How Analog and Digital Recording Works
Digital music was the sole function of CDs when they were originally launched in the early 1980s.
You must first grasp how digital recording and playback works and the distinction between analog and digital technologies in order to comprehend how a CD works.
Analog and digital recording will be examined in this post so that you can fully grasp the differences between the two methods.
In the Beginning: Etching Tin
In 1877, Thomas Edison is credited with developing the first apparatus for recording and playing back sound.
To mechanically store an analog wave, he employed a very basic technique.
Diaphragm-driven needle on Edison’s original recorder produced analogue signals on a thin sheet of foil in Edison’s first record player.
When you talked into Edison’s invention, the needle “recorded” what you said onto the tin by turning the cylinder.
There was also a corresponding vibration in the needle as it swung in response to the diaphragm’s movement; these vibrations were recorded on the can.
The needle was moved over the scratched groove during recording to play back the sound.
In order to hear the recorded sound, a needle was driven into the tin, which caused the diaphragm to wiggle and play the sound.
Emil Berliner expanded on this system in 1887 and created the phonograph, which is also a mechanical instrument that utilizes a needle and diaphragm for recording sound.
Using flat records with a spiral groove on the gramophone made mass manufacture of records much easier.
The current phonograph operates in the same way, except instead of directly vibrating a mechanical diaphragm, the needle’s signals are amplified electronically.
It’s hard to tell from the picture, but it looks like Edison’s needle is scratching something on the cylinder.
It’s a representation of the vibrations you make with your voice in analog form.
Saying the word “hi” will produce the following waveform on a graph, for exampleRather than on tinfoil, this waveform was captured electronically, but the principle remains the same.
The position of the microphone’s diaphragm (Y axis) over time is depicted in this graph (X axis).
The diaphragm is vibrating at a rate of 1,000 oscillations per second, which is extremely fast.
Edison’s tinfoil was etched with waves like these.
The waveform for the word “hello” is quite complex, as can be seen.
A pure tone is nothing more than a sine wave vibrating at a specific frequency, such as this 500-hertz wave:
Scratching on tin is surely a clear and uncomplicated technique to storing and playing back an analog wave.
The fidelity of the simple technique is poor, which is a drawback of this strategy.
This can be seen, for example, in Edison’s phonograph, which records the intended signal alongside a great deal of scratchy noise.
The needle’s passage over the grooves of a phonograph wears it down over time; if you play it frequently, it will eventually break down (and eventually erases it).
If you want to generate a high-fidelity recording on CD or any other digital recording method, you need to make sure that the original and the replicated signals sound identical (the recording sounds the same every single time you play it no matter how many times you play it).
Recording data digitally is a way to achieve both of these objectives.
An analog-to-digital converter does the conversion (ADC).
A digital-to-analog converter transforms the numerical stream into an analog wave before the music can be played back (DAC).
In order to create the sound, the DAC amplifies and feeds the analog wave produced by the DAC to speakers.
As long as the numbers are not corrupted, the DAC will create the same analog wave every time.
If the analog-to-digital converter sampled at a high rate and produced precise values, the analog wave produced by the DAC will also be highly comparable to the original analog wave.
If you know more about the analog-to-digital conversion process, you can appreciate why CDs have such excellent fidelity.
Take a sound wave and sample it with an ADC, for example.
Using the assumption that each tick on the horizontal axis represents a thousandth of a second, consider the following common waveform:
It’s possible to manipulate two variables when you sample a wave using an ADC:
Controls the number of samples taken per second (sampling rate).
Controls how many distinct gradations (quantization levels) are possible while obtaining a sample.
We assume that the sample rate is 1,000 per second and the precision is 10 in the following figure.
Samples are shown in the form of green rectangles.
The ADC examines the wave every one thousandth of a second and selects the nearest value between 0 and 9.
Along the bottom of the image, you’ll see the number you chose.
These numbers represent the original wave in a digital form.
To obtain the blue line displayed in this picture, the DAC uses these numbers to duplicate the wave:
Since the blue line lacks most of the original red line’s information, the faithfulness of the replicated wave must be poor, as can be seen from the comparison.
This is the error due to sampling.
The sampling error can be reduced by increasing the sampling rate and precision at the same time.
A two-fold increase in both speed and precision is shown in the accompanying graph (20 gradations per second at a sampling rate of 2,000 per second):
At 40 gradations at 4,000 samples per second, the rate and precision have been doubled again.
Fidelity can be clearly seen when the rate and precision improves, as the original wave and DAC’s output become more identical.
Fidelity is vital in CD sound hence the sampling rate is 44.1100 samples per second and the gradation count is 65,536 in order to achieve this.
To most human hearing, the sound is “perfect” when the DAC output matches the original waveform at this level.
CD Storage Capacity
The CD’s sampling rate and precision result in a lot of data being generated.
The ADC’s digital numbers are saved on a CD as bytes, and each of the ADC’s 65,536 gradations requires two bytes.
Recordings are being made of two distinct sound streams (one for each of the speakers on a stereo system).
There are 44,100 samples/(channel*second) * 2 bytes each sample * 2 channels * 74 minutes * 60 seconds/minute = 783,216,000 bytes of digital data to be stored on a CD.
How many bytes are in there?
It’s not an easy feat to fit so many bytes onto a cheap piece of plastic that can withstand the damage most CDs are subjected to, especially considering that CDs were originally released in 1980.