在线音乐服务需要改进音频

[This article appeared in the April/May issue of 流媒体 magazine under the title "Sound in the Cloud."]

最后的 year was the year of Facebook (or at least the year of Mark Zuckerberg) the movie (和 the ubiquitous nerd). 但2011年将成为流媒体音乐之年, 在这个庞然大物的背后并没有单一的个性, 甚至连史蒂夫·乔布斯都没有, whose streaming music proposition for iTunes is reportedly launching this year. It will have to compete with a couple dozen other streaming music entities. 这包括掌舵巧妙的潘多拉, which has been cutting deals to get under our skins through whole-house automation systems 和 automotive audio. Spotify, the Swedish service that gave music streaming its best traction in Europe with 10 million subscribers, 一直推迟到美国的时间.S.美国仍然是世界上最大的音乐市场，音乐销售额为4美元.2009年为60亿美元. 最后的.fm，美国.K. CBS于2007年收购了该服务, topped 40 million users 和 offers a well-rounded array of content besides music. Others have been waiting for the techno-entertainment culture to migrate from ownership of file-based media to being able to pull it down from anywhere, 任何时候.

The shift from downloading to streaming is tectonic by digital st和ards, 和, 就像之前从光学媒体到下载的转变, it will be accompanied by much teeth gnashing 和 lawyer wrangling as royalty payment st和ards are worked out as much in the courts as in the smoke-free back rooms of Palo Alto, 加州.. In fact, London’s Financial Times asserts that attempts by Spotify 和 others to launch in the U.S. have been delayed mainly by the complexity of negotiating licensing terms with record companies 和 music publishers, 而不是底层技术.

流媒体音乐的声音

但 what the transition means for the audio quality of music is less murky to parse. 就像下载文件找到一个发烧友频道一样, 增加了256Kbps的版本，甚至还有无损WAV, FLAC, ALAC文件在互联网上传播, streaming comes along 和 presents music with an even broader l和scape.

“Wouldn’t it be nice if one of these days people who write music 和 people who write code could actually get together with each other?” asks Robert Reams, a tech entrepreneur who sold his most recent company, Neural Audio, to DTS, Inc. 两年前. He says that the processing of music is going to have to change to accommodate streaming. Reams says that spatial (imaging) 和 temporal (timing) differentials between the left 和 right axis of music are to blame for most of the artifacts that lossy codecs such as MP3 和 AAC create when they process music.

“Dynamic spatial 和 energy offsets within the content affect the coding efficiency of modern lossy codecs,Reams解释道. “Any time the content is mixed [in such a way as to create these conflicts], 声音可能被渲染成不必要的瑕疵. 线性格式[i].e., 磁性介质]在草率的空间映射方面是宽容的, 感知无关谱, 和噪音. 有损编码则不是.”

Reams所指的补偿是作为音乐组合的一部分而创建的, 哪一个本质上是不平衡的. Differences of as little as 1 dB between the same 信息 in each stereo channel can cause the codec to view it as an error rather than a work of art. 在理想的情况下, the effect of a codec should be addressed at the point the music is mixed (和 MP3 developer Fraunhofer-Gesellschaft debuted something exactly like that at the NAMM Show in Las Vegas in January, 我们马上就会讲到). 但, 大量承认, with the biggest news of last year being the arrival of The Beatles on iTunes, the vast majority of music that will be streamed will be legacy content, 从1岁到50岁都可以.

“如果主甲板的死角没有完全对齐, the audio on the left channel may not be perfectly synchronized with the audio on the right, 在这之前或之后. An offset of even one sample can generate enough energy to induce a problem with the [codec] processing,他解释道.

干净的流媒体音乐

蒂姆·卡罗尔，线性声学公司的创始人和总裁., 哪家公司生产用于流媒体和文件传输的处理设备, emphasizes that starting with a clean track is crucial to music surviving the streaming process with as much integrity as possible. “Most codecs are pretty benign these days—AAC at 224Kbps compared to what we’ve been used to is pretty spectacular, 和 the HE-AAC data rate can be lower but still maintain the same quality,他说.

然而, the data rates needed to stream fully lossless music files are high 和 are not going to be the norm in the streaming environment 任何时候 soon, 尤其是手机用户. 因此, 卡罗尔的压力, the music-mastering stage of the record-making process will be critical, 和 it will have to work even harder to avoid migrating the problems caused by the ongoing “loudness war” to the streaming environment. 这个短语指的是没有说出口的话, 但是非常真实, 音乐内容发行商之间的竞争(i.e., record labels) to digitally master 和 release recordings with higher real, 和感知, 响度等级. 唱片公司和艺术家们都在寻找能在听觉上脱颖而出的唱片, 只要大声一点, 无论是在电台还是在俱乐部. The practice goes back decades, to the days when Motown Records founder Berry Gordy Jr. would have his staff engineers analyze the top 10 records every week, 包括它们的相对音量, 这样他就可以有一个摩城单曲响度的基准. 结果是一张吵闹的唱片, 感知来说, 但它往往是声音的糊状物, 在艺术上来说, with the dynamic range of each individual instrument blunted 和 subsumed by the force of the track’s overall level.

“当你掌握了一个(音轨)到剪辑的地步, 每一个瞬态, 就像踢鼓, 变成了方波, 和 the processing engine is going to have the hardest time trying to code that, 因为[剪辑]会产生干扰编解码器的谐波,卡罗尔解释道. “That’s just bitrate wasting, because the codec is having to work harder. 编解码器会因剪切工件而阻塞. 在像流这样的低比特率环境中, 这个游戏的名字从一开始就是清洁, free of clipping 和 square waves 和 overloads; stuff you can’t recover from.”

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