Codec Takes On Spectrum-Efficiency, Audio-Quality Design Challenges

A new DSP-based evaluation platform demonstrated by CSR is aimed at its low-latency, high-quality apt-X Live codec. This technology, suited for professional audio applications, helps overcome the spectrum-efficiency and audio-quality challenges involved in digital wireless microphone and in-ear monitoring system design.

Digital wireless microphone systems offer numerous performance advantages and end-user benefits over conventional analogue designs. An important attraction is their potential to offer more efficient use of the radio-frequency spectrum for a given audio quality.

Spectrum-efficiency is fast-becoming a make-or-break design issue, due to ongoing moves by government authorities to downsize the radio-frequency band allocations for legal operation of wireless microphones in the U.S. and elsewhere. A key factor toward achieving this spectrum efficiency is the use of low-latency audio-compression technology as an essential subsystem in the signal-processing chain.

CSR’s apt-X Live codec is designed for the requirements of digital wireless microphones.  The patented architecture provides significant audio data-rate reduction and an enhanced degree of error tolerance, all with low latency.

The apt-X Live evaluation platform comprises separate digital radio transmitter and receiver modules. It’s designed to demonstrate how the low-latency audio codec subsystem within a typical digital wireless microphone system can be implemented using apt-X Live.

The programmable DSP-based digital radio transmitter module takes a mono channel audio input (16bit resolution, sampled at 48kHz) and compresses it in real-time using the apt-X Live algorithm, running at 2.5bits per PCM sample to produce a compressed bitstream at 120kbps. This bitstream is digitally modulated using FSK onto an 868MHz RF carrier. It’s then transmitted over the air to a similar DSP-based receiver module that demodulates the bitstream, and decompresses the data to reproduce the original PCM audio signal.

The audio codec can deliver a single channel of 24bit, 48kHz audio in 120kbps with a codec latency of less than 1.9ms. The framework of the audio coding will provide a higher resilience to random bit errors. Additional techniques employed within apt-X Live also reduce the significance of these errors and the magnitude of the audible effect on certain signals.

TAGS: Digital ICs
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