compatible with Bluetooth 5.2. Other features built into the suite include touch control and integrated voice assistant technology (Figure 6).
Figure 6: The reference design of TC-35030-000 TWS earplug has a BA tweeter for good high-frequency sound and a 10 mm dynamic bass unit for solid bass. (Image source: Knowles)
The BA tweeter provides a response well above 20 kHz. Comparing the tweeter output of Knowles products with a typical 8 mm dynamic speaker, the BA tweeter unit provides greater tweeter output and extension required for high-quality audio, including the ability to support hearing personalization or enhancement (Figure 7).
Figure 7: The figure shows the comparison of high frequency response between Knowles‘ BA tweeter and dynamic loudspeaker. (Image source: Knowles)
epilogue
Advances in wireless semiconductors and codecs have changed the face of earplugs. Consumers now expect their in ear TWS devices to have deep bass, refined treble and wide dynamic range. In addition, users expect to have advanced functions such as ANC and personalized voice, and are no longer willing to endure such effects as blocking.
In order to better meet the frequency response requirements of TWS earphones, designers need to use a dual drive design with dedicated tweeters and bass units instead. Although this is technically challenging, Knowles‘ TC-35030-000 TWS earplug reference design can help. It combines the BA tweeter, bass unit and MEMS microphone, providing a good basis for designing high-quality audio earplugs, so as to achieve obvious product differentiation.
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