Film is rearranged metal particles on tape which are read by an electromagnet. CDs are discs with burned pits in them which are read by a laser. What makes one analog and one digital?



Film is rearranged metal particles on tape which are read by an electromagnet. CDs are discs with burned pits in them which are read by a laser. What makes one analog and one digital?

In: Technology

CDs pits represent binary data, on or off. Film’s particles (or grain) can be any color, making it analog.

A good way to determine if something is analog vs digital is how the smallest unit of measurement can be measured. If it can only be on or off then it’s digital, but if it can be on, off, or some value in between then it’s analog.

CDs are digital because the data is stored in a discrete fashion: binary, 1 or 0. Film is stored in an analog fashion because the data is recorded as a gradient (of brightness, color, etc), so it isn’t discrete.


Do you mean audio cassette?

In any case, the audio cassette if that’s what you mean, represents a smoothly varying signal, which corresponds to the magnetic field on the tape.

WHen recorded, the audio signal is directly fed into a magnetic head, and that head (which is just a coil of wire) creates a changing magnetic field, which magnetises the tape in a way which directly represents the analog audio signal. There’s no converting happening, except between electrical and magnetic.

With a CD, what’s encoded in the pits and lands is digital data, which has to be fed into an digital to analog converter to turn it back into the sort of signal that can be fed out of a speaker.

Digital just means there’s an extra step in the middle to translate everything into 0s and 1s.

Film uses the physical properties of the metal and clever manipulation of chemistry to store an image.

CDs uses a laser and the physical properties of light to store an image, but the image was first translated into 0s and 1s (which correspond to the lands and pits on the disc)

Edit: More examples after I saw you meant audio.

With a magnetic tape, the soundwaves that were recorded get directly transformed into magnetic fields of varying strength. The tape essentially store “magnetic waves” that are a 1 to 1 transformation of the original sound waves.

With a cd, those sound waves are first translated into 0s and 1s before being transformed into lands and pits. With out a computer that knows the code, those 0s and 1s are meaningless.

Sorry, my title is poorly worded:

For the sake of simplicity, I’m talking just about audio. Cassette tapes and CDs.

Audio tape contains varying degrees of magnetism. It is a lot like turning a dial on an adjustable light. Because the amount of magnetism at a certain spot is measured “as is”, and then comes out of the speakers as an analog signal.

The different between an analog and a digital signal is that a digital digital has a certain number of levels it can be at. These are numbered, and depend on how many bits (1s or 0s) you use. I5 totems or that if you write out as many 2s as you have bits, it tells you how many levels it has.

A bit is a 1 or a 0, but it can also be an off or an on, or any two choices. For a CD, the choices are “laser reflected into sensor” and “laser not reflected into sensor”. Writable CDs change the color on the CD so that it absorbs or reflects the light because it’s transparent and lets light through. Grooves work because the laser bounces off a different height, and misses its target.

These offs and one, 1s and 0s, are then grouped together into chunks. For 8 bit Audio, the chunks are 8 bits wide, etc.. These 1s and 0s are then converted into sound.

This is rarely done directly, but is in some cases like files that end in “.wav”. Mp3s on the other hand, read the CD as a bunch of code. It contains less data because it turns out that you can get really close to the original sound by using techniques like “wavelets”, which you can think of as digital bells all combining together to make your song.

But long story short, analog stuff is a range that is measured directly, and digital data can be written as bits, or numbers, or letters.

For the record you can do analog CDs. Video Disc – the old standard with disc as big as vinyl records – were analog recordings. The laser’s reflection would vary in intensity to make the analog signal.

Making CDs digital was a conscious decision. That video disc is subject to gradual degradation with damage from handling, the effects of time and environment, etc. CDs are digital – there’s less confusion about whether something is a 0 or a 1 without significant physical damage. Furthermore being digital we can use math to repair some damage. CDs include error correction codes allowing a few confusing or erroneous bits to be identified and reconstructed.

As such, CD audio is generally always perfect, or is permanently and unquestionably damaged. As opposed to somehow degraded, like muffling or such.

You are confusing film with tape. Film is read by shining a light through it while a tape is read by an electromagnet. The difference between analog and digital is how many different levels the signal can have in its raw format. In an analog format there is no limit to the granularity of the data other then those imposed by the material. For example a film can be fully transparent to completely opaque and everything in between. However the raw format on a CD is either fully flat or a full depth grove. There is no half way in between. The problem is that the laser in the CD is not arranged in such a way that it can pick up more then two levels. However if you for example were able to get a needle to read the depth of the groves you could store analog data in the same format, this is what vinyl records did. It is the analog version of a CD. Similarly there are digital formats of tapes as well, firstly the Digital Audio Cassette which were a consumer grade product but it is the same technology behind the tape as in hard drives and those are digital. Tapes is also very popular for enterprise systems for huge amounts of storage because it is the cheapest form of data storage.

The advantage of storing things in a digital format is that the data is there even if there is some change in the medium which cause its signal level to change. You know there are only two valid signal levels so if you get something in between you can just round it to the closest valid level and you have not lost any data. This means you can have a lot of signal interference without losing any of the data.

An analog signal is a continuous signal that represents physical measurements. Digital signals are time separated signals which are generated using digital modulation. It uses a continuous range of values that help you to represent information. Digital signal uses discrete 0 and 1 to represent information.6

It’s all about the encoding.

In a magnetic tape, the strength of the magnetic field is directly related to the strength of the sound – so to play it, all you have to do, is turn it into electricity, multiply it by the volume, and turn it into sound. The magnetic field is a direct analog to the sound produced.

By contrast, a digital signal is encoded. It’s basically a series of numbers, and the strength of the digital signal has nothing to do with the strength of the sound produced – as long as it’s strong enough to be understood, the system will work. So, to produce sound, the computer reads the signal, calculates the sound represented by the signal, and directs a DAC (digital-to-analog converter), to produce a sequence of voltages analogous to the sound.

The reader has to know what to do when they read the tape/CD. It’s perfectly possible to make a reader that reads tape as digital, or reads CD’s as analog. If the reader reads as digital, then it converts the measurement to a 1 or 0, otherwise the measurement directly converts to an audio voltage.