As a music enthusiast, you must be familiar with the various digital audio formats right? But do you know why digital audio files are categorised into these formats in the first place? There’s more than meets the eye when it comes to digital music playback and if you want to know more, then ask this – What is Audio File Compression?
If you’re not familiar with audio signal processing, then today’s article might be a little off-putting. But don’t you worry too much, as I will only be touching the surface of audio data compression. You will learn more about the principles behind the compression process, and also the various attributes of different formats. So, let us begin!
Data Compression – A Vital Process
Data compression is a process that involves encoding information using fewer “bits” than the original representation. In layman terms, it is the process of reducing the size of an audio file. There are two types of data compression algorithms used for audio and video, and they are known as “lossy” and “lossless”. I will explain more about them as we move forward.
Compression actually helps to reduce the resources needed to store and transmit data. The process is subject to a “space-time complexity” trade-off. For example, a compressed video file may require expensive hardware in order to be decompressed fast enough as the file is being viewed, and choosing to decompress the video in full before watching it may require additional storage.
There are various data compression methods used, and all of them involves trade-offs among various factors such as the degree of compression, the amount of distortion introduced (when using lossy data compression), and the computational resources required in order to carry out the compression and decompression process.
Here are the topics that we’ll be looking into:
- Lossless compression
- Lossy compression
- Practical uses
You may have heard of digital audio files such as “Apple lossless”, “FLAC” and “WMA lossless”. These files have actually been compressed using algorithms that exploit statistical redundancy, in order to represent data without losing any information (making the process reversible). Lossless compression is possible because most real-world data exhibits statistical redundancy.
Statistical redundancy basically refers to the “wasted space” used in transmitting data. For example, an image may contain several pixels of the same colour. Thus, instead of coding “red pixel, red pixel, …” the data may be encoded as “279 red pixels” (total number of red pixels). This is a basic example of run-length encoding, which is one of many methods used to eliminate redundancy.
Unlike lossless compression, lossy compression algorithms actually allow some “acceptable” loss of information. This method of compression is based on the principle of saving storage space by taking out insignificant details from the original data source. Lossy data compression methods takes into consideration the listener’s perception of the audio file in question.
In lossy audio data compression, theories of psychoacoustics are employed when removing inaudible components of the audio signal. Human speech is also compressed using specialized techniques such as “speech coding”. There are various lossy audio coding formats such as “MP3” and “AAC”, and they all have different audio and speech compression standards.
The main purpose of audio data compression is to reduce the transmission bandwidth and storage requirements of audio data. Audio compression algorithms are implemented in software as “audio codecs”. Lossy audio compression algorithms will be able to produce a higher data compression, but at the cost of fidelity. However, this might be useful in various situations.
For example, a typical compact disc (with a total size of 640MB) is able to store approximately one hour of uncompressed high fidelity music, which is equivalent to less than 2 hours of lossless music, or 7 hours of music compressed in the lossy “MP3” format. On the other hand, a digital sound recorder can store around 200 hours of high quality speech in 640MB.
In professional audio work, it is ideal to work with uncompressed or lossless audio files. Working with a lossy audio file will typically produce an inferior final product, as compared to a compressed final mix that originated from an uncompressed file. Other than in sound editing and mixing, lossless audio compression is also utilised for archival storage, or producing master copies.
Despite lossy audio files not being suitable for audio engineering purposes, they are still very commonly used in a wide range of applications. Other than in MP3 players and computers, lossy files are used in most video DVDs, digital television, streaming media on the internet, satellite and cable radio, and increasingly in terrestrial radio broadcasts.
That’s all I have for you folks today. I hope that the information presented here is enough to give you a strong understanding of the different audio file formats!
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