jitter是如何被发现,被公认的
偶尔读到一篇美国著名音响评论家,音响专业人士Robert Harley (TAS的主编)写的文章Time is Everything (时间就是一切). 里面有一大段是讲述人们是如何从对jitter一无所知,到最终公认jitter的经历,我觉得很有启示性,在这里选译出来,给大家参考. 直播- -.. 搬个板凳听课 大家知道,在数码音频刚问世的阶段,整个音响界普遍大声为数码音频叫好,认为模拟音频可以马上走进历史垃圾堆了. 当时的人们因为还看不到数码音频的缺点,普遍认为数码音频是完美的.在文章里,Robert Harley这样写道:
The advent of digital audio was heralded by proclamations that the sound-quality variability inherent in analog systems was a thing of the past. Once an audio signal had been digitized, the conventional wisdom held, it was immune to degradation. If the bits were the same, the sound was the same. Digital audio either worked perfectly, or it didn’t work at all.
数码音频的诞生使得很多人认为,模拟音频系统里的音质可变性(音质会逐渐劣化的现象)会成为历史. 一旦一个音频信号被数字化,人们普遍认为,它的音质就不会再劣化. 只要数码的比特数保持不变,声音就是不变的. 数码音频要么将完美地工作,要么就根本不能工作(错误).
This was, at first glance, a startling advancement over analog systems, which introduced slight (or not so slight) cumulative distortions at every turn. Put an analog signal down a piece of wire and it degrades, not to mention subjecting the signal to every other component in the signal path.
初看起来,数码音频的这个特点,使得它对于模拟音频系统来说,是一个巨大的进步. 模拟音频系统会引入失真,并且失真会逐步累加. 将一个模拟音频信号引入一根导线,它就开始劣化,更不用说将信号引入HI-FI系统中的其他音响器件.
But beginning in the mid-1980s critical listeners reported hearing differences where none should have existed.1 Using observational listening techniques, audiophiles noticed musically significant variations between coaxial and TosLink connections, brands of digital cables, and even in the directionality of digital cables themselves. It was an easy matter to prove that the bitstreams were identical—how could the sound change? If the sound is different then the signals must also be different, but in what way were the signals different? What was this mysterious “X factor” that caused identical digital bitstreams to exhibit an analog-like variability?
但从八十年代中期开始,一些细心的听众开始注意到,他们听到了原本不该存在的音质差异. 音响发烧友们注意到同轴和光纤连接会造成显著的音质差异,并且不同品牌的数字线,甚至数字线的不同接线方向,都引起音质的不同. 很容易验证,不论是什么情况,数字音频流是相同的. 那么怎么可能音质不同呢? 假如听到的声音不同,那音频信号肯定是不同的.但信号怎么会不同呢? 这个神秘的,造成相同的数字流竟然会出来不同音质的"X因素"是什么呢?
[ 本帖最后由 小白 于 2009-11-14 16:10 编辑 ] 好，大家看一下。
上次哪位兄弟作电影播放的比喻很形象啊。 This conflict between what audiophiles heard and what the audio academics told us was impossible quickly became a wedge between the high-end and the audio-engineering establishment, particularly members of the Audio Engineering Society (to which I, too, belonged at the time). The “bits is bits” view of digital’s perfection was an article of faith among the AES faithful. The idea that bitstreams with the same “ones and zeros” could sound different when converted to analog by the same digital-to-analog converter was viewed as the epitome of audiophile lunacy, and the high-end community was subject to open ridicule, scorn, and hostility. For example, at an AES conference I attended in London in September, 1991, John Watkinson, a respected engineer, author of several textbooks on digital audio and video, and a Fellow of the AES, used his time addressing the society to attack audiophiles on this point: “Somehow I can’t conceive of an audiophile ‘one’… You can only say that once, which is a problem if you have to publish a hi-fi magazine every month. It leaves an intellectual vacuum…When the term ‘audiophile’ replaced ‘hi-fi freak,’ I immediately thought of necrophiles (sic) and pedophiles. Perhaps I wasn’t far off.’”
