So I was watching this (https://www.youtube.com/watch?v=WkIR23emsWY) by technology connections and I just realized that I have no idea how CDs work. The video is about how some CD players can read audio of a CD where the data is just copied directly onto a CD with a computer whereas older players only had hardware compatible with CDs formatted as …. digital audio?
I was trying to figure it out on wikipedia, how is a commercial CD you’d buy in a store different from a CD with files copies onto it?
I remember my dad used to use a burning tool to make images. Why are they called images? How is an image different from a CD?
Why are some CDs readable and writable but some aren’t?
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The Audio CD format (often known as Red Book audio standard) defined a very specific way of storing audio as a stream of samples. It’s not a filesystem of files, it’s just audio written as a digital stream with a defined table of contents at the beginning.
“MP3 CDs” as described are a filesystem containing files, the playing device must be able to understand that filesystem and to be able to decide the compressed audio files back to a stream that can be sent to a speaker.
A CD image is a file that contains an entire filesystem of a disc in a single file. This format of data storage is particularly convenient when you want to make a disc that, at the software level, is an exact duplicate of the original source.
When CDs were first brought to the mass market in the early 80s, they were a form of high-end music media, competing for the audiophile space. The distinguishing factor was that they were able to deliver uncompressed audio. As digital audio production took on in the 80s, this meant the CD you listened to was pretty much the exact waveforms the producer heard with the final mix.
These CDs were initially incredibly expensive (an album in 1985 usually went for $20, that’s nearly $60 in today’s dollars), but you got what you paid for – a CD could hold up to 650mb of storage, which was a unthinkable amount of data back then. The caveat here was that CDs were pressed at the factory, and couldn’t be rewritten.
However, by the mid 90s, the production costs of CDs were dropping quickly, and the average person was buying CDs regularly. As costs dropped, another technology that took off was the writable CD – a blank CD anybody could write to, provided they had a burner and the requisite software. That initially was around $1000 in the early to mid 90s, but that price quickly dropped too.
Writable CD media, coincidentally, rose around the same time as the mass adoption of the internet. It didn’t take long before people to put two and two together and realize that they could “rip” a CD of its contents and share it to people over the internet. The problem here is going back to the media – CDs, being the audiophile media, usually contained uncompressed audio. A full length album could easily fill all 650mb. A typical hard drive of the mid 90s was around 500mb; nevermind that most people were also on a 56k modem – averaging .056 Mbps.
The compromise here was that eventually people selected a new file type for the ripped media to share online, one that could drastically reduce audio file sizes. That’s where MP3 came in – it was a file type that could drastically reduce the file size through compression at the cost of audio quality, but that was obviously a trade people were willing to make then in order to share and download music over the internet.
The factory pressed CDs used the Red Book Audio standard, which was a very specific way that you had order and structure the contents in order to be able to be read by a CD player. A “normal” CD player at the time, therefore, couldn’t actually play the MP3s because it wasn’t the Red Book that the player expected. Manufacturers quickly begin to understand the demand though, and added the ability for CD players to begin playing MP3s. Eventually companies realized that the “CD” part of this whole process was unnecessary – which led to the creation of the MP3 player and the rest is history.
Commercial CDs are physically stamped. There is a long groove of high points and low points (lands and pits) that spiral from the center of the disk to the edge that are interpreted as 1s and 0s based on whether or not a laser shone on them reflects back to a sensor or not. Writeable optical media is ink-based. A new CD-R is shiny. When you write data to the CD, the ink is “burned” to make non-reflective spots. So instead, shiny/not shiny are your 1s and 0s. Eventually, the ink will fade. This is why you should back up any optical media you may have created.
(Fun sidenote: CD players used a red laser. If we want to pack more data on the disc, we need the lands and pits to be closer together. This means we need a shorter wavelength of light to read them. In the visible spectrum we all know as ROYGBIV, red is a longer wavelength. Blue is shorter. Hence. Blu-ray.)
A commercial CD that you’d buy in a store, like a music album, is usually what’s called a “pressed” CD. During manufacturing, data is physically stamped into the disc in the form of microscopic “pits” and “lands”. These represent the digital information (as zeros and ones) that is read by a laser in your CD player. These discs are read-only; you can’t change the data on them.
