CDs: Friend or Foe?
I threw out hundreds of floppy disks the other day, some of them dating back to the Vietnam War. It was an emotional moment, ditching those little plastic fellas, but they were taking up valuable space, and their contents have long been transferred to CD-ROMs (each of which holds about 500 floppies' worth of data). But then I came across an article in a Dutch PC magazine called PC Active that froze me in my tracks. It said (well, an automated translation of it said, because my grasp of Dutch is limited) many CD brands can't be relied on to store data for more than a year or so, even when you store them properly (out of sunlight, at room temperature, with back issues of The Countryman magazine for them to flip through). Yikes. Could this be true?
Well, after exhaustive research, I can safely say: sort of. The answer is a tad complicated and involves understanding what we do when we burn a CD, but bear with me because it's kind of interesting. (I know, I know, this is where half of you skip to the last paragraph.)
First, what exactly is a CD? It's basically a piece of see-through plastic with very thin layers of other stuff on top. The plastic is there for protection. It's what's in the layers that is important. (A pre-recorded CD -- whether it's a music CD or a software CD-ROM -- is a bit different from one you burn at home. We're talking here about a blank CD -- what's called a CD-R, or CD Recordable.)
Beneath the plastic is a thin layer of dye, which is designed to react to certain frequencies of light. In other words, the dye is basically being cooked by the CD writer's laser -- hence the term "burning." Remember, all this is going on to store digital data -- binary ones and zeros -- so all the laser has to do is change the dye's appearance to create a one, or not change it to produce a zero.
Below the layer of dye is another thin layer, of either gold or aluminum. When the laser reads a finished CD-R, instead of cooking the dye, the laser beam will bounce off the aluminum or gold layer. If there has been no change in the dye, the laser beam will bounce back up directly at itself, generating a burst of electrons in the laser head that are converted to binary digits. If there has been a change in the dye, the laser beam will get confused and scatter in different directions. The not-bouncing-back-to-the-beam-head thing provides the other part of the binary puzzle.
So, why do we need to know all this, I hear you ask? Well, the difference in CD quality that our Dutch friends spotted largely comes down to the quality of the dye and the metal layers. First off, the dyes: I'm going to have to use a couple of long words here, but I promise not to overdo it. There are three main types: phthalocyanine, which is transparent; cyanine, which is blue; and metalized azo, also blue. That's why, depending on whether the metallic layer is aluminium or gold, the CD-Rs you see in shops are sometimes green, blue or gold. OK, that's the last time I'm going to write phthalocyanine, but am I the only person who thinks that's a great name for a drink?
Given these different components, and the complexity of the CD-writing process, you can see why the quality of CDs might vary. Phthalocyanine CDs (sorry, I wrote it again -- that's really the last time, I hope) are supposed to last up to 100 years, but of course, no one knows for sure that they do. Cyanine CDs are supposed to have a higher writing speed -- meaning you can burn them more quickly -- but last only between 10 and 20 years.
All this said, here's my advice to avoid having a cupboard full of CDs you can no longer access.
? Don't buy cheap CD blanks unless they're for disposable stuff: MP3s for a car trip, transporting files from the office.
? Back up your valuable data regularly to good-quality CDs and make two copies.
? When you burn a CD, follow the recommendations on the box. Different CDs work best at different speeds: Burning slowly isn't necessarily better. If your software has a "verify" function to check that the data on the CD you've burned is OK, use it.
? Avoid sticking labels or writing on your CDs. Doing so can affect their spinning speed, and hence their lasers. Also, over time, the chemicals in the pen, the glue or the label can seep through the CD's outer protective lacquer and damage the aluminium and dye.
? Store the CDs in a jewellery box in a cupboard away from sunlight and high temperatures. Ultraviolet light and oxygen can react with the dye and disrupt the data.
? In case of natural or man-made disasters, keep your CD back-ups far away from your computer -- and I don't just mean across the room. Keep them in another building, another postal district, even at your mom's place.
The harsh truth here is that CDs, though more durable than some other media, are perhaps a bit too fragile to be relied on 100%. I'll give you some alternative ways to save your data another time, but for now, a simple rule of thumb: The cheaper the blank CD, the less chance your data has of making it to the end of the year. Now, who's up for a glass of phthalocyanine? Oops.