发烧友们靠耳朵听到的现象,和音响学术界认为这不可能的观点,使得HI-END发烧圈和专业音响界,特别是AES(音响工程学会,Robert Harley也是该学会的成员)的会员,两个群体之间出现观点分歧."bits is bits"的观点,即认为数据相同,声音就不可能不同的观点,当时在AES看来是一个基础信念. 当时AES认为那些说相同的0和1竟然会出现不同音质的人,只能是典型的"发烧疯子". HI-END发烧圈饱受嘲弄,讽刺和敌视. 举个例子,我1991年9月参加了AES在伦敦的一次会议.在会上,一位受人尊敬的音响工程师John Watkinson,他曾写过好几本关于数码音频和数码视频的教科书,同时也是AES的成员,做了一次讲话,以攻击发烧友们. 他说道: "我无法想象出一个发烧级的1 ...... 你只能说那个数据相同,音质必定相同的1. 当然后者会造成一些问题,因为你必须每月出版一册HI-FI杂志. 当人们用'发烧友'这个词来指代'发烧怪人'时,我立即想到了necrophile(恋尸者)和pedophile(恋童者),也许我并不是太夸张." 题外话,我必须指出一下,这位受人尊敬的John Watkinson先生不知道今天是否还健在,是否承认jitter,但他这番话中对发烧友的恶意攻击,把发烧友比喻为恋尸者和恋童癖,是非常过分的. Nonetheless, critical listeners accepted the reality of what they heard, and high-end cables designers developed better-sounding digital cables. The academics continued to reject the idea of an analog-like variability in sound quality between two identical bitstreams. No researchers wanted to touch the subject for fear of ridicule from their colleagues. None of the test-equipment manufacturers thought about investigating the phenomenon—or developing instruments to measure the effect. Sonic differences were, in their view, purely the result of overactive audiophile imaginations.
然而,细心的听者们还是接受了他们耳朵听到的事实. 发烧线材制造商们造出了音质更好的数码线. 学术界继续排斥那种认为数码音频也会象模拟音频一样音质出现变化的观点. 学术研究者们都不愿意触及这个问题,因为担心被同行们耻笑. 测试仪器制造商们都没有想到去研究这个现象,或者开发出能测试这个现象的仪器. 他们认为,有人所听到的音质差异,只是反应过度的发烧想象.
I learned during a 1989 press trip to JVC’s laboratories in Japan that timing inaccuracy—called “jitter”—in the digital-to-analog conversion process introduced an analog-like variability in digital playback. JVC had developed a circuit it called the “K2 Interface” that reduced jitter. The engineers explained the fundamental principles involved, along with K2’s circuit details, and then proceeded to demonstrate the salutary effect of the K2 circuit. I had one of those “Ah, ha!” moments when I first understood how timing inaccuracy could cause two identical bitstreams to sound different. The JVC engineers had known about jitter for years and treated it as simply another engineering challenge to overcome.
在1989年去拜访日本JVC公司实验室的过程中,我了解到了,数字-模拟转换过程中时间的准确度——称为jitter——会造成重放时音质的变化,就象在模拟音频系统中的音质变化那样. JVC开发了一个叫做K2的界面,以减轻jitter. 工程师们向我解释了一些基本理念,以及K2电路的细节,然后向我演示了K2电路的实际正面效果. 我开始惊讶地了解到时间的不精确会导致两个相同的数字音频流出现不同的音质. JVC的工程师们知道这个已有数年,把这个问题当成一个必须克服的工程障碍.
The “paradigm shift”that transformed jitter from audiophile lunacy to textbook orthodoxy began with an October, 1991, AES paper titled “Is the AES/EBU/SPDIF Interface Flawed?” by Malcolm Hawksford and Chris Dunn. (The AES/EBU interface is the professional implementation of the consumer SPDIF interface.) Hawksford was a professor at Essex University, and Dunn one of his students. (Hawskford is now Director of the Centre for Audio Research and Engineering and Director of Postgraduate Studies within the Department of Electronic Systems Engineering.) Hawksford and Dunn laid out in precise and fascinating detail exactly how the digital interface can introduce an analog-like variability in sound quality while preserving bit-for-bit accuracy. Hawksford was no wild-eyed audiophile making claims about the audibility of a mysterious phenomenon; he was (and is) a highly respected researcher and university professor with a large body of original research. The paper shocked the audio community. Working audio engineers, who assumed the digital interface they used on a daily basis was sonically transparent, became alarmed and began experimenting themselves. Audio academics grudgingly acknowledged the phenomenon described by Hawksford and Dunn, but questioned (at least initially) how that phenomenon translated to sound-quality variations.
