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AUDI O & VIDEO PROCESING Full-duplex data streaming via the analogue audio connector By Alex Costa and Helmut Theiler Fig. 1: Block diagram of the digital multiplexing demonstration system. Active noise cancellati on (ANC) technology has been a hit with consumers. To date, it has been implemented mainly in stand-alone ANC headphones and earphones. Consumers have been prepared to pay a premium for headsets that enable them to listen to music undisturbed by ambient noise. Now, mobile phone manufacturers are looking to ANC as a way to differentiate their products and provide a superior audio experience in both voice calls and media consumption. The most economical and convenient way for them to implement noise cancellation is by hosting the noise-cancellation circuitry inside the mobile phone itself. But the ambient noise must be sampled at the headset, not at the phone (which could be inside the user’s pocket and insulated from the sources of noise). This then presents a considerable difficulty: how to get two (left and right) sampled noise signals from the headset to the phone, and the inverted (noise-cancelling) signal back from the phone to the headset via the standard 3.5mm audio connector found on nearly all headsets. This standard analogue audio connector is usually a fourpole device. The four wires are used for left and right headphone speakers, a single microphone output, and ground. The microphone wire (mic-wire) also typically carries the supply for the microphone amplifier in the headset. In today’s mobile phone applications, the single microphone channel carries the user’s voice signals in normal voice call mode. When the 3.5mm audio connector is used in conventional analogue mode, then, there is no route for the two sets of noise samples to be transmitted from the headset to the mobile phone for signal processing. Now ams has developed a new digital multiplexing technique that, in effect, creates extra channels in the mic-wire. These channels can be used in ANC applications to carry the noise samples from two or four additional left and right microphones in the headset to the mobile phone. In other applications, the extra channels might be used for low- or mid-rate data transfers to audio accessories, for instance to enhance them with data displays, sensor data and other additional features. Of course, the conventional analogue functionality of the audio connector may be maintained, so standard headsets can still be used, although they will not support the enhanced features. This article describes the implementation of this new technique for full-duplex data streaming via the 3.5mm audio connector. Simultaneous voltage and current modulation The digital microphones widely used today supply the audio signal as a serial Σ-Δ-modulated bit stream based on an oversampling clock. This makes a digital technique for multiplexing signals to provide full-duplex communication on the mic-wire feasible. The challenge is to avoid interference between the up-link and down-link signals on the same wire, while providing a high enough bit-rate to meet consumers’ demands for high audio quality. The successful method that ams has developed combines voltage and current modulation on the mic-wire: one provides the up-link and the other the down-link. To prove the effectiveness of the technique, ams has produced a fully operational demonstration system. This system, which includes ANC functionality, can be linked to a mobile phone or MP3 player with a standard 3.5mm audio connector. It offers data rates of around 2Mbits/s in the up-link and Alex Costa and Helmut Theiler are Design Engineers at ams AG - www.ams.com Fig. 2: The ams demonstration system, showing the master board (bottom), the peripheral circuit (top) with Volume Up, Mode and Volume Down buttons, and a headphone set. www.electronics-eetimes.com Electronic Engineering Times Europe February 2014 31


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