MP3

• the quality depends not only on the encoding file format, but also on the quality of the psycho acoustic of the encoder. Typical layer 1 encoders use a very simple psycho acoustic which result in a higher needed bitrate for transparent encoding.
• Layer 1 encoding at 384 kbit/s even with this simple psychoacoustic is better than Layer 2 at 192...256 kbit/s
• Layer 3 encoding at 112...128 kbit/s is worse than Layer 2 at 192...256 kbit/s.

• Layer 1: excellent at 384 kbit/s
• Layer 2: excellent at 256...320 kbit/s, very good at 224...256 kbit/s, good at 192...224 kbit/s, should not be used below 160 kbit/s
• Layer 3: excellent at 224...256 kbit/s, very good at 192...224 kbit/s, good at 160...192 kbit/s, should not be used below 128 kbit/s

The algorithm of the MP3 format uses, at its heart, a hybrid transform to transform a time domain signal into a frequency domain signal:

• 32 band polyphase quadrature filter
• 36 or 12 Tap MDCT, size can be selected independent for subband 0...1 and 2...31
• alias reduction postprocessing

An important feature of MP3 is that it is lossy -- meaning that it removes information from the input in order to save space. As with most modern lossy encoders, MP3 algorithms work hard to ensure that the sounds it removes cannot be detected by human listeners, by modelling chacteristics of human hearing such as noise masking.

However, experienced listeners can tell the difference from the original at 192 kbit/s, and even at 256 kbit/s on some of the less powerful (and obsolete) encoders. If your aim is to archive sound files with no loss of quality, you may be more interested in lossless audio compression such as FLAC, SHN, or LPAC -- these will generally compress a 16-bit PCM audio stream to approximately 50-75% of the original size (depending upon the characteristics of the audio itself).

There are also some non-lossy (lossless) audio compression methods used on the internet. While they are not similar to MP3, they are good examples of other compression schemes available. These include:

• SHN
• FLAC
• Monkey's Audio

Though proponents of newer codecs such as WMA and RealAudio have asserted that their respective algorithms can achieve CD quality at 64 kbit/s, listening tests have shown otherwise; however, the quality of these codecs at 64 kbit/s is definitely superior to MP3 at the same bandwidth.

Thomson claims that its mp3PRO codec achieves CD quality at 64 kbit/s, but listeners have reported that a 64 kbit/s mp3PRO file compares in quality to a 112 kbit/s MP3 file and does not come reasonably close to CD quality until about 80 kbit/s.

The Xiph.org Foundation, the developers of the Vorbis algorithm used in the new Ogg format, claims that Vorbis somewhat exceeds MP3 and WMA sound quality while infringing no patents, and provides a web page with listening tests to demonstrate this.

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