Audio File Formats

1. Audio File Formats By Craig Stewart

2. Properties of File Formats • An audio file format is form of storage file used to store digital audio data on a computing system. • This data storage can be grouped into data that has been compressed or data that is in a raw, unedited or compressed form. • Compressed data however, can be broken down into two separate further groups, a ‘lossless’ compressed file or a ‘lossy’ compressed file.

3. Compressed V. Uncompressed • As with anything else there are reasons why to use something for a situation and reasons not to use something for a situation and audio file formats are no different, however, the number of different formats is near endless at this point and going over them all one by one is time that could easily be used for more ‘important’ things such as eating packets of skittles or updating your status on failbook. So I will cover the major points that are more necessary.

4. Uncompressed… • Uncompressed audio files are as the name implies audio files that have had no compression coding placed on the file so the audio is in it’s raw form without any loss at all between it and it’s original master. • Uncompressed audio files are generally used for storing masters of a production in an audio system. Obviously if anything has been lost on the master then it will be past on to any copies derived from it and using an uncompressed format is the best way to avoid this. • However, the one big flaw with using a technique such as this is the fact that as it is still contains the data uncompressed it takes up a lot more space than a compressed file format, it might be necessary to introduce an external hard drive into your system in order to contain all the masters.

5. Lossless (Compressed…) • Lossless is a form of compressed file formatting it is the line between ‘uncompressed’ and ‘lossy’ file formats. • In this file format the compression that takes is very slight and is in theory a more ‘intelligent’ compression format in the way that it goes about removing frequencies from format as it will actively judge through out the track what frequencies have little or no effect in the course of the song and remove them from the file. • In theory, they allow for the music to be as whole as it is in an uncompressed file with a reduced burden in terms of storage space. • However, the problem with skimming off things such as ‘silence’ is that these things take up very little space anyway and therefore not much is actually taken off the file (a few MB at most), so why use it when you may as well use uncompressed?? Or maybe you think that those few MB will matter when it all stacks up??

6. Lossy (Compressed…) • ‘Lossy’ audio file formats is an umbrella term for audio file formats that shave a considerably large volume of frequencies from the file in question in the conversion process that cannot be restored even if you reconvert back to the original format. • This enables a massive reduction in file size at the cost of a much greater volume of frequencies in the file being shaved. This due to the simplifying of data complexity. • These are the most popular commercial formats as they allow for easy storage of media in portable devices such as MP3 players etc. • However, due to the large scale removal of frequencies there is a chance that the audio’s quality will be slightly effected, however, this often negligible due to the fact that most people will not be listening out for it in everyday situations.

7. .wav (Uncompressed…) • Waveform Audio File Format is an uncompressed form of file formatting (also known sometimes as WAVE but more commonly known by it’s file extension ‘.wav’) and is one of the standards for storing raw audio data in a computing system. • It is also readily found on the internet when in pursuit of audio files that contain full quality of sound aka original masters of songs. • This has been seeing a decline in recent years, however, with the dawn of the MP3 and it’s suitability for the casual listener as well as it being pushed by all the major music file stores.

8. .wav (continued...) • It is however, widely used in audio editing where time constraints between compression and decompression of files etc. are considered of great importance. They are also occasionally used in radio broadcasting especially by those who rely on ‘tapeless’ systems, the BBC in fact actually implement such a system. • ‘.wav’ has a limit placed on it just to make things that more confusing of up ton 4GB caused by limitations placed by the ‘file size header.’ Despite this not being an issue for CD quality audio it can be a problem for high definition audio.

9. Apple Lossless • ‘Apple Lossless’ aka ‘ALAC’ aka ‘ALE’ (extension ‘.m4a’ is a codec developed by scene kid gods Apple Inc.) • It is involves the ‘lossless’ data being stored within an MP4 container . • This file appears most commonly in iPods, iPhones and most likely whatever iVariant they will come up with next. • Apple Lossless uses roundabout ‘40% - 60%’ of the original file’s data size. This varies wildly depending on how complex the music being stored is.

