SUPPLEMENT CONNECTORS & ENCLOSURES


DATA-RICH 3.5MM JACK VIES WITH USB-C FOR HEADSETS


Horst Gether from ams AGexplores a new accessory communication Interface standard T


he 3.5mm phone jack is a well- established standard in the audio


industry. Invented in the 19th century, it was originally used in telephone switch boards, and then made its way into mobile phones, tablets and personal computers to connect audio and communication headsets for phone calls or listening to music. Ever since its invention, this 3.5mm four-pole accessory device has provided a rather limited set of functions. The 3.5mm jack’s capabilities might not have changed in recent years, but the environment in which they are used certainly has. Smartphones have become incredibly powerful computing devices. And more and more users are demanding that the headset they used with their phone should have an Active Noise Cancellation (ANC) capability. Now manufacturers are making a new


demand: for an ANC headset system that can operate with no battery inside the ANC headset’s accessory, or dongle. One method for achieving this is connect the accessory to the phone via a USB interface, which can supply power from the phone’s battery to the dongle. But phone and headsets makers want to create designs that have a low bill-of- materials cost and a small form factor. In this solution, there is a lot of duplication – both the mobile phone and the accessory have their own audio codec, DSP extension, amplifier and power management unit.


S8 MARCH 2018 | ELECTRONICS


LOWER SYSTEM COST AND SMALLER FORM FACTOR Now ams has invented a new interface standard to be used with the 3.5mm audio jack, offering full backward compatibility to existing 3.5mm ports. The new interface technology, called the Accessory Communication Interface, or ACI, uses the microphone (MIC) signal line to provide extra functionality, and turn it into a digital bi-directional interface. The goal within the development of the


ACI was to use the computation power inside the mobile phone rather than having duplicated blocks inside both the phone and the accessory. Figure 1 shows the implementation of an ACI system for a 3.5mm audio jack system and a USB-C based system with an analogue audio output. All these options need is an ACI master inside the mobile phone. Eventually this master device may be integrated into the audio codec, to reduce system cost even more. Now the audio codec, headphone


amplifier, microcontroller and power management unit are no longer required in the accessory. All the buttons for user


Figure 2: ACI system overview


Figure 1:


ANC headset systems based on a 3.5mm audio jack (left) or USB-C interface (right) and powered by ACI technology


interface control and the microphones for speech and active noise cancellation are directly connected to the ACI slave chip inside the accessory. This collects all sensor, microphone and button information and transfers it to the host. The audio signal processing is handled by the audio codec’s Digital Signal Processor (DSP) for transmit-path noise reduction, just as it does for receive path noise reduction. A detailed block diagram of the ACI system is shown in Figure 2. In order to eliminate the battery inside the accessory, the ACI system supports power extraction on the slave side, at output currents up to 100mA. The supply voltage in a range from 1.4V to 1.9V can be used to power microphones or additional sensors that are connected via I2


C to the ACI slave device. The communication between the ACI


master and an ACI slave is based on a half-duplex time-multiplexed modulation scheme. There is a DC offset of typically 3.3V on the signal line, in order to provide power to the AS3445B. The microphone data as well as metadata are then directly modulated on the 3.3V supply rail. Some 83% of the overall bandwidth is used to transfer low latency signals, normally microphone signals. Another 10% is used to synchronise the master and slave clocks. The last 7% provides the metadata transfer for I2


C communication to the


ACI slave, for direct bit transfers and for interrupts.


The ACI technology can be used in another way in headsets. There are many interconnections between left and right ear cups, and the number of available cables is limited by the available space in the headband. In such a case, ACI could be used with a single additional wire to connect multiple microphones, buttons and sensors located on the second ear cup to the main PCB on the first ear cup, where the main processor of the headset is located.


ams


www.ams.com T: +49 71 21514 860


/ ELECTRONICS


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