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Sound Quality Top
Sound quality is best defined as the quality of audio output from electronic devices. More specifically, it is gauged by the accuracy of an electronic or analog device as it records or projects sound waves. Accuracy is dependent on range, rate, and conversions. For example, if the range of a sound falls outside the range of human hearing, the quality will be affected. Also known as timbre, some of the characteristics used to describe quality include pitch and loudness.
In terms of physics, the characteristics of sound contributing to its quality are harmonic content, attack and decay, and vibrato. Harmonic content relates to the quantity and quality of harmonics in a sound. Attack and decay relate to amplitude; they measure the initial and then decreasing quality of a sound when an instrument is played or somebody sings a note. Decay is sustained, whereas attack is immediate and short-lived. Vibrato is used to describe changes in pitch and frequency.
Many digital audio formats allow quantifying of sound quality. The most common of these are MP3 and Ogg Vorbis. Because they can measure quality, they also help the encoder determine whether or not there is excess data in the sound file that can be discarded. For MP3s, the quality is defined based on the rate of the sound. Rate is defined as the amount of information detected per second of sound; a higher rate means a higher-quality recording. Ogg Vorbis files, on the other hand, judge quality based on the decimal value of the sound.
There are two main types of CDs. The standard type is a 16-bit. SACDs are not as common, but produce an overall higher sound quality. Their bit length is significantly smaller; they only use 1-bit. However, they nonetheless produce better sound because the rate of the sound is much higher. While a 16-bit CD has a sample rate of 2.8224 MHz, an SACD has a sample rate of 44.1 kHz. This example demonstrates that, while all factors are important in determining sound quality, some are more vital than others, and can compensate for deficiencies in other areas.
As the case of SACDs demonstrates, the sample rate is extremely important for sound quality, and can also make up for a limited bitrate. Some audiophiles also argue that the filtering and editing necessary for 16-bit and 24-bit CDs damages sound quality, and that SACDs have better quality because they do not require the same sort of processing. Given that many forms of digital sound editing, including filtering, can do some damage to sound quality, their hypothesis is not improbable.
When using an electronic output device such as a CD player, it is possible to boost the sound quality emerging from that device with a converter, especially with high-quality ones. But as with many attempts at altering sound qualities, improvement in one area might mean compromise in another. The output device may not be equipped to handle the converter, which can degrade the output signal. However, this problem is fairly rare, especially as audio technology becomes more and more developed.
Audio Formats Explained Top
When audio is captured from its source, it is then modified and transformed into a notably different file of a certain format. The modification also allows for easy distribution of the audio file electronically. Audio files come in various formats; each one being different from the other in terms of how the audio source is modified.
Modifying the original audio source essentially means sampling the audio voltage at regular intervals and storing the sampled value in digital format with a certain resolution. The sampling rate, the resolution used and the number of channels (ex. in stereo, there are 2 channels) are the distinguishing parameters of different digital audio file format.
Audio formats are grouped in three major categories:
a. slightly compressed file format, ex. WAV, AU, AIFF
b. lossless compression format, ex. lossless WMA, Apple lossless, WavPack, FLAC, Monkey’s Audio, TTA
c. lossy compression format, ex. MP3, OGG (Vorbis), AAC, lossy WMA
Most of the examples given above are the names and, at the same time, the file extensions of the different audio formats. Some of the formats are described in brief below.
WAV – This is one of the most popular file formats of audio data. WAV files are just slightly compressed thus it gives the best audio quality and large file size. WAV files’ size makes it unsuitable for sharing over the internet. Its slight compression also makes it compatible with Mac OS and Windows. With the WAV format’s flexibility in storing any sampling rate or bitrate combination, it becomes the preferred format in storing an original recording.
AIFF - The "Audio Interchange File Format" was developed by Apple for storage of sound in the data fork of Macintosh files. It has been adopted as a standard audio format by the OMFI (Open Media Format Interchange) group for cross-platform media exchange, which includes Silicon Graphics, Avid Technology and others.
Mac-based professional digital audio recording systems and multimedia applications such as Macromedia Director, Adobe Premiere and Movie Player allow importing and exporting of AIFF files. Using this format, audio can be stored as mono or stereo, 16-bit or 8-bit, and at a wide range of sample rates.
