Free Talks in MP3 Audio
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A note on MP3 Quality
When performing audio encoding, such as creating an MP3 data stream, there is a
trade-off between the amount of data generated and the sound quality of the
results. The person generating an MP3 selects a bit rate, which specifies how
many kilobits per second of audio is desired. The higher the bit rate, the
larger the MP3 data stream will be, and, generally, the closer it will sound to
the original recording. With too low a bit rate, compression artifacts (i.e.,
sounds that were not present in the original recording) may be audible in the
reproduction. Some audio is hard to compress because of its randomness and sharp
attacks. When this type of audio is compressed, artifacts such as ringing or
pre-echo are usually heard. A sample of applause or a triangle instrument with a
relatively low bit rate provide good examples of compression artifacts. Most
subjective testings of perceptual codecs tend to avoid using these types of
sound materials, however, the artifacts generated by percussive sounds are
barely perceptible due to the specific temporal masking feature of the 32
sub-band filterbank of Layer II on which the format is based.
Besides the bit rate of an encoded piece of audio, the quality of MP3 encoded
sound also depends on the quality of the encoder algorithm as well as the
complexity of the signal being encoded. As the MP3 standard allows quite a bit
of freedom with encoding algorithms, different encoders do feature quite
different quality, even with identical bit rates. As an example, in a public
listening test featuring two early MP3 encoders set at about 128 kbit/s,[54] one
scored 3.66 on a 1–5 scale, while the other scored only 2.22. Quality is
dependent on the choice of encoder and encoding parameters.
This observation caused a revolution in audio encoding. Early on bitrate was the
prime and only consideration. At the time MP3 files were of the very simplest
type: they used the same bit rate for the entire file: this process is known as
Constant Bit Rate (CBR) encoding. Using a constant bit rate makes encoding
simpler and less CPU intensive. However, it is also possible to create files
where the bit rate changes throughout the file. These are known as Variable Bit
Rate The bit reservoir and VBR encoding were actually part of the original
MPEG-1 standard. The concept behind them is that, in any piece of audio, some
sections are easier to compress, such as silence or music containing only a few
tones, while others will be more difficult to compress. So, the overall quality
of the file may be increased by using a lower bit rate for the less complex
passages and a higher one for the more complex parts. With some advanced MP3
encoders, it is possible to specify a given quality, and the encoder will adjust
the bit rate accordingly. Users that desire a particular "quality setting" that
is transparent to their ears can use this value when encoding all of their
music, and generally speaking not need to worry about performing personal
listening tests on each piece of music to determine the correct bit rate.
Perceived quality can be influenced by listening environment (ambient noise),
listener attention, and listener training and in most cases by listener audio
equipment (such as sound cards, speakers and headphones). Furthermore,
sufficient quality may be achieved by a lesser quality setting for lectures and
human speech applications and reduces encoding time and complexity. A test given
to new students by Stanford University Music Professor Jonathan Berger showed
that student preference for MP3-quality music has risen each year. Berger said
the students seem to prefer the 'sizzle' sounds that MP3s bring to music.
An in-depth study of MP3 audio quality, sound artist and composer Ryan Maguire's
project "The Ghost in the MP3" isolates the sounds lost during MP3 compression.
In 2015, he released the track "moDernisT" (an anagram of "Tom's Diner"),
composed exclusively from the sounds deleted during MP3 compression of the song
"Tom's Diner", the track originally used in the formulation of the MP3 standard.
A detailed account of the techniques used to isolate the sounds deleted during
MP3 compression, along with the conceptual motivation for the project, was
published in the 2014 Proceedings of the International Computer Music
Conference.
