Solutions for audio fidelity and functionality of portable devices

In the mobile market, audio fidelity and functionality continue to drive innovation to meet the needs of listening to music, watching movies or making phone calls. Therefore, semiconductor suppliers have always faced challenges and need to create audio solutions to meet these requirements for audio ports and headphones. This article will introduce some new solutions, including reducing various types of clicks and pops and meeting different world standards for headphones.

In the past few years, headset manufacturers such as Bose, Sennheiser, UlTImate Ears, Monster, and Harman Kardon have formed alliances with major smartphone manufacturers (including HTC, Samsung, and Apple) to launch new products repeatedly. Product diversification has expanded the basic earphones of the past into earphones that can help smartphones to answer calls, increase / decrease volume, play / pause and microphone functions.

Next-generation headphones now include active drivers that improve audio fidelity through noise reduction and elimination. Smartphone manufacturers must ensure a smooth and comfortable customer experience, allowing different headphones from various vendors to be used for each device. To meet this customer demand, manufacturers and semiconductor suppliers work together to help apply these headset accessories.

Another challenging aspect is that different smartphone models may have completely different audio codec solutions. For example, if you disassemble the iSuppli and iFixit of the Samsung Galaxy SII GT-i9100 smartphone, you will find one type of audio codec, while in the Samsung Galaxy S III, you will find another audio codec. The characteristics of the two audio codecs are different, including noise reduction, parametric equalization, and integrated headphone capacitors. Although these are different audio codec devices, they must be suitable for all previous headsets, and the performance is "relatively" the same. Therefore, many smartphone manufacturers must use supplementary audio jack detection devices to allow the use of audio accessories and maintain the same performance.

From the perspective of the 3.5mm audio jack and the attached headphones, the problem is further analyzed. Obviously, users need to insert non-original headphones or OEM headphones into the mobile device. At this time, because the headset is being plugged in, there cannot be any signal transient on the left or right interface of the headset plug. This produces audible clicks and pops, also known as plug-in clicks and pops. Once the headset is connected, the phone must perform the headset detection process before applying the appropriate audio and control signals. Testing includes verifying the polarity of 3-pole or 4-pole headphones, active or passive headphones, and GND and MIC plugs. During this particular detection stage, there must not be any audible clicks or pops. After the plug is determined, the audio codec or baseband will be notified, and then audio and control signals can be applied to the headset. In the working mode, the detection mode cannot reduce the audio fidelity.

Plug insertion and removal

When plugging or unplugging the audio plug, when the two audio amplifiers are active, there may be pops or clicks due to the MIC bias. This is usually due to incorrect isolation timing or bias gating. When inserting or removing the plug, the terminals scrape through these biases, resulting in popping and clicking sounds. Implementing a more advanced jack detection circuit can solve these two problems. The traditional jack detection circuit uses an N / O (normally open) or N / C (normally closed) mechanism to switch to baseband GPIO (general input and output) in a jack with a pull-up resistor. For example, with an N / O mechanism switch without a plug inserted, the logic level is high; when the plug is inserted, the ground path is connected to a pull-up resistor, resulting in a low logic level. To debounce this circuit, use a parallel capacitor to create an RC (resistance capacitor) circuit.

Usually, this RC debounce time is about 500ms, and this is the problem. The set debounce time does not provide flexibility for slow plug insertion, and the debounce time of the plug is the same as the insertion timing. Usually implement more advanced detection circuit, such as Fairchild's FSA8008 audio jack detection and configuration switch, can solve these problems. This device uses a 0.5μA pull-up current source in the jack's mechanical switches and other logic to solve these problems. During the insertion of the plug, the device implements debounce logic. If at any time the jack detection logic causes interference due to slow or partial insertion, the debounce time will be reset (see Figure 1). Before activating the MIC bias and amplifier, this ensures that the plug is fully inserted.

Solutions for mobile audio fidelity and functionality

Figure 1 Jack detection reset debounce when interference is caused by slow / partial plug insertion

Furthermore, the FSA8008 uses a switch to isolate the MIC bias voltage, ensuring that there is no MIC potential on the jack until it is fully inserted into the plug. The time of plug removal is as important as the timing of insertion. In this case, the unplug timing must be much faster than the insert time. By determining the reference characteristics of the jack, the correct timing required can be determined. For example, when unplugging, the correct plug terminal can be connected to the GND terminal of the jack, we determine the time required for a plugging feature is less than 100us.

For additional tolerances, the FSA8008 implements a 30μs unplug time, making the device suitable for many jack types. When the plug is unplugged, the jack detects triggering and the device quickly turns on the MIC switch to isolate the MIC bias and communicate with the baseband to disable the audio amplifier. By implementing more advanced jack detection and gating or isolation, the timely MIC bias and audio amplifier can eliminate popping and clicking sounds caused by plugging and unplugging.

Open Mobile Terminal Platform (OMPTP) and Cellular Phone Industry Association (CTIA) audio plug standards

Solutions for mobile audio fidelity and functionality

Figure 2 The slow conduction time of the crossbar solves the click and pop

A major trend in headsets today is the choice between OMTP (Open Mobile Terminal Platform) and CT IA (Cellular Phone Industry Association) plug standards. These two standards switch the third and fourth terminals. To allow mobile devices to use any standard operation, crossbar switches such as Fairchild's FSA8049 devices are required to route between the MIC and GND terminals of the jack. If slow on-time is not implemented, this solution may cause popping and clicking sounds. When switching between terminals, if the audio amplifier is still active and grounded, the amplifier will generate current spikes, which will cause the speaker to click and pop. By implementing a slow on-time in the switch, or using an FSA8049 cross switch with adjustable on-time, the di / dt on the speaker may be significantly reduced, thereby eliminating popping and clicks. When the audio jack is driven by two different sources, any audio jack can be used. When the switch is activated and you switch from source A to source B, if the signals do not match, the transient will cause audible clicks and pops. There are ways to solve this situation. One is the proprietary SignalWise â„¢ technology that allows signal matching before conversion. In Figure 2, we can see sources A and B again, but this time the conversion occurs when they are equal, and there will be no transients that cause clicks and pops.

in conclusion

As headset manufacturers continue to push their products globally, we will continue to see additional features added to the 3.5mm jack. The jack must identify and adapt to all commercially available accessories of different standards in order to provide a consistent user experience. Semiconductor suppliers have created multiple solutions to deal with clicks and pops and other issues to maintain audio fidelity.

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