MP3 – Yet another popular and most widely used audio file format, MP3 paved the way for easy and effortless music file sharing over the internet. MP3 stands for Mpeg-1 Layer 3 or Mpeg Audio Layer 3. As per the categories above, MP3 is one kind of lossy compression. With the MP3 compression, a user can reduce a regular music CD into a tenth of its original size. The sound quality is also changed with the compression, yet the difference is sometimes not too audible. Audio streaming is also possible with MP3 which means that a user need not complete the download of the file just to be able to listen to a few parts of the music.
WMA – Windows Media Audio, or WMA, is Microsoft’s counterpart for MP3. Windows Media Audio is integrated with Windows Media Player which is also installed with a new Windows installation. Microsoft claims WMA to offer CD-quality audio at only a fraction of its size. WMA also do not allow further distribution of copyright-protected songs which made it the more preferred format over MP3 of music corporations. WMA is also known in the World Wide Web for its streaming capabilities.
AAC – Advanced Audio Coding was supposed to be the successor of MP3. It was created by the same people who made MP3 as the answer to the limitation of the latter format in lower sample rates. However, it did not meet its goal as loss of quality is still noticeable even at 96kbps with the aac format. Also, there is almost no software that supported this file type.
OGG Vorbis – OGG is another audio file format yet it is an open source project which makes it free of patent. Its development started in 1993 and still continues. With every new development, the quality of sound OGG offers gets better. New versions are also backwards compatible. This file type offers encoding at different sample rates, as well as multi-channel compression.
Real Audio – This format has the file extension .ra and is common to radio stations being aired over the internet. Real Audio is generally a streaming audio format hence the small file size. However, if a real audio file is played over a slow internet connection (ex. dialup connection), there is a great loss in the compression and the sound quality is not that good.
MIDI – This file format is not like the other formats mentioned. Midi is not a compressed audio, instead, it is the medium that lets computers and musical instruments communicate. Midi files come with file extension .mid and are just small files, around 30 to 60 kb for each song. You can download thousands of free midi files in the free midi file section on our site.
RMF - The company Beatnik developed the RMF (Rich Music Format) as part of its strategy to create a cross-platform audio standard for the web. RMF files can contain MIDI as well as other audio information, such as sampled sounds. To play an RMF file, you need the Beatnik Plug-In
Digital Video Compression Explained Top
If you use digital video, file size is an important concern because digital video files tend to take up a lot of storage space on your hard drive. The answer is compression—making files smaller.
With text files, size is less important because the files are full of “spaces” and can be compressed very tightly—a text file can be made at least 90 percent smaller, resulting in a high compression ratio (the ratio of compressed data to uncompressed data). Other file types, like MPEG video or JPEG photos, hardly compress at all because they're in a format that's tightly compressed to begin with.
Why is digital video compressed?
Digital video is compressed because it takes up a staggering amount of room in its original form. By compressing the video, you make it easier to store. Digital video can be compressed without impacting the perceived quality of the final product because it affects only the parts of the video that humans can't really detect. For example, there are billions of colours, but we perceive only about 1024 shades. Since we can't discern the subtle difference between one shade and the next we don't have to keep every color. There's also the matter of redundant images—if every frame in a 60–second video has the same chair in the same spot, why save the data of that chair in every frame?
Compressing video is essentially the process of throwing away data for things we can't perceive. Standard digital video cameras compress video at a ratio of 5 to 1, and there are formats that allow you to compress video by as much as 100 to 1. But too much compression can be a bad thing. The more you compress, the more data you throw away. Throw away too much, and the changes become noticeable. With heavy compression you can get video that's nearly unrecognizable.
When you compress video, always try several compression settings. The goal is to compress as much possible until the data loss becomes noticeable and then notch the compression back a little. That will give you the right balance between file size and quality. And remember that every video is different—some videos look great highly compressed, others don't. You'll have to experiment to get the best results.
