Audio Processing on ARM Cortex-M4 for Automotive Applications

ITTIAM Systems – White paper – ARM Cortex-M4<strong>Audio</strong> <strong>Processing</strong> on ARM ® Cortex-M4for Automotive ApplicationsIntroduction–By Pradeep D, Ittiam Systems Pvt. Ltd.Automotive infotainment systems have become an integral part of the in-car experience. Theseinclude an array of features ranging from multimedia playback and telematics to Internet browsing andcomplex human-machine interfaces turning the automobiles into connectivity and entertainmentdestinations. Music has been the most preferred form of entertainment in automobiles due to its nonintrusive nature. Satellite radio, live news, weather feeds, streaming from mobile devices, rear-seatentertainment and multiple such features have become a necessity for automobile manufacturers todifferentiate their offering from their competitors.In this whitepaper, we discuss the automotive audio system, elaborating on the functions andprocessing requirements for audio, followed by analysis of the Cortex-M4 core, the newly announcedARM core in the Cortex-M series for automotive audio system.In-Car <strong>Audio</strong><strong>Audio</strong> entertainment undeniably plays a large role in automobiles. Over the years the audiosystems have changed from simple car radio to an integrated entertainment system supportingmultimedia, next generation digital radio, connectivity to personal devices like the mobile phones, andvoice activated or touch based User Interface (UI). The customer’s expectation has also changedsignificantly, demanding personalized, high fidelity audio.Digital Radio, a necessity - The advent of digital broadcast brings in wider coverage and better qualityradio. Radio has been the preferred form of entertainment in-car as it brings in an unlimited supply ofmusic with ‘the element of surprise’ as well as the latest news. A wide variety of standards such DAB,DAB+, DVB-T, DRM, HD-radio, ISDB-T etc are in use in different parts of the world to transmit digitalradio. Most of these standards use MPEG audio format for encoding and transmission.Compression formats involved - The media player needs to support a variety of compressed audioformats like MP3, AAC, WMA and Vorbis in different file/container formats. The media player should becapable of playback from a multitude of storage media ranging from CD/DVD drives, USB sticks, HardDisk, SD card and also from a range of personal devices like mobile phones, portable media players likeApple® iPod, Microsoft® Zune etc.<strong>Audio</strong> rendering challenges - The acoustic sound stage, the listening environment coupled with thedemand for hi-fi sound quality in the car poses a significant challenge in the acoustics design and audioprocessing. Additionally, the environment noise, both due to the engine and the surrounding, results in1 | P a g e

ITTIAM Systems – White paper – ARM Cortex-M4Processor Requirements for <strong>Audio</strong> <strong>Processing</strong>In this section, we will analyze the processor requirements for efficient audio processing. Theaudio processing modules can be broadly classified into 3 categories – MAC (Multiply and accumulate)intensive code, control code, mix of MAC and control code. Each category has specific traits anddifferent processor capabilities for efficient audio processing. The table below lists the traits andprocessor requirements for audio processing discussed in the earlier section.<strong>Audio</strong> <strong>Processing</strong> ModulesCategory Example Traits Processor RequirementSignal <strong>Processing</strong> module High precision multiply High precision MACMACintensiveFiltering modules Repetitive operations SIMD capabilityE.g. FIR, IIR FiltersDifferent bit-widths for theinput dataMixed bit-width arithmeticWindowing modules Saturate arithmetic Saturation logicControlcodeEntropy decoding Code flow data dependent Bit manipulation operationsE.g. Huffman, Run length Input data treated unsigned Unsigned operationsBit stream processing Bit stream operations Compact code sizeMix ofcontrol andMAC3 | P a g eTransform modules MAC, MSU operations DSP instructionsE.g. FFT, MLT, Filter bank Operations on twiddles Packed data processingTable 2 – <strong>Audio</strong> processing modules and their processor requirements for efficient processingFrom the table, the processor requirements for audio processing can be summarized as high precisionMAC, DSP instructions, SIMD capability, mixed bit-width arithmetic, packed data processing andsaturated arithmetic.ARM Cortex-M4In this section, we will explore the features of the Cortex-M4, the latest processor core fromARM. The Cortex-M4 core, in the Cortex-M series of microcontrollers, is a successor to the muchsuccessful Cortex-M3 processor. Inheriting all the features of Cortex-M3, like high code densityadvantage due to Thumb®-2, Cortex-M4 scores over other microcontrollers with new features additionslike DSP instruction set and floating point unit (FPU). The Cortex-M4 features along with sampleinstructions and corresponding cycles are listed in Table 3.Features: In audio processing, since the bit-width of operands depends on factors like filter stabilityand input data precision, mixed bit-width arithmetic operations which allow operands of different bit-