10. Apple Lossless (continued…) • This audio file format is a good one to use for many reasons including the fact it is widely accepted by a large portion of computing programs including, QuickTime, iTunes and Real Player. • It is generally used for the archiving of and reproduction of music on the consumer level. • It is also an open and royalty-free codec meaning that if you are a decent script kiddie you can manipulate and knock the code about until it meets your demands without any nasty legal ramifications.

11. Apple Lossless (continued…) • However, having said that it is readily useable with many programs it is uncannily not readily useable on windows media servers for obvious reasons and although widely accepted by major programs it is very selective of what programs it does use due to it’s proprietary nature.

12. MP3 (Lossy) • ‘MP3’ (Powerword MPEG-1 or MPEG-2 Audio Layer III) is a lossy form of data compression encoding. It is the most common form of encoding used in consumer audio products despite being nearly 20 years since it’s inception because of it’s ‘hack and slash’ attitude to what data is being retained at full and what is not causing a massive reduction in the amount of storage used making it suitable for technologies such as portable media players.

13. MP3 (continued…) • As revolutionary as the MP3 format was to the music industry there is as always ‘nay sayers’ that will try to shoot the pretty balloons, in this case, the argument is because of it’s ‘sloppy’ coding process the MP3 codec can often ‘hack’ unnecessarily deep into the data of the track being converted affecting the reliability and overall quality of the audio. • There is obviously the possibility of this occurring in the process, however, it is unlikely to affect the casual listener’s experience, if it will have any effect it will be in music experiences with much broader frequency ranges such as large sweeping orchestral compositions.

14. MP3 (continued...) • You do, however, have a degree of input in the ratio of audio quality v. storage consumption as in most cases you are quite free to manipulate the bit rate of your encoder so that it can more accurately represent the music it is attempting to encode, some people are able to distinguish between a gap like 256 kbps to 192 kbps, others cannot just to make you aware that these differences do exist. The general scope of these encoders is from a high of 320 kbps which is very close to CD quality down to a low 60 kbps an extreme murky compression coding. A simple example of bitrate in everyday use Youtube’s bit rate (Youtube uses a low bitrate in order to minimize bandwidth thus keeping costs down) of about 96kbps.

15. Error Detection & Correction • This phenomena occurs during data transmission, channels magnetic, tapes and optical discs. It involves the data involved being interpreted wrong or data being complete missing. It is caused by ineffective storage devices and communication lines. • Data communications channels are especially susceptible to this effect. • Some storage devices will also handle the transition between components worse than others. • The purpose in this is to reduce the potency of these errors as much as possible, hence less damage to the audio.

16. Error Detection & Correction • A form of error detection is a ‘parity check’ it involves sorting a data stream into rows and columns, then depending on the bits present and type of parity check (a parity check be either odd or even) a bit will be added to the word being monitored. If odd parity check is being administered then the bit will be a ‘1’ in order to make the word odd or a ‘0’ to signify the word is originally odd. • From here the failed bit can easily be flipped due to relative ease simplicity involved in binary, however, with this simplicity an error can just as easily be missed in the case of multiple errors in a word rendering the word back to ‘odd’ so in that case it is unreliable.

17. Error Detection & Correction • Another form of error correction is the ‘cyclic redundancy check’, a mathematical technique with an extremely high accuracy rate (over 99.9%). It involves the whole data stream being considered as a single whole and then divided by a pre-set number, this number remains constant throughout the process. In this technique the resulting quotient is discarded so that only the remainder remains this figure is then packaged and sent along with the data and is received at the other end. • The idea is that if an error has indeed occurred then the number will have in some way been effected. This would then result in some remediating action being taken.

18. Error Concealment • Interleaving helps the error correction process by breaking the data stream into small words thus breaking the burst errors into smaller chunks as well then randomizes the stream before returning it to it’s original form leaving the burst errors scattered effectively breaking them down and minimizing their effect on the data.

19. Error Concealment • Error concealment is the digital audio system’s attempt at making errors in the system seem less audible to the listener, this involves interpolation of a signal, this function never actually being used on raw data however. There various levels of this ideal: • Zero Order involves simply ‘looping’ in order to cover the missing or incorrect sample. • First Order involves the rogue sample being replaced with a mathematically derived sample from the mean value of previous and subsequent samples. • Muting involves simply setting the missing sample to ‘0’.