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As you can see from the three clear definitions above, genre is the most controversial word in this topic.

Genre is definitely of great importance in describing music, one of the most widely used descriptors for a given piece of music. But though important and popular, the concept of genre turns out to be inherently ill-defined, since many attempts at defining it end up to be circular or heavily subjective.

A pertinent remark: Music Genre Recognition is conceptually twice as hard as other recognition problems, precisely because the word genre is difficult to define. In optical character recognition for example, we have a fairly accurate definition of what a character is, so the problem is just to try to recognize these characters. The same in optical note recognition and most other recognition problems. In Music Genre Recognition, however, we need to figure out first a definition of the word genre, then try to recognize it, given a piece of music.

We will see that many approaches to solve this recognition problem greatly depend on the definition adopted for 'genre' and, vice-versa, the definition of 'genre' often follows from the kind of approach used (again, circular things!!)

People tend to classify things into categories, since, usually, it makes life less complicated... A 'Music Genre' can be broadly thought of as a category of music in which pieces share some characteristics in common. If we try to think for a while what those characteristics could be, we rarely come to a unique clear answer.

Try to think for a minute what makes Elvis Presley's "It's Now or Never" (Rock' n Roll genre) different from Bach's "Prelude No. 1 in C Major" (Classical genre) ..

We can make a list of the differences and similarities we notice between these two genre representatives:

Similarities	Dissimilarities
Both have the piano instrument involved	"It's now or never" has more than one instrument while "Prelude" has one instrument
Both have a noticeable melody	"Prelude" has no drums in it
	"It's now or never" 's tempo is faster than "Prelude" 's tempo
	"Prelude" has no singing in it
	"Prelude" is calmer and more relaxing than "It's Now or Never"

But are we sure we listed all the characteristics? Are we sure the characteristics for "It's Now or Never" are present in all or almost all other Rock 'n Roll music pieces?

Is a 10 seconds-length sample enough to let us draw the most important characteristics of the piece? Are we influenced by some cultural knowledge about the artist when drawing characteristics?...As you might have noticed, this table is subjective, and it says very little if not nothing about the definition of "Rock 'n Roll" or of "Classical". You could come up with a completely different table.

Comparisons like this one maybe have to be made over thousands of music pieces to get a finer definition for a particular genre, and relations have to be established between similar pieces, and this is of course very far from possible.

This is not the only problem we face in trying to define genres. We face the problem of sub-genres, and sub-sub-genres, and (sub)^k-genres where k is an undetermined integer that takes values according to a mapping from the set of fantasies to the set of integers. Also, sub-genres can belong to different genres, and some sub-genres can also be genres...!!

In Classical music we have a sub-genre called "Baroque", which refers to the music composed in the Baroque period (1600-1750). A sub-genre of Baroque could be an "Invention" (created by Bach) for example, and a sub-genre of an "Invention" could be a "2-voice Invention". Another sub-genre of Baroque could be a "Prelude". But "Prelude" could be a sub-genre of the "Romantic" genre, like in Chopin's preludes. Now "Romantic" can also be a sub-genre of a "Modern" music, where "Modern" would be the sub-genre of music pieces composed in the 19^th century period. As if this mess is still not enough, people have also created genres beginning with the prefix "pseudo".

Now you could also have genres that combine many other genres. So "Relaxing" Music can include "Classical" Music but can also include "Soft pop" music or can include Yanni's music (which would be a "Relaxing Post-Modern Neo-Classical" genre of music). Also, "Instrumental" music is a a very broad genre which clearly applies to hundreds of other genres (which then would be considered sub-genres...aargh).

But still, to make things even messier, some genres are not always agreed upon. "Relaxing" is a genre that includes classical music pieces. Every time I listen to classical music, I feel relaxed, but I can prove that my neighbor does not. (The proof is simple: Unless he is following the beat, the wall knocks I hear every time I listen to classical music clearly indicate that.)

So genre is intrinsically ill-defined, and this is not unexpected. As a matter of fact, the etymology of the word "genre" dates back to 1770, from the French "genre" which means "kind, sort, style," (from etymonline) the latter words which by definition are vague and broad..

Not to mention, also, the fact that industry manipulates genre to promote sales, as noted in [1].

Apart from the genre problem, we also have a problem in the word "music". Now it is true that we all have an idea about what music looks like (or hears like...), but we really don't know the border between music and noise/sound. A famous example by the American composer John Cage (1912-1992) is a piece of music called "Williams Mix" (1952). I remember last year having strongly hurt my Music History professor when he made us listen to "Williams Mix" and I asked him if that was considered music...It really sounded like sound/noise to me. The prof. told me that Cage spent 12 months composing this piece and wrote 192-pages of scores for that piece, and hence he did not devote all this time to produce sound/noise. Here is a sample of "Williams Mix" (if you hear sounds that seem coming from your window, well they are not, they come from this music piece): .

You might be wondering so far, given the level of pessimism we have reached, whether you should continue to read this webpage... Solutions have been proposed, but they are still all far from the results we would have liked to expect, in comparison to the results achieved in other recognition problems.

However, many good approaches to deal with this problem in the best possible manner have been proposed.

As [2] puts it, genre can be an intentional concept and an extensional concept. By intentional, it is meant that we tend to derive genres based on the prior cultural

knowledge we have about a certain song or artist. By extensional, it is meant that genre can be viewed also as a property of the music piece, much like timbre or tempo.

In the first view, "St. Anger", by Metallica, would be considered as metal rock because it is by Metallica and we all have prior knowledge of what kind of songs this group makes. In the other view, "St. Anger" would be considered as metal rock because of the distorted electric guitar, the way the drum beats sound, the fast tempo and the 'scream-like' singing involved.

In the following section (Why? - Motivation), we give a more tangible study of existing genre classifications and motivate (even more!) the problem by presenting a manual genre classification process that has been done by the authors of [2]. In the section that follows (How? - Approaches), we present 2 approaches to automatic genre recognition: the first is called prescriptive approach (based on prior genre assumptions) and the second is called emergent approach (based on objective similarity measures).