AAC Coding Algorithms
AAC for ACCESS Licensed by Fraunhofer IIS
Overview
When Comrex developed BRIC Technology (Broadcast Reliable Internet Codec) it was mindful of the challenges that broadcasters would encounter in the field when trying to use a variety of wired and wireless IP circuits. The main challenge when using public Internet connections is that thousands and thousands of users are in contention for use of the same resources. To make effective use of this worldwide data infrastructure we developed some very aggressive, intelligent coding algorithms that are capable of allowing transmission of high quality audio over very constricted data channels at extremely low data rates. While BRIC-HQ1, BRIC-HQ2 and BRIC-ULB (these are the standard coding algorithms found in the ACCESS Stereo BRIC IP Codec) are more than suitable for the majority of remote broadcast applications they are not what some would consider "STL quality" audio.
In order to provide both 20 kHz STL quality audio as well as future compatibility with codecs from other manufacturers, Comrex has implemented MPEG-4 AAC, MPEG-4 HE-AAC, MPEG-4 HE-AAC v2, MPEG-4 AAC-LD and MPEG-4 AAC-ELD licensed by Fraunhofer IIS for ACCESS. These algorithms are available as an optional AAC Option package for ACCESS codecs. AAC-ELD algorithms are included as part of the AAC Option package but requires ACCESS Firmware version 2.5 or later. Please contact your favorite Comrex dealer for pricing of the AAC Option for ACCESS.
Deployment
The Comrex AAC Option adds encoders for all the listed AAC modes. All ACCESS units running version 2.0 firmware and above will have decoders enabled for AAC by default, without the AAC Option. Comrex customers who purchase the AAC Option for ACCESS will receive the upgrade as a license key via email. The license key is serial number specific and can only be installed to a single ACCESS device for which the license was purchased. Units can also have the AAC option pre-installed at the factory. For more details please contact sales@comrex.com.
AAC-ELD algorithms are available in ACCESS firmware version 2.5 or later. Existing AAC Option owners will simply need to update their AAC Option enabled ACCESS units with the 2.5 firmware update. Once complete, the AAC-ELD algorithms will be made available in the list of available encoders in the Profile setup menu. For the latest ACCESS firmware updates, go to www.comrex.com/updates.
AAC Coding Algorithms
AAC or Advanced Audio Coding is part of the MPEG-4 international standard for coding of audio and video in a digitally compressed format. This standard is described as a continuation in the development of the previous standards, providing a much more efficient method of encoding than its predecessors MPEG-2 Layer 2 and Layer 3.
The Comrex AAC package for the ACCESS codecs includes:
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AAC - Provides near transparent audio at a range of data rates. AAC has been extensively tested by critical listening studies and has been judged to be virtually indistinguishable from the original audio. |
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HE-AAC - Enhances the AAC standard by adding Spectral Band Replication. This is a way of delivering equivalent performance to AAC at lower rates. SBR is applied to high frequencies of audio, while normal AAC is applied to the lower frequencies. This is the standardized version of what has been known in the industry as AACPlus™ HE-AACv2 - Enhances the HE-AAC standard by adding a special form of parametric stereo coding to further reduce data rates. |
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AAC-LD - Requires higher data rates but provides near transparent voice and music with low delay. |
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AAC-ELD - Combines the best aspects of low-delay audio coding with Spectral Band Replication to reduce bitrate. This combination creates a coding algorithm that is uniquely suited to real-time IP transmission. Using AAC-ELD with ACCESS allows full-fidelity high quality audio transmission at a data rate that easily fits into satellite and 3G wireless upload channels. One way ACCESS transmission delay using AAC-ELD is an easily-managed 50 milliseconds. |
| Mode | Bit Rates (kb/s) | Audio Bandwidth | Coding Delay |
|---|---|---|---|
| AAC | 64, 96, 128, 256 | 20 kHz | 300 ms |
| HE-AAC | 48, 64, 96 | 20 kHz | 300 ms |
| HE-AACv2 | 18, 24, 48 | 20 kHz | 300 ms |
| AAC-LD | 96, 128, 192, 256, 384 | 15 kHz | 80 ms |
| AAC-ELD | 24, 36, 48, 64, 96 | 20 kHz | 50 ms |
Additional Reading
More information on AAC is available on Fraunhofer IIS website at www.fraunhofer.de
For a highly technical, deeper dive into advances in coding technology, visit www.wikipedia.org