Contents 1 Helix Fixed-point HE-AAC (aacPlus) decoder 2 Key Features 3 Technical Specifications 3.1 Average CPU Usage 3.2 (1) Tested with ARMulator, simulated zero-wait-state memory 3.3 Notes: 3.4 Memory Usage 4 Frequently Asked Questions 4.1 What features are currently not supported in the aacPlus decoder? 4.2 Where is the code in CVS? 4.3 Where does the build system put the code in my local source tree? 4.4 How do I play .mp4/.3gp streams that contain AAC-encoded streams?

Helix Fixed-point HE-AAC (aacPlus) decoder

The Helix Fixed-point HE-AAC decoder provides an optimized 32-bit fixed-point implementation, which will be dual-licensed under RCSL/RPSL. No extra copyright license is required beyond the standard Helix license terms, although patent licenses are not included and must be acquired from the appropriate AAC licensors.

The Helix AAC decoder is highly optimized for ARM processors and meets or exceeds the performance of other commercial implementations. It also features one of the first widely available fixed-point implementations of SBR.

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Key Features

• MPEG-2 and MPEG-4 low complexity decoding (intensity stereo, MS, TNS, PNS)
• Spectral band replication (SBR), high-quality mode
• Mono, stereo, and multichannel modes
• ADTS, ADIF, and raw data block decoding
• MP4, RM, and other file formats supported via Helix client

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Technical Specifications

Average CPU Usage

1. MPEG4-LC AAC (SBR not used)

Sample Rate	Channels	Bit Rate	Processor Model (1)
			ARM9TDMI-REV2	ARM9E	StrongARM1 (uncached)	XScale (cached)
22 KHz	1	64 Kbps	7 / 9 MHz	5 / 7 MHz	6 / 8 MHz	6 / 8 MHz
44.1 KHz	1	96 Kbps	11 / 13 MHz	9 / 11 MHz	9 / 11 MHz	10 / 12 MHz
44.1 KHz	2	128 Kbps	19 / 23 MHz	17 / 21 MHz	17 / 20 MHz	18 / 21 MHz
44.1 KHz	2	320 Kbps	23 / 30 MHz	21 / 28 MHz	21 / 27 MHz	22 / 28 MHz

2. MPEG4-LC HE-AAC (SBR enabled, "high quality" mode)

Sample Rate	Channels	Bit Rate	Processor Model (1)
			ARM9TDMI-REV2	ARM9E	StrongARM1 (uncached)	XScale (cached)
22 kHz base/44 kHz output	1	48 Kbps	32 / 34 MHz	27 / 29 MHz	27 / 29 MHz	28 / 30 MHz
22 kHz base/44 kHz output	2	64 Kbps	58 / 60 MHz	49 / 52 MHz	49 / 52 MHz	51 / 53 MHz

(1) Tested with ARMulator, simulated zero-wait-state memory

Notes:

• Compiled using ARM ADS 1.2
• Tested with ARMulator, zero-wait state memory
• Results are average CPU usage, typical content
• first MHz value is when TNS is off, second value is when TNS is on

Memory Usage

1. MPEG4-LC AAC (SBR not used)

• ROM (const globals) = 27128
• RAM (heap, max 2 channels) = 20656
• Code size - x86 (MSVC++ 6.0) = 22654
• Code size - ARM (ADS ARM9TDMI) = 20120
• Additional RAM required per channel = 4104

2. MPEG4-LC HE-AAC (SBR enabled, "high quality" mode)

• ROM (const globals) = 37580
• RAM (heap, max 2 channels) = 79580
• Code size - x86 (MSVC++ 6.0) = 47822
• Code size - ARM (ADS ARM9TDMI) = 41764
• Additional RAM required per channel = 17198

Notes:

• Memory usage constant for all sample rates, bit rates
• Does not include input buffer (encoded data) or output buffer (PCM)
• There is no zero-init or read-write global data
• All values are in bytes

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Frequently Asked Questions

What features are currently not supported in the aacPlus decoder?

• main or SSR profile, LTP
• coupling channel elements (CCE)
• 960/1920-sample frame size
• low-power mode SBR decoding
• downsampled (single-rate) SBR decoding

Where is the code in CVS?

• See the Helix Datatype project page: http://datatype.helixcommunity.org
  The CVS root is /cvsroot/datatype, and the module name (path) is aac/codec/fixpt/decoder

Where does the build system put the code in my local source tree?

• datatype/aac/codec/fixpt

How do I build the fixed-point AAC implementation through the "ribosome" build system?

• In the build menu, use one of the following bif files: helix.bif (HEAD) or hxclient_1_5_0_cayenne.bif
• Select the target "datatype_aac_codec"
• Make sure the define "HELIX_CONFIG_FIXEDPOINT", is either in the profile or config (cf) file that you are currently using.

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How do I play .mp4/.3gp streams that contain AAC-encoded streams?

• In the build menu, use the same bif as you have used above
  
• Select the target "splay". This builds a command-line player through which you can play .mp4/.3gp streams
• For instructions to check out source, go to Quickstart Guide
  

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Where do developers go to acquire their aacPlus patent license?

• For information regarding patent licensing for AAC, please see Via Licensing's website.
• For information regarding patent licensing for aacPlus, please see Coding Technologies' website.

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