20-04-2012, 03:34 PM
MULTIMEDIA TRAINING KIT: INTRODUCTION TO DIGITAL AUDIO HANDOUT
mmtk_digital_audio_handout.doc (Size: 130 KB / Downloads: 28)
Introduction
You have heard about MP3s, RealAudio and wav. You know that they are about sound, but don’t know much more than that. This guide aims to give you some basic information about digital audio – we will be talking a lot about MP3/RealAudio format – as well as where, why and how can you use them and where to look for further information about this topic.
Types of audio files
There are two most important parameters you will be thinking about when working with digital audio: sound quality and audio file size.
Sound quality will be your major concern if you want to broadcast your programme on FM or short wave.
If you are creating audio for exchange over the Net
(uploading/downloading), or to include it on a web site, file size will be one of your greatest concerns – one needs a very strong modem, a good connection and a good computer to download big files, which will narrow your audience significantly.
Incidentally, the size of an audio file influences your computer performance – big audio files take up a lot of hard disc space and use a lot of processor power when played back.
There are other settings that influence the sound quality and the file size of a digitised file (such as length and digitising settings), which you will learn about when you investigate digital audio further.
These two parameters are interdependent – the better the sound quality, the bigger the file size, which was a big challenge for those who needed to produce small audio files, but didn’t want to compromise the sound quality.
That is why people and companies tried to create digital formats that would maintain the quality of the original audio, while reducing its size.
Which brings us to the three most common audio formats you will come across: wav, MP3 and RealAudio.
wav files
When importing audio material into your computer, you will most probably by default save your digitised file as wav. Wav files are proprietary Microsoft format and are probably the simplest of the common formats for storing audio samples.
Unlike MP3 and other compressed formats, wavs store samples "in the raw" where no pre-processing is required other that formatting of the data. Therefore, because they store raw audio, their size can be many megabytes.
The quality of a wav file maintains the quality of the original. Which means that if an interview is recorded in the high sound quality, the wav file will be also high quality. This is why some radio stations use wav files for their FM broadcasts.
Because of its size, wav files are not suitable for using on the Web. They are not recommended for use in archiving audio either, as they take up a lot of disk space (unless disk space is not a concern).
In order to reduce the size of a wav file, but to preserve the sound quality as much as possible, you can convert it into an MP3 or RealAudio file.
MP3s
MP3 is fast becoming the industry standard for listening to audio files on the Web. This is mainly because the audio files are highly compressed whilst retaining a high level of sound quality. For example a raw audio file of 36MB will become a high quality MP3 file of 3.6MB. You can squeeze around 10 hours of audio in MP3 format onto a CD (which usually only stores around one hour of non-compressed audio).
In making MP3s "perceptual" encoding is used, which means that only the sounds one can hear are saved, while the sounds a human ear won't pick up are discarded. It makes the MP3s smaller but still very high quality.
MP3 is short for "MPEG Layer-3". MPEG stands for "Moving Picture Experts Group" and refers to the experts who developed MPEG - a set of industry standards for digitally encoding video and audio information.
MPEG Layer-3 is the part of the MPEG standard that describes how to create and store high quality audio on a computer using very little space, and how to play it back with very high fidelity. MPEG Layer-3 audio files are often referred to as "MP3s", since each audio file is usually named with the three-letter extension .mp3.
RealAudio
RealAudio was developed by the computer company Real Networks in 1995. It was one of the first audio formats, and remains one of the most popular and versatile formats in use today. They developed a way of "streaming" RealAudio – which means that the audio file starts playing "as it downloads". This means you don't have to wait for the whole audio file to download before you listen to it (so it's quicker and more accessible). But there is a drawback to "streaming" – it means that your connection to the internet has to be continuous and if your connection goes down when you are listening to streaming audio, so does your audio feed. You will recognize RealAudio by these extensions: .ra, .ram, .rm.