Bit rate explained Top
Bit rate describes how much information there is per second in a stream of data. You might have seen audio files described as “128–Kbps MP3” or “64–Kbps WMA.” Kbps stands for “kilobytes per second,” so the higher number represents more data: 128–Kbps MP3 audio files contain twice the data as 64–Kbps WMA files and take up twice the space. (Although in this case, however, the two files would sound about the same. The reason? Some file formats use data more effectively than others, and the sound quality of a 64–Kbps WMA file is as good as the 128–Kbps MP3 file.) The important thing to understand is that the higher the bit rate, the more information, and thus the more effort it takes to decode that information, and the more space the file requires.
Selecting the proper bit rate for your projects depends on the playback target: if you're making a VCD for playback on a DVD player, the video must be exactly 1150 Kbps and the audio 224 Kbps. A typical Pocket PC running at 206 megahertz (MHz) can work with MPEG video up to 400 Kbps—anything above that will cause it to sputter during playback.
Compression strategies
There are many different approaches and strategies used to squeeze digital media files down to manageable size. Here are some of the most common ones:
Psychoacoustic audio compression
The word psychoacoustic looks complicated, but it simply means “the way the brain interprets sound.” All forms of compressed audio use powerful algorithms to discard audio information that we can't hear. As an example, if I shout at the top of my lungs and also lightly tap my foot, you'll hear my voice but you probably won't hear my foot tapping. By getting rid of that foot tapping sound, there's less information and a smaller file size, but it will sound the same to your ear.
Psychovisual video compression
Psychovisual video compression is similar to its audio counterpart. Instead of discarding audio that we can't hear, psychovisual models discard data that are eyes don't need. An uncompressed clip that shows a chair in the same location for 60 seconds repeats the same data for that chair for each frame. With psychovisual video compression, the data for that chair from a single frame is stored and reused in subsequent frames. This type of compression—called “statistical data redundancy”—is one of the mathematical tricks that WMV, MPEG, and other video formats use to compress video while retaining good quality.
Lossless compression
The term lossless means “no loss of data.” When a file is compressed in a lossless fashion, 100 percent of the data is still there, much like when you zip a document—the document file gets smaller, but all the words are still there when you unzip it. You can save lossless video over and over without any loss of data—compression simply squeezes that data into a smaller space. Lossless compression saves less space because you can compress data only so much before you have to start discarding information.
Lossy compression discards data in order to achieve a lower bit rate. Psychoacoustic compression and psychovisual compression are lossy technologies that result in smaller files that contain less of the original source data. And every time you save your file in a lossy file format, it discards more of the data—even if you're saving it in the same format. A good rule of thumb is to move to a lossy format only as the very final step in your project.
Video Compression Explained Top
Today’s digitized videos come in an array of file formats. Let’s take a look at three of the most popular, and briefly discuss their differences.
.AVI files. AVI stands for audio/video interleaved. This file format is the most popular computer video format, and it is defined by Microsoft. The file sizes tend to be large compared to some of the other formats. These files will play in most of the popular media players. Generally, they are used to edit video or when a high resolution version of the video is needed.
.MPG, .M1V, .M2V files. MPG (or MPEG) stands for Moving Picture Experts Group. This file type utilizes standards that have been, and are being, developed by the Moving Picture Experts Group. They are a common file type and are generally much smaller in size than .AVI files.
.MPG most commonly denotes a file with MPEG-1 compression. The most common resolution for this file type is 352x240, but other resolutions may be used (the MPEG-1 files produced by WorshipFilms are 640x480). .M1V usually denotes a video with MPEG-1 compression that does not contain any audio. MPEG-1 files will play in almost any media player. They are generally used when limiting the file size is the main objective.
One confusing element is that .MPG can also denote a file that utilizes MPEG-2 compression. These files are generally higher in resolution (most commonly 720x480), and are often used in the creation of DVDs. .M2V generally denotes a video with MPEG-2 compression that does not contain any audio. The file size of MPEG-2 files is generally larger than MPEG-1 files. MPEG-2 files will only play in media players that have a MPEG-2 video codec installed.
.MOV, .QT files. These files extensions denote a Quicktime media file. Apple Computer developed this file type for the creation and viewing of multimedia content. Generally, most media players will play a Quicktime file that is version 2.0 or earlier, but later versions require player software that is proprietary to Apple. File sizes are generally comparable or slightly larger when compared to MPEG-1 files (depending on compression settings).
In our next article, we will conclude our discussion of file formats by examining .WMA, .RM, .ASF and .MP4 files.
