by June 2, 2003 0 comments



MP3 has become the de-facto standard for distributing music by digital means. Despite Microsoft’s best efforts, neither WMA nor any other standard has quite captured the public’s imagination as much as MP3. But, looking beyond all the hype reveals that MP3 is now an ‘old’ technology. The MPEG-1 Layer 3 (MP3) standard is more than a decade old, and some of the advances made in audio-encoding techniques (keeping in line with the ‘faster, smaller, better’ motto of modern-day technology), makes it look very dated indeed. Step in, AAC (Advanced Audio Coding).

AAC is an encoding technique that was first proposed as a part of the MPEG-2 standard. It has been designed from scratch, which means it does not suffer from any of the pitfalls of the MP3 standard and can fully implement the improvements that have been made in encoding techniques over the years. AAC is also sometimes called MPEG-2 NBC (Non Backward Compatible) because is not backward compatible with MP3 (hence, not with MP1 or MP2 either). This means that you need to get a separate player to play your AAC files–a small price to pay for having a better standard. 

Apple for AAC
A big push for AAC in recent times came when Apple announced its iTunes online Music store, with music being available for download in the AAC format at 99 cents for a single and $9.99 for an album. The fact that the store sold a million units in a week from its launch could be put down to the novelty of the pricing. But, if it sustains, then AAC could get the big push it was waiting for. Currently, the service is available only on Macs with a Windows version slated for a later release.

AAC supports bit rates of 8 Kbit/s to 512 Kbit/s (even higher in some cases). Sampling frequencies range between 8 kHz and 96 kHz, with support of 1-48 audio channels. AAC has been optimized to run best at bit rates of around 128 Kbits/s; the quality it offers around this mark is unmatched by any other audio format. At higher bit rates (>160 Kbits/s), it is second to only the MusePaCk format (which is rather weak at lower bit rates) and at lower bit-rates (<128 Kbits/s) to the Ogg
Vorbis.

This is quite an impressive overall showing. AAC files of the same quality have
a 30% size advantage over their MP3 counterparts. Conversely, an AAC file encoded at the same bit rate as a MP3 file, sounds noticeably better.

The AAC encoding process, though, is not entirely dissimilar to that of MP3. It provides lossy compression using, like all other coding schemes, the signal-masking property of the human ear, which cannot differentiate between two sounds if they are very close to each other. This is why the encoding techniques can ‘get away’ with taking only x number of samples of the audio signals a second, thus, changing it from a continuous to a discrete signal and greatly reducing the amount of information that is to be encoded, without any noticeable change to the user as long as x is sufficiently large. AAC encoding is a very slow process, perhaps one of the trade-offs of it being so effective. AAC decoding, though, is faster and less resource hungry compared to that of MP3.

AAC is an initiative of the group of companies that were behind MPEG as well–Dolby, Fraunhofer (FhG), AT&T, Sony, and Nokia, amongst others. But, unlike MPEG, these companies (www.aac-audio.com) have not made the AAC method of compression available for free. This is the main reason that is stopping AAC from taking off in a big way. AAC, though, is at the core of the next MPEG standard, MPEG-4, and that can only help it gain acceptability. Apple, with Quicktime 6 and its implementation of the MPEG-4 specification, is one of the vendors banking on AAC to take digital audio to an era of high quality with smaller files. Size does matter!

Kunal Dua

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