把jitter正式从"发烧疯子的理念"转变为教科书正统理论的过程,开始于1991年10月,当时AES发布了一篇名为"AES/EBU/SPDIF数码界面有缺点吗?"的论文. 作者是Malcolm Hawksford和Chris Dunn. Hawksford是Essex大学的一位教授,Dunn是他的一名学生. 这两位作者在文章中给出了精确而引人的细节,解释数字传输界面是如何导致音质变化,而同时从数据说又是无比精确,一个比特也不差的.Hawksford并不是一位激进的发烧友,声称可以听出别人听不到的神秘现象. 他是一位受人尊崇的学术研究者,大学教授,进行了大量的实际研究工作. 这份论文震动了音响圈. 音响工程师们,原本认为他们每日里使用的数码界面是完全"透明"的,被这篇论文惊醒了,开始了各自的试验. 音响学术界的人士很不情愿地承认了Hawksford和Dunn描述的现象,不过提出质疑(至少一开始是这样),这一现象是如何导致音质变化的.
[ 本帖最后由 小白 于 2009-11-14 19:03 编辑 ] By the mid 1990s, jitter, and its audible effects, became established fact. Test equipment manufacturers offered jitter-analysis devices. Other academics published papers on jitter. Manufacturers of professional audio equipment began touting the low jitter in their products. Today, it’s as though the raging debates of the late 1980s and early 1990s never happened—jitter is now accepted as a source of degradation in digital-audio recording and reproduction.
到九十年代中期,jitter以及它造成的音质差异,已经是一个被大家认定的事实. 测试仪器制造商们搞出了jitter分析仪器,更多的学术研究者发布了关于jitter的论文. 专业音响器材制造商们开始宣传他们产品的"低jitter". 到了今日,上世纪八十年代晚期到九十年代早期的那阵争辩,似乎已烟消云散,从未发生过. jitter已被公认为数码音频录音和重放的音质劣化的一个源头.
1 The first such published report I’m aware of is Stereophile founder J. Gordon Holt’s review of the Sony CDP-605ES in 1986, in which Gordon reported—almost incidentally—that the coaxial connection sounded better.
注1: 我所知的第一篇报道,是Stereophile杂志的创办人J. Gordon Holt在他1986年写的一篇SONY CDP-605ES的评论文章里,几乎是在无意间指出,数码同轴连接的音质更好. 鸡特儿渊远流长啊，截个楼，等吵架。;P .
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︶ ︶ ︶ 翻译完了. 感叹一句: 当时发烧友和学术界之间的分歧,最后看来还是发烧友赢了,其实是耳朵赢了. AES,这个专业音响界的权威组织,至少一开始他们的理念是完全错误了.
当然发烧友只能给出一个现象,不能去做理论的阐述和解释. 理论性的工作还是要专业人士去做.
还有很值得记住的一个事实是,JVC的工程师们虽然默默无闻,也没有发表什么论文,但其实早已认识到了jitter的存在,并且早已在设法克服它.
所以我个人一直是这个信念: 1) 发烧友的耳朵不可辱. 2) 我们的理论,是在不断进化,丰富中的. 对于数码音频,我们远不能说已全部掌握和控制. 当年我们是婴儿,现在也只是少年而已. 3) 走在最前面的,最领先的,往往是一些厂家的工程技术人员. 他们不写论文,也基本不会参与发烧友的讨论,但他们是做实际工作的人,是主客观结合的人(既做主观聆听,也必须做客观测试). 我们从一些著名的数码音频厂家的产品中,可以看出他们的工作成果,从侧面了解他们的发现.
[ 本帖最后由 小白 于 2009-11-14 17:34 编辑 ] 白板的英文功力可以啊:lol 原帖由 zsmzsm 于 2009-11-14 22:12 发表 http://bbs.headphoneclub.com/images/common/back.gif
白板的英文功力可以啊:lol
有一处没翻对，另有几处没忠于原文意思。但不影响整篇内容的理解，也算是集功劳、苦劳、疲劳于一身 :lol 顺带做个广告：对数字接口Jitter感兴趣的，可以去买一本本月刚出的《无线电与电视》，里面有我的一篇关于数字接口与Jitter关系的文章。
