Reducing the complexity of sub-band ADPCM coding to enable high-quality audio streaming from mobile

White Paper

Reducing the complexity of sub-band ADPCM
coding to enable high-quality audio streaming
from mobile devices
a technical white paper by Neil Smyth and David Trainor, APTX www.aptx.com
1 | © APT Licensing Ltd., 2009 White Paper
ABSTRACT

The number of consumer audio applications demanding high quality audio compression and communication
across wireless networks continues to grow. Although the consumer is increasingly demanding higher
audio quality, devices such as portable media players and wireless headsets also demand low computational
complexity, low power dissipation and practical transmission bit-rates to help conserve battery life. This
paper discusses research undertaken to lower the complexity of existing high-quality sub-band ADPCM
coding schemes to better satisfy these conflicting criteria.

INTRODUCTION

The widespread popularity of portable media devices has allowed consumers to enjoy audio/video
entertainment, games and online content at their convenience. The only limitation is the awkward necessity
of wires to connect these devices to displays, earphones or car/home entertainment systems. Wireless audio
streaming isolates the speaker device, allowing users to place their mobile devices in a safe or withdrawn
location while they enjoy convenient wireless audio streaming.

A major obstacle to wireless audio streaming is that the mobile device and/or speaker system are typically
power restricted. If the application involves video playback there is also a requirement for low latency in
order to provide acceptable lip-synch. These requirements have led to the use of ADPCM coders rather than
perceptual coders in wireless audio/video devices due to their relatively low complexity and latency. As the
storage capacities of these devices continues to grow consumers increasingly exploit the quality benefits of
increasingly higher bit rates to compress their audio content. As consumers become increasingly accustomed
to high quality wired audio playback they will demand that this quality is maintained when using the
convenience of wireless streaming. All of these factors lead to the opposing design constraints of high
quality and low complexity.

A number of audio compression algorithms are used for wireless audio streaming, these include Bluetooth
SBC [1] and Enhanced apt-X [2]. The Philips developed SBC algorithm was selected as a mandatory codec for
use in Bluetooth to insure interoperability between products. Bluetooth SBC is a frame-based variable rate
APCM codec with low complexity processing overhead, an adaptive quantization step size and lower latency
than optional Bluetooth codecs such as MP3 and AAC.


2 | © APT Licensing Ltd., 2009 White Paper

Enhanced apt-X is a frameless ADPCM codec (see Figure 1 for a basic overview) with a fixed compression
ratio of 4:1, an adaptive quantization step size and predictive differential coding. The use of prediction and
differential coding in such an ADPCM codec provides reduced levels of quantization noise and thus quality
improvements over an APCM codec. However, a significant obstacle to deployment of ADPCM codecs in
comparison to APCM is the additional computational cost associated with the prediction process.
(a) ADPCM sub-band codec
(b) ADPCM encoder
(c) ADPCM decoder Fig. 1: ADPCM Sub-band codec block diagram The difference in latency between the SBC and Enhanced apt-X codecs should also be considered. The
frameless structure of Enhanced apt-X provides extremely low latencies. In contrast the framed SBC codec
requires buffering of frame data prior to encoding thereby consuming additional algorithmic delay. When
considering the impact of framed and frameless coding, the buffering and transmission delays incurred in
the wireless transmission system should also be considered. If the wireless transmission system is designed
3 | © APT Licensing Ltd., 2009 White Paper
to accommodate low latency operation the low latencies associated with a frameless streaming codec such