In our last article, we began to examine the most popular file formats for digital video. Now, let’s take a look at four more and briefly discuss their attributes.
.WMV files. WMV stands for Windows Media Video, and refers to a video codec set developed by Microsoft. In earlier versions, Microsoft utilized a form of MPEG-4 technology in developing this codec, but later versions has relied more on Microsoft’s own technology. It is a widely used file format on the internet because of its relatively small file size, and is now used in other media players beyond Windows Media Player (referring to version 9).
.ASF files. ASF is short for Advanced Streaming Format. It is a very compressed format that contains streaming audio, video, slide shows, and synchronized events. It provides a continuous stream of data, so that even lengthy videos begin playing almost immediately. It is not necessary for the entire file to download first. This is a popular internet file format, especially for online seminars or other lengthy subjects.
.RM files (also .RA or .RAM). A .RM file is a Real Media file, which (of course) is played in Real Media Player. These files are generally intended to be used as streaming files, and cannot be played in other media players.
.MP4 files (or .M4V). MP4, or MPEG-4 is a format developed by the Motion Picture Experts Group that allows you to combine multiple media streams into one file. Initially, the MPEG-4 format looked a lot like the Apple Quicktime format, but has since been significantly changed and improved upon. It is an end-user format that is used most often for streaming applications and mobile devices (PDA’s, cell phones, etc.). The great appeal of this format is its general quality level in relation to its small file size.
Hopefully, these two articles will serve as a basic reference for you on video file formats, and help you to more quickly identify file types in the future.
DivX Top
DivX is known to create a balance between quality and file size with its highly efficient compression abilities. And because of that, it s one of the codecs used for ripping where audio and video are copied from a source to the PC hard disk for archiving and transcoding.
The commercial DivX competes with Microsoft's Video for Windows in WMV, Apple's QuickTime in the MOV and RealNetwork's Real Video in the RMM file formats. An open source version released by Xvid solutions in 2001 is the Xvid file format.
While DivX has long been renowned for its excellent video quality, its free and open source equivalent Xvid today offers comparable quality, also based on MPEG-4 Part 2 (MPEG-4 ASP). In a series of subjective quality tests at Doom9.org, the DivX codec has been successively beaten by Xvid every year since 2003. Confusion clarified
DivX are two different things from two different companies. One is DIVX created by Circuit City, a US electronics retail giant that attempted to market a DVD rental system that used special players and discs. And the other is the DivX multimedia codec trademarked and marketed by DivX, Inc. which is actually a reference to the failed Circuit City system.
A short history
DivX roots can be traced back to 1998 as a hacked version of Microsoft's MPEG4 version 3 which is inferior to the MPEG4 that we know today. It was a French hacker Jerome Rota who, rather than modify his video resume which could not play on the new Windows Media Player at that time, reverse-engineered the MPEG4 format together with a German Hacker Max Morice to come up with an MPEG4 format encapsulated in AVI instead of the ASF it originally had. It only took them a week. Between 1998 and 2002, the DVD hacking community had independent hackers enhance the format that later came to have the Divx with a smiley emoticon J attached as version 3.0.
In 2000, Rota was hired by Jorda Greenhall to form a company called DivXNetworks (later renamed to DivX, Inc.) based in the French Riviera. The association resulted in the OpenDivX codec a year later. Its source code was open to anyone and could be downloaded from the projectmayo.com website. The following year, the two left for San Diego and developed the OpenDivX software to become DivX version 4.0. Other developers took the Encore2 software to enhance the open source OpenDivX to arrive at the rival Xvid format.maintained by Xvid Solutions, Inc.
The DivX Company continued to enhance the DivX software that in 2002 has taken on the fifth version. By 2004, the features of the DivX format are as complete as we know it today. In May 2007, the Windows Vista version DivX 6.6 for the PC and the Mac was released.
The advantages and benefits of using DivX
DivX found itself at the heart of video piracy in the late 90s as its format became widely popular for ripping copyrighted DVD materials for bootleg replication and distribution. A number of generic DVD players as well as branded ones are claiming to play DivX materials
What's so appealing about the format is that it's free. Same with the software players you can use to play it with. It belongs to the open source community together with Xvid offering competitive if not better quality.
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