On the other hand, when you copy files onto a CD with a computer, that process involves “burning” the data onto the disc. This happens with a special laser that changes the surface of the CD-R (CD-Recordable). Unlike pressed CDs, these discs have a special dye layer that the laser alters to simulate the same pit-and-land pattern used in commercial discs. When your computer burns the data, it uses this dye to create marks that a CD player can read. CD-RWs (Rewritable) go a step further: they use a special dye that allows the laser to erase and rewrite the data multiple times, however after a few rewrites, these discs can wear out.
Now, these are the differences on the hardware level. However, there are different ways how the information can be stored on the software level. Pure digital audio (also called a “red book” CD) is basically the digital equivalent to an old analog record. It contains only the raw audio amplitude samples from the audio track similar to how an anlog record contains the track as a groove. Its a very simple format, and it can only store raw, uncompressed audio. Music CDs from a store typically are in this format and and old CD players can only play back this.
Contrary to the red book CDs it is also possible to create a full file system on a CD, similar to a USB thumb drive. This way it is possible to store arbitrary files on the CD, not only audio. And the audio that you do save can be in an abitrary format, even compressed like MP3. Reading those CDs is much more complex which is why old CD players cannot do it.
A CD image is just a big file on your computer containing a one-to-one copy of the bits on a CD. This way you can store and copy a CD without having to open or understand its contents.
Man I feel old.
The official CD albums had an uncompressed, digital representation of the music you’d listen to. Think bars instead of a wave. It tells the speaker what signal to output as it reads through it.
MP3 files tossed into a CD on a home computer worked differently. More like read only storage; it had a file system, with a list of the contents, maybe even folders, and the mp3s just tossed on there as files. The most basic CD players couldn’t understand any of this, they were just expecting a sequence of bars to convert into a digital signal.
> I remember my dad used to use a burning tool to make images. Why are they called images? How is an image different from a CD?
An image in this context is a file representation of a storage media. A CD in file form, basically. The burner takes all your songs and creates an image, that then gets burned into the CD.
> Why are some CDs readable and writable but some aren’t?
CD burning is just that: the laser on the reader is physically burning the disk’s surface as it writes to it.
A writable CD needs certain chemical properties to be able to be rewriten. A read-only disk physically couldn’t be written into again because you couldn’t “erase” the old information from it, short of rendering it useless by “burning” its entire surface.
Some early rewritable disks could only be burned a certain number of times before they would go bad.
The original music CDs were basically like vinyl records, but instead of a record player with a needle it is a CD player with a laser.
Record players are really dumb. You put the needle in the groove and the record spins playing everything in the groove then lifting up at the end to stop it. To skip songs you had to look for the empty grooves. They need that exact format to work.
CD players weren’t fancy computers. They had very basic instructions to read the info that was directly translated into analog audio.
Computers can read info from different places on the disc and could decide complicated information and convert it to whatever needed to be played.
Since you’re already watching Technology Connections, I highly recommend going through his series on all the different ways we’ve recorded audio and video.
Let’s start with how digital audio works in general.
Imagine a [suspension bridge](https://cdn.britannica.com/76/22076-050-4F3391DC/suspension-bridge-forces-lines-tension-compression.jpg) and you wanted to record the shape of that top cable. One way to do that is to sketch the same shape on paper. The paper version is an analog of the shape, rising and falling in the same way. You can recreate the bridge by reversing the process and making the cable take the same shape as your drawing.
Digital audio is different. Instead, look at the vertical cables and pretend you measured each one and wrote down the length. What you wrote down is no longer an analog of the top cable, but you can still recreate that shape by cutting new vertical cables and laying a top cable down so it matches those. That’s how digital audio works.
Once you have that list of numbers there’s all sorts of ways to store them. A standard CD is very simple, with all the numbers for each song just written down one after another. There’s a directory that says where to find the tracks, and that’s basically it.
For an MP3 CD, the numbers aren’t just a simple list. They’ve been compressed. Basically, an algorithm finds ways of storing the numbers in ways that use less space. This can get very complicated, but imagine only writing down the difference between the heights of each cable, rather than the total height. That’s a smaller number that needs fewer bits to write down. The player decompresses this data back to the original number list before it reconstructs the audio. This makes a file like you have on a computer and can include metadata like artist, album, title, etc.
How this MP3 file is stored on the CD is different, too. This is a data CD, with an actual file system, file names, folders, etc. It’s far more complicated and capable. Which means it also needs a more capable computer to access and use the data.
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