[Technology Report]
Audio Codecs—The Entertainment-DSP Connection
Standard compression algorithms and proprietary post-processing code challenge DSP chipmakers to deliver more MIPS at fewer watts.
There are audio codec ICs that comprise an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC) around some processing hardware, but they are the tip of the proverbial iceberg. These days, most engineers think of audio codecs in terms of compression and decompression algorithms that run on DSP platforms. In the range of audio frequencies, "voice" codecs suit digital telephony, while "audio" codecs (as the term is generally understood) fit entertainment audio.
In that sense, audio codecs evolved from Dolby Labs' noise-suppression technologies into Dolby's and others' more comprehensive compression schemes. They also emerged from the International Standards Organization's Motion Picture Experts Group (MPEG), a working group that develops standards for coded representation of digital audio and video.
While voice-codec technology is somewhat static (no pun intended), audio-codec technology continues to evolve (Fig. 1). For one thing, there's a trend toward ever more channels of surroundsound. On top of that, add in the technologies for simulating multichannel audio in binaural systems and post-production techniques to replicate the "presence" of specific live concert venues. Then there's the challenge of doing all that processing—not with DSP engines running off of the fat power supplies inside AV receivers or DVD players, but rather off the batteries in a cell phone or PDA. Combine it all, and you have an interesting story in terms of different but converging applications areas—personal audio and home theater.
PERSONAL AUDIO Randy Cole, chief technology officer for Texas Instruments' portable audio/infotainment business, notes that the most ubiquitous audio codec in the personal audio space is MP3 (Fig. 2). Once limited to PCs and portable media players, it's now closing in on the cell phone industry as makers continuously search for new functions that induce end users to upgrade every six months.
MP3 is a standard published by the International Standards Organization (ISO). It was the third in a series of MPEG-1 codecs developed by MPEG. MPEG-1 had three layers, each of which included the previous layer. So, layer 3 incorporates layers 1 and 2. Out of all this, MPEG-1, layer 3, popularly known as MP3, offered a low enough bandwidth and data rate that it could be used in portable applications.
Over the past decade, MP3 has been the dominant audio codec for downloading music. But Apple's iPod has thrown the spotlight on a new MPEG development called the Advanced Audio Encoder, or AAC. MPEG began working on AAC in the mid-1990s as part of its next-generation MPEG-2 effort, incorporating the best new ideas developed at AT&T, Dolby, Fraunhof, and Sony. Originally, it was to be backward-compatible with MP3, but that goal could not be achieved.
Therefore, because the industry was heavily involved with MP3 and no one would commit resources to make a large amount of audio content available for the new scheme, it languished. That is, it languished until Apple picked the MPEG-4 version of AAC for the iPod. (The next MPEG group after MPEG-2 was MPEG-4, skipping 3. MPEG-4 AAC, which is what Apple uses, is an enhanced version of MPEG-2 AAC with a lower data rate and improved quality.)
Other proprietary coders exist beyond MP3 and AAC. They have some penetration in PCs and personal media, but they're less important in the cellular space because the cellular makers prefer standard encoders and their fixed royalties. One familiar proprietary encoder is Windows Media Audio (WMA). It is primarily used in PCs, where it competes with MP3 and AAC. However, it is flexible in terms of data rate (low to high with appropriate differences in quality). Now, there also is WMA-Pro, the multichannel version, and Microsoft recently announced a lossless WMA.
The other significant proprietary encoder is Dolby Digital, also known as AC3. That's the coder used for DVDs and (in the U.S.) digital TV. Until recently, it ran at data rates too high for the Internet or cell phones. Coming to the rescue, though, is a new version that drives the data rate lower.
According to Mohsin Imtiaz, marketing manager for TI's performance audio business, the primary codecs in the home-theater arena are Dolby and DTS. Dolby has announced Dolby Digital Plus, which targets HD DVD and Broadcast. But there's crossover with portable standards such as MP3, AAC, and WMA. For next-generation DVD, Microsoft is pushing WMA.
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