› Digital Audio Technology

How Digital Audio Technology Works


Like many others, I've always felt that the growth of digital audio technology is one of the biggest reasons why you're able to create and produce music in your own home studio. Without the technology and the massive use of computers and other digital devices, you'd still find music being recorded in large expensive studios using analog tape machines.


The growth of digital technology has meant that it's become relatively cheap and easy to record and produce music at home, and to achieve great results as well. Long gone are the days where you needed very large sums of money to record and produce your music in expensive commercial studios.


I think it's important for modern-day recording engineers and producers to be aware of how the digital world works, so here I've outlined the basic principles behind the technology.




Digital Audio Technology and Binary Numbers


In very basic terms, a digital audio system is able to take a continuous analog signal and substitute it for a sequence of individual samples, all without losing any of the important information from the signal. The original analog signal can be rebuilt later on, from the original samples that were taken. 


The binary number system is used to transmit and store data

Digital audio technology systems use the binary number system for storing and distributing information. Decisions and operations in a digital system take place through the use of boolean algebra, and it's a very fast and efficient way for digital devices to work. It's especially important for computers.


The system uses 'bits' of data, either a 0 or a 1. This is binary coding. In a digital system, the 0 equates to the voltage being turned off, and the 1 means the voltage is turned on.


Each digit (a digit is referred to as a bit) in a binary code represents a different value. So for example, imagine this binary number (this is an 8-bit dataword, equal to 1 byte):


  • 0010 1101


  • Reading from right to left, each number represents the values of 1, 2, 4, 8, 16, 32, 64, 128. This pattern would continue if the binary number was longer.


  • Each new value is double the previous value, and the pattern continues like this.


  • So, from the above binary number...

0

0

1

0

1

1

0

1

128

64

32

16

8

4

2

1

…we can see that this binary number would equal 45 (32 + 8 + 4 + 1).




Sample Rate


The sample rate of a digital audio technology system relates to how often the analog signal is sampled each second.  CDs use a sample rate of 44.1kHz. This means that the original analog audio signal has been sampled 44,100 times per second.


Audio on DVDs have a sample rate of at least 96kHz, or even 192kHz, one of the reasons why DVDs are said to have higher-quality audio than CDs.


Digital audio technology works by sampling analog waveforms at regular time intervals, and each reading is stored and transmitted digitally as a binary number


The sample rate also governs the highest audio frequency that can be sampled and stored (the range of frequencies that can be stored is known as the bandwidth). As humans, our hearing limit lies between 20Hz-20kHz, so in theory the system we use needs to be capable of reproducing frequencies up to this upper limit of 20kHz.


To accurately record audio signals digitally, the sample rate must be double the value of the highest frequency that we would like stored and reproduced. So if we would like our system to play signals of 20kHz, we must use a sampling rate of at least 40kHz.


This is why CDs use a rate of 44.1kHz - it's so we can capture all of the frequencies that we can potentially hear and to help create a high-quality listening experience.


A sample rate of 192kHz can capture and record audio signals all the way up to 96kHz.



This frequency value of half the sampling rate is known as the 'Nyquist frequency', after Harry Nyquist, an engineer who worked in early digital communications.


The Nyquist frequency used on a CD is 22.05kHz, which is half of 44.1kHz.


This method helps to prevent something called 'aliasing', whereby signals above the Nyquist frequency get into the system and cause distortion



Every time the audio signal is sampled (for example 44,100 times per second), the voltage level is taken and recorded as a binary dataword.


The higher the sampling rate, the more samples are taken each second, which means that the conversion of the analog waveform into the digital domain will be more accurate and more detailed (one of the reasons why 96kHz audio is seen to be higher-quality than 44.1kHz audio).


Of course, this also means that higher sampling rates need more storage space to hold this extra information. DVDs require a lot more room compared to CDs.




Bit Depth


The second major area of digital audio technology is the bit depth.


When a sample of the audio waveform is taken, the voltage value is stored as a binary number. This voltage value represents the amplitude of the audio wave.


The bit depth of each sample determines how accurate the digital readings are, so a higher bit rate will mean a more accurate and better quality digital signal


The size of this value is set by the bit depth - the bit depth sets how large each binary word is. So for example, CDs use a 16-bit system. This means that each binary word that is recorded and stored is 16 bits long. DVDs use a 24-bit system - each stored dataword is 24 bits long.


So the 24-bit word will store more information in it - which means the system's resolution will be higher. The more bits there are, the more accurate the sample reading will be. The resolution is higher.


Imagine a large stack of piled paper in front of you, with each sheet representing one digital reading:


  • A 16-bit system would be represented by a height of 22 feet of paper, and there are 65,536 levels that can be stored and read (each level is known as a 'quantization level').
  • A 24-bit system would be represented by a height of 5,632 feet of paper, and there are 16,777,216 quantization levels that can be stored and read.


So the 24-bit system is much more accurate at storing an analog signal in the digital domain. This is one of the reasons some CD albums have been re-mastered and re-released as DVD-audio - the marketing departments at the record labels were keen to show-off the higher audio quality that you'd get with this newer format.




Final Thoughts


The sampling rate and bit depth of digital recorder systems are the terms that are most often heard when it comes to digital recording in the studio. These ideas can relate to recording music in a DAW, as well as the qualities of CD and DVD audio. Knowing what these names and numbers mean and what the theory is behind them will give you more of an understanding about the digital world that we're surrounded by.


I hope I haven't lost you too much in this discussion about digital audio technology - I know that these sorts of topics can appear confusing at first as it took me quite a while to fully grasp them. But I also hope you stick with it and try to understand it, as I think this kind of knowledge can only help you in your studio.



 

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