ITTIAM Systems – White paper – ARM Cortex-M4widths become essential. The packed data operations allow operands to be packed in registersalleviating any register crunch in kernel loops of complex modules like FFT. DSP instructions with SIMDcapability perfectly exploits the repetitive nature of signal processing modules. The high precisionmultiply and accumulate (MAC) unit takes just 1 cycle for multiplying up to 32 bit data with another 32bit data giving 64 bit accumulated result. These features of Cortex-M4 are advantageous for audioprocessing, where high precision is required to maintain ‘perceptually transparent’ audio quality.ARM® Cortex-M4 Processor featuresProcessor Feature Cycles Instructions ExplanationHigh precision MAC unit 1 SMULL, SMLALDSP Instructions 1 SMUAD, SMLAD64 bit result from 32 bit multiplied with 32bit data, with or without accumulation32 bit result from addition of dual packed 16bit multiply, with or without accumulationMixed bit-widtharithmetic1SMULWB,SWLAWT32 bit result from 32 bit multiply with packed16 bit data, with or without accumulationPacked data processing 1 SMULBB, SMLATB32 bit result from packed 16 bit multiply, withor without accumulationSIMD capability 1 SHASX, SADD16 Dual 16 bit arithmetic operationsSaturated arithmetic 1 QADD, SSAT Addition with saturation, Signed SaturateTable 3 – Cortex-M4 features and example instructions and cyclesWith new features like DSP instructions with SIMD capability, high precision MAC unit, Cortex-M4 is amicrocontroller with DSP capabilities, a Digital Signal Controller (DSC). Traditionally audio processing isimplemented on a Digital Signal Processor (DSP) while General Purpose Processor (GPP) takes care ofthe system features. With the introduction of M4, audio processing can be efficiently implemented onCortex-M4 since it is ideally suited for audio processing application, like automotive audio solution.Ittiam <strong>Audio</strong> Solution on Cortex-M4Ittiam’s audio solution on Cortex-M4 processor comprises of low power audio codecs and highquality post processing components. The typical performance for few of the audio components onCortex-M4 processor is given in Table 4.The audio codecs have been optimized by making the best use of the DSP instructions and SIMDcapability that Cortex-M4 processor offers. The performance optimized audio solution enables efficientuse of processor MHz resulting in a significant amount of free processor time. This would enable theprocessor to be used for other applications alongside. Similarly, the post processing components achievehigh quality without overhead in performance by efficiently utilizing the high precision single cycle MAC4 | P a g e

ITTIAM Systems – White paper – ARM Cortex-M4unit. These features of Cortex-M4 together with high code density due to Thumb-2, enables Ittiam’saudio solution to achieve high quality at low power.ITTIAM <strong>Audio</strong> SolutionTypical MHz onCortex-M4<strong>Audio</strong> DecodersMP3 DecodeAAC DecodeWMA Decode7 – 10 MHz<strong>Audio</strong> Encoder MP3 Encode 12 – 15 MHzLoudness Compensation<strong>Audio</strong> Post <strong>Processing</strong>Parametric Equalizer (5 bands)Dynamic Range Control4 – 12 MHzChannel FaderTable 4: Processor resource utilization for audio codecs and post processing on Cortex-M4All the above performance numbers are assuming zero-wait state memory configuration. Theperformance on hardware will vary with actual memory configuration. These typical performancenumbers are for stereo mode and 44.1 kHz sampling frequency configuration.SummaryIn this whitepaper, we discussed about a typical In-Car audio infotainment application, a few keysystems and their requirements. Later, we analyzed the processor requirements for performing efficientaudio processing. The features and capabilities of ARM Cortex-M4 processor for audio processing wereexplored. We saw that Cortex-M4 is well suited for audio processing given its low power and superiorperformance.5 | P a g e

ITTIAM Systems – White paper – ARM Cortex-M4References:1. The Future of <strong>Audio</strong> Entertainment in the Car – part 1, part 22. ARM Cortex-M4 datasheet, literature and other marketing collateralsDisclaimer: This white paper is for informational purposes only. Ittiam makes no warranties, express, implied orstatutory, as to the information in this document. The information contained herein represents the current view ofIttiam Systems on the issues discussed as of the date of publication. It should not be interpreted to be acommitment from Ittiam, and Ittiam cannot guarantee the accuracy of any information presented after the date ofpublication.Complying with all applicable copyright laws is the responsibility of the user. Without limiting the rights undercopyright, no part of this document may be reproduced, stored in or introduced into a retrieval system, ortransmitted in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise), or forany purpose, without the express written permission of Ittiam Systems.Ittiam Systems may have patents, patent applications, trademarks, copyrights, or other intellectual property rightscovering subject matter in this document. Except as expressly provided in any written license agreement fromIttiam Systems, the furnishing of this document does not give you any license to these patents, trademarks,copyrights, or other intellectual property.© 2010 Ittiam Systems Pvt. Ltd. All rights reserved.All other copyrights, brands, trademarks and product names are the property of their respective holders.6 | P a g e