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#1 Small Things Come In Lossy Packages |
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It's no surprise to most of us that, by adopting digital acquisition, processing and distribution, we've settled for audio that is audibly degraded relative to its analog antecedents. Emerging acquisition standards, such as high-resolution PCM and DSD, have brought back the easy listening of analog to digital audio. While discriminating engineers sweat the details during production, distributing the finished product is another matter entirely. Distribution, until recently, dictated degraded quality. Take Justin the Freshman. He's quite happy listening to MP3's "near-CD quality" just as his forefathers were quite satisfied with their hideous 8 track tapes. Since today's distribution hot button is the net, something's gotta go when Justin’s lucky to have 56 kbaud. Whether its telecom, broadcast or optical delivery, there's usually not much bandwidth available for our audio data. We can't all afford xDSL just yet, but we can use some discrimination when the client asks us to "make it fit." The search for ways to stuff audio through pitifully puny pipes started with short-word-length PCM, ADPCM and IMA, and advanced to the current quagmire of "standards," MPEG-1 Layer III (MP3), RealSystem G2 and QuickTime 3 (QT3) — all of which share the ability to take PCM source files and squeeze out the inherently redundant data, resulting in a smaller file that usually sounds acceptable. "Sounds acceptable?" Now that's being polite. But, all is not lost. Of late, corporate brain trusts have been coming up with new, widely deployed codecs that actually sound good! Restricted carrying capacity certainly isn't new. Born in the Analog Era, there are several bandwidth–challenged analog standards you may recognize: NTSC and PAL television, AM/FM radio and pre-digital Plain Ol’ Telephone Service (POTS). Those standards relied on peculiarities of the human perceptual system to deliver just enough information to convey a message over a channel with less than full bandwidth. Current distribution channels for digital audio, from satellite TV to CD-ROM, are largely supporting lossy codecs for the same reason. Some examples: All of the codecs mentioned above are classed as perceptual coders. Though invented in the late 1980s at Bell Laboratories, these algorithms have continued to morph just as Bell Labs has morphed into Lucent after the breakup. Engineers at Dolby, QDesign, Lucent and the Fraunhofer Institute for Integrated Circuits (FgH) among others, have thrown every trick in their multidisciplinary book at the development of modern versions. The algorithms these companies developed, as in the analog days of yore, provide bit rate reduction without compromising perceived fidelity. The sidebar shows some features of the low-data-rate king, QDMC v2, and the high-rate winner, ePAC, along with AAC. ePAC, is a highly refined dark horse with the power of Lucent behind it. MP3 is included for reference only as its old-school performance is considerably poorer than the other three.
* (Comments) - IIPP stands for integrated intellectual property protection mechanisms such as watermarking and encryption. Also, WMA is not a codec but is a framework for several specialized codecs. How do perceptual encoders work? Let's look at AAC, as its not cloaked in secrecy like QDMC and ePAC. Both ePAC and MPEG-2 Advanced Audio Coding incorporate algorithms from Lucent's PAC, dating from 1992. AAC is one of the codecs of the MPEG-2 standard and is a subset of MPEG-4, the new unified family of ISO standards for delivery of rich media. Major contributors included the FgH, AT&T, Dolby and Sony. As with other offerings, AAC employs perceptual subband/transform coding, whereby the signal is first transformed from the time domain into the frequency domain using a variable window or block length. The encoder then applies a psychoacoustic model to estimate whether, in any particular band of frequencies, the signal strength is above or below the perceptual threshold relative to the adjacent bands. If the signal is above the masking threshold, a spectral coefficient or value is generated to represent the signal in that band. Masking threshold means the amplitude threshold below which a spectral component will be "hidden" or masked by louder components at frequencies nearby. Its a brain thing, just go with it. Once all the valid coefficients are determined, AAC applies additional mechanisms to enhance coding efficiency, including: joint coding, which removes monaural redundancy in a stereo signal; temporal noise shaping prediction, which distributes quantization noise over time; and lossless Huffman coding. You can be sure that lossy codecs won't be going away anytime soon, based on the proliferation of terrestrial (and satellite) distribution of rich media, the emergence of solid-state personal stereos and record labels—large and small—betting on a hybrid revenue model of on–site advertising with the enticement of free track downloads. So, when a project comes knocking that allows your input, do what you're paid for. Use your ears first, then make an educated choice from the new codec menu. To help you get a feel for which one is appropriate, I've posted sample files on my site for your evaluation. Stop by www.seneschal.net and follow the Papers & Articles link to take a listen (see below). Though I've written for Mix as far back as 1987, "The Bitstream" is my first column for this magazine. I'll try to provoke some thought about the technological foundation for our industry and answer questions you may or should have. The conflux of audio and computers forms the basis for discussion and there's plenty out there to cover. I'll also, on occasion, wander into other subject areas while eluding my editors… by Oliver Masciarotte Bio — Oliver Masciarotte is an audio craftsmensch and new-media consultant. He recently returned from Japan where he worked to bring up a new, integrated DVD-A/V installation for Prime Mix/Tokyo and the HMC. Take a listen, then you decide… Click to go to the Audio Samples page. Related Links - Players & Servers Players & Broadcasters - The MP3Car site, despite the name, has a good listing of commercial and home-brew mobile and fixed hardware players and a crappy listing of player software Servers Icecast has a streaming audio server for Intel, being developed under the GNU open source umbrella... Liquid Audio started the whole secure download trend. These days, they’re long in the tooth and short of viable offerings. Microsoft has it's Windows Media Player. Its cross-platform but their server is anything but. Nullsoft's SHOUTcast Server is available for the following platforms: Windows 9x/NT/2000, FreeBSD, Linux, and Solaris. For open source audio-related projects, such as Darwin servers and LAME encoders, see Seneschal’s Linux, Open Source & Unix Audio links. Links - Music Downloads & Streams Angry Coffee is a "technical information resource for novice to expert users who want to learn about and develop audio on the Internet." Bearshare, an enhanced Gnutella file sharing tool for Win. Gnutella, a true peer-to-peer network, lives on. imesh, another enhanced P2P application for Win. Listen.com has multiformat music for download. Mouse Jam has QuickTime-enhanced interactive music. MP3.com has, guess what? Beware, they will spam you to distraction!
Napster, now in a coma, combined a chat client with an MP3 player, tossing in other features to make an interesting product. Interesting enough to clog campus networks and send Intellectual Property zealots into a tizzy... Links - Various Flavor Software provides engineering services for building MPEG 1 through 4 products and services Replace the for–pay CDDB with the open source freedb€works for most well–coded players and rippers The ISO's Motion Picture Experts Group home page has a comprehensive FAQ and other resources. Music Enforcer checks "all servers on all services for illegal material - giving you enough identifying information to either submit a ban on the users from the service, or to hold them responsible in a court of law." The Xiphophorus company is a distributed group of Free and Open Source programmers working on OggSquish, a group of related multimedia and signal processing projects. | ||||||||||||||||||||||||||||||
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