Decoder free mp3

MPEG (Moving Picture Experts Group) audio and video have been with us now for more than 10 years. The MPEG group was formed in the late 1980s to create standards for the compression of digital audio and video signals. In 1992, MPEG became a standard as agreed upon by the International Standards Organization (ISO) and the International Electrotechnical Commission (IEC). MPEG 2 became a standard in 1994 and added the ability to encode content at lower bit rates (16 kbps, 22.05 kbps, 24 kbps) and to encode a signal according to psychoacoustic models. The psychological models exploit masking and threshold effects in human hearing to decrease the amount of encoded data below the audibility threshold. For video encoding, MPEG may decrease the number of bits per frame for less complex pictures because the human eye will not notice the loss of quality. There are three layers to the MPEG specification that apply to audio (only) encoding - layer one, layer two, and the most recent, layer three.

Interestingly, the most prolific users of MP3 are musicians and Web surfers who have all but created an online culture around the exchange of MP3 (music only) files, so most of the information gleaned about this cutting-edge technology has been through its application on the Internet. Although it may be difficult in the infancy of a new trend to determine its ultimate effects on the A-V world, what is clear is that MP3 is here to stay, and it is evolving. The format of this transmission, compression and decoding scheme will no doubt undergo many changes over time to suit the needs of its users and exploit what will be an ever-changing computer hardware marketplace.

So what does this technology mean to the installation and contracting professional? MPEG offers a unique way of transmitting and receiving audio and video data over the Internet. In the future, manufacturers will be able to design devices that can be updated with such new program material as music and messages as well as program material that is developed, modified and mixed via computer. An advantage to this technology is its media - it does not wear out like tape or other magnetic media, nor does it require more permanent media to be produced, such as CD-ROM or DVD. It can be continuously updated via an Internet connection, thereby avoiding costly on-site service. Venues where extremely high-fidelity program material is not paramount, such as background music applications or video in such environments as restaurants, bars and retail applications, are likely to benefit the most at a point in time sooner than other venues as the technology develops further.

Although MPEG 2 delivers audio and picture quality equivalent to TV studio standards, it is not perfect. MPEG is not a loss-free encoding or compression scheme, and its application results in a loss in signal quality. A way to understand compression loss lies in understanding that you do not get 100% signal quality after compression, but by setting the encoding parameters in advance of the compression so that you can ensure that the material you encode and subsequently transmit, download or read from a CD is of a high enough quality for your target audience. Another point to remember is that the encoding setups will differ with different applications, using different schemes for broadcast, downloading over the Internet, or producing a CD.

Defining the technology

A single piece of hardware (or software) that can do both encoding and decoding is sometimes referred to as a codec (encoder/decoder). In audio, the current MPEG specifications are broken up into layers, termed 1, 2, and 3. The popular abbreviation, MP3, refers to audio layer three encoding. No one seems to know why 128 kbps MP3 became the choice for downloading files from the Internet instead of 128 kbps MP2. In all likelihood, it happened this way because MP3 is a more recent development than MP2, and although MP3 is a higher revision version than MP2, people sometimes assume MP3 is superior. It is a fact that MP3's predecessor was audio layer two or MP2 encoding, and many people believe it to be superior to MP3 at bit rates of 128 kbps or higher.

Higher fidelity encoding, however, requires more resources, and this means more bandwidth and an increased demand for data storage space. Higher layers increase the amount of audio data compression and the complexity of encoding the audio signal. It is less mathematically intensive (and therefore takes less time) to encode a signal on layer one as audio than it is to encode the same signal on layer two as audio. The layers are hierarchical, so a layer three decoder should be able to decode layer two audio. Layer three is built on the features of layer two, adding a modified FFT (Fast Fourier Transform) and a modified discrete cosine transform to the encoding process. Encoding for layer three is more computationally intensive then layer two or layer one. The more complex encoding schemes and algorithms of the higher layers can improve audio quality, despite having greater compression. Even with increased compression and a lower bit rate, layer three audio encoding offers equal ! or greater quality then layer tw o audio encoding. For the overall effectiveness of MPEG audio's different layers, refer to table 1.

Economy of scale: time vs. audio quality

Increased computation time on the encoding side is a small price to pay for the quality and compression that even MP3 affords. Thus, MP3 encoding is starting to be applied at even the professional audio level. For example, 4 minutes of audio from a standard audio CD requires about 40 MB of disk or server space. The equivalent MP3 or MP2 file encoded at a 128 kbps constant bit rate takes up about 4 MB of space, a tenth of the space (a 10:1 compression ratio).

Some audiophiles describe the quality of MP3 audio at 128 kbps as not being even remotely close to CD. Most people, however, hear 128 kbps constant bit rate MP3 audio as comparable to a Dolby B or Dolby C cassette recording of a state-of-the art CD; there is a reduction in the dynamic range and some loss of highs and imaging, but content will remain a far cry from unlistenable. Different codecs can provide varying levels of audio quality, and more importantly, such encoding parameters as the encoding model or algorithm, where to cutoff low frequencies, and the choice of stereo modes can affect the sound quality any MP3 encoder will produce. Decoders can vary in quality in similar ways.

Tradeoffs: bit rate vs. bandwidth

An important consideration in encoding audio is the relationship between audio quality and bit rate (or bandwidth) and how much space the data requires on disk or in memory. If you encode at lower bit rates, audio quality can suffer, but lower bit rates are better suited to slower speed network and transmission lines. Similarly, files encoded at lower bit rates also take up less size in memory or to data storage. If you are willing to double your bandwidth from 128 kbps to 256 kbps, then constant bit rate MP2 or MP3 audio is fairly close and perhaps indistinguishable from CD quality. The 4 minute selection example mentioned earlier now requires about 8 MB of disk space when encoded in 256 kbps constant bit rate MP2 audio, or you get a 5:1 compression ratio.

Further, doubling the bandwidth from 128 kbps to 256 kbps to increase the audio quality halves the compression ratio from 10:1 to 5:1 and doubles the storage to contain the entire file all at once on disk or in memory. Broadcasters will also need to rent or buy faster network connections to transmit audio at higher bit rates.

Downloading

Downloading audio means receiving audio data from a server over the network. Downloads of files usually require the entire file to be copied to disk before anything can subsequently be done with the file (like playing it). Therefore, downloading files of MP3 music means the end user waits for the entire file to be copied over the network to his local disk before playing it. If you are transmitting data to a client with a 56 kbps modem, the 4 minute, 4 MB MP3 file will take about 10 minutes to download, assuming the network connection between your computer and the server does not encounter severe degradation or bottlenecks. The 8 MB MP2 file would take twice as long, or about 20 minutes to download, but compare this to the amount of time it would take to download the original 40 MB CD audio file - 100 minutes.