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. As a result, semiconductor suppliers have been challenged to create audio solutions to meet these requirements for audio ports and headsets. 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 push new products. Product diversification has expanded the basic headsets of the past into headsets that help smartphones answer calls, increase/decrease volume, play/pause, and microphone.
Next-generation headsets now include active drives that improve audio fidelity through features such as noise reduction and cancellation. Smartphone manufacturers must ensure a smooth customer experience, allowing different headsets from various vendors to be used for each device. To meet this customer's needs, manufacturers work with semiconductor suppliers 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, and in the Samsung Galaxy S III, you will find another audio codec. The two audio codecs have different characteristics, including noise reduction, parametric equalization, and integrated headphone capacitance. Although these are different audio codec devices, they must be applied to all previous headphones and the performance is "relatively" the same. As a result, many smartphone manufacturers must use complementary audio jack detection devices that allow the use of audio accessories and maintain the same performance.
This problem is further analyzed from the perspective of the 3.5mm audio jack and the attached headset. Obviously, the user needs to insert a non-original headset or an OEM headset into the mobile device. At this point, there is no signal transient on the left or right side of the headphone plug as the headset is being plugged in. This produces audible clicks and pops, also known as plug-in clicks and pops. Once the headset is connected, the handset must perform the headset detection process before applying the appropriate audio and control signals. Detection includes verification of 3- or 4-pole headphones, active or passive headphones, and GND and MIC plug polarity. There must be no humming or popping during this particular detection phase. Once the plug is determined, the audio codec or baseband is notified and the audio and control signals can then be applied to the headset. In the operating mode, the detection mode does not reduce the audio fidelity.
Plug insertion and removal
When an audio plug is inserted or removed, a popping or click may occur due to the presence of the MIC bias when the two audio amplifiers are active. This is usually due to incorrect isolation timing or bias gating. When the plug is inserted or removed, the terminals scrape through these offsets, causing pops and clicks. Implementing a more advanced jack detection circuit solves both of these problems. The conventional jack detection circuit switches to the baseband GPIO (General Purpose Input Output) using an N/O (Normally Open) or N/C (Normally Closed) mechanism in a jack with a pull-up resistor. For example, with an N/O mechanism switch that is not plugged in, the logic level is high; and when the plug is plugged in, the ground path is connected to the pull-up resistor, resulting in a low logic level. To debounce this circuit, use a shunt capacitor to create an RC (resistance capacitor) circuit.
Usually, this RC debounce time is about 500ms, and this is where the problem lies. The set debounce time does not provide flexibility for slow plug insertion, and the plug debounce time is the same as the insertion timing. More advanced detection circuits, such as Fairchild's FSA8008 audio jack detection and configuration switches, are usually implemented to solve these problems. This device uses a 0.5μA pull-up current source in the mechanical switches and other logic of the jack to solve these problems. During the insertion of the plug, the device implements debounce logic that resets the debounce time if the jack detect logic interferes at any time due to slow or partial insertion (see Figure 1). This ensures that the plug is fully inserted before activating the MIC bias and amplifier.
Figure 1 Jack detection reset debounces due to interference caused by slow/partial plug insertion
Moreover, the FSA8008 uses a switch to isolate the MIC bias, ensuring that there is no MIC potential on the jack until the plug is fully inserted. Plug removal time is just as important as insertion timing. In this case, the extraction time must be much faster than the insertion time. By determining the reference characteristics of the jack, you can determine the correct timing required. For example, when removing the plug, before the correct plug terminal can be connected to the GND terminal of the jack, we determine that the time required for a plug-in feature is less than 100us.
For additional tolerances, the FSA8008 implements a 30μs pull-out time, making the device suitable for many jack types. When the plug is removed, the jack detection triggers and the device quickly turns the MIC switch on, isolates the MIC bias and communicates 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 eliminates pops and clicks from plug insertion and removal.
Open Mobile Terminal Platform (OMPTP) and Cellular Telephone Industry Association (CTIA) Audio Plug Standard
Figure 2 Crossbar slow turn-on time solves clicks and pops
A major trend in today's headsets is the choice between OMTP (Open Mobile Terminal Platform) and CT IA (Cellular Phone Industry Association) plug standards. These two standards switch between the third and fourth terminals. To allow the mobile device to operate in any standard, a crossbar such as Fairchild's FSA8049 device is required to route between the MIC and GND terminals of the jack. If a slow on time is not implemented, this solution can cause pops and clicks. When switching between terminals, if the audio amplifier is still active and grounded, the amplifier will generate a current spike, causing the speaker to beep and pop. By implementing a slow on-time in the switch, or using the FSA8049 crossbar with adjustable on-time, the di/dt on the speaker can be significantly reduced, eliminating pops and pops. Any audio jack can be used when the audio jack is driven by two different sources. When the switch is activated and switched from source A to source B, if the signal does not match, the transient will result in a click and pop. There are ways to solve this situation, one is the proprietary SignalWiseTM technology that allows signal matching before conversion. In Figure 2, we can see the sources A and B again, but this time the conversion happens when they are equal, so there are no transients that cause clicks and pops.
in conclusion
As headset manufacturers continue to push their products globally, we will continue to see other features added to the 3.5mm jack. Jacks must identify and adapt to all commercially available accessories of different standards to provide a consistent user experience. Semiconductor suppliers have created a variety of solutions to deal with issues such as clicks and pops to maintain audio fidelity.
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