光盘－－朋友还是敌人？
那天，我扔掉了几百盒电脑软盘，其中一些的年代可以追溯到越战时期。当时真有些舍不得扔掉这些可爱的小家伙，但它们占去了宝贵的空间，而里面的内容早已被转移到光盘上了（每张光盘能存储相当于约500张软盘的资料。）但随后，我在一本名为《活跃的电脑》(PC Active)的荷兰语杂志上读到了一篇文章，它改变了我的看法。这篇文章说，即使你的存储方式没有问题（让光盘避光、保存在室温下，以及用过期的杂志把它们分门别类地放好，便于浏览），许多光盘存储资料的时间也不会超过一年。这难道是真的吗？
不过，当我进行了详尽的研究后，我可以肯定地说：上述论断有一定的道理。其中的原因有点复杂，并涉及到刻录光盘的整个过程，但我还能忍受，因为它比较有趣。
首先，光盘究竟是什么？它就是一张表面带有超薄涂层的透明塑料片。塑料只是起保护作用，涂层里面的内容才是关键。（预先录制好的光盘，如音乐碟片或软件光盘，与你在家自己刻录的光盘有点不同。我们这里所说的是空白的光盘，即可读写的光盘。）
光盘塑料的下面是一层薄薄的染料，能对特定频率的光线起反应。换句话说，这层染料主要与光盘刻录机的激光起反应，因此有“烧录”一说。记住，所有这些都是为存储数字化数据也就是二进制的1和0。因此，激光只需要改变染料的结构以便创造出1，或是维持染料的结构不变以便创造出0。
在染料下面是另一层物质，或者是黄金或者是铝。当激光读一张已写好的光盘，而不是与染料起反应时，激光束会与这层黄金或铝接触。如果上面的染料没有变化，激光束会自己反弹回去，在激光头处产生能被转化为二进制数字信号的电子簇。如果染料起了变化，激光束会受到干扰，向四面八方散射，进而产生混乱。
那么，为什么我们要了解这一切呢？上面那篇文章认为，光盘质量的不同主要取决于染料和金属层质量的高低。首先是染料：我将使用几个复杂的词，但我保证不会大量使用这种词。染料主要有三种：透明的苯二甲蓝染料、蓝色素以及金属化偶氮基，后面两种都是蓝色的。这就是为什么商店里的空白光盘是绿色、蓝色或金黄色的，具体颜色取决于染料下面的金属层是铝还是黄金。我不会再使用苯二甲蓝染料这个词，但我想用它作饮料的名称会很不错。我是不是第一个这么想的人呢？
由于染料的不同，以及光盘刻录过程的复杂性，你就会明白光盘质量何以会千差万别。据说，采用苯二甲蓝染料（对不起，我又用了这个词）的光盘寿命能长达100年，当然，没人知道这是否属实。据说采用蓝色素的光盘的刻录速度要快一些，但寿命只有10到20年。
综上所述，我将给你几条建议，以免你有一柜无法再使用的光盘。
除非进行一次性处理，例如录制供驾车出游时欣赏的MP3音乐，或把办公室里的文件转移到家中，否则请不要购买廉价的空白光盘。
定期在高质量的光盘上备份重要资料，并拷贝两份。
当你刻录光盘时，遵循产品说明书上的建议。不同的光盘达到最高效率需要的速度不同：慢慢刻录并不一定意味著更好。如果你的软件带有“校验”功能，能检查录制在光盘上的资料是否正确，请使用这项功能。
避免在光盘上粘贴标签或写字。这样会改变光盘以及激光的转速。此外，随著时间的推移，墨水、胶水或标签中的化学物质会渗透光盘外层的保护漆，损害铝和染料。
把光盘放置在柜子中的珠宝盒里，远离阳光和高温。紫外线和氧气会与染料起反应，破坏光盘中的数据。
为了防止自然灾害或人为破坏的侵袭，让备份光盘尽量远离电脑，这并不仅仅是说换个房间。把备份光盘放在另一幢大楼、另一个邮区，甚至放在你母亲那里。
严峻的事实是：虽然光盘比其他一些介质更经久耐用，但它依然有弱点，不可以百分之百地相信它。今后，我将给你提供一些其他保存资料的建议，但目前的一个经验之谈是：空白光盘的价格越便宜，你的资料就越有可能放不到年底。来，让我们来喝一杯苯二甲蓝染料吧？
