This is only going to be a brief overview, because if I go into too much detail it will end up as a doctoral thesis.
I should also point out that these are generalisations of eras, because there are always songs which go against the norm. I'm just focusing on what was the more common style of the time.
It's difficult (if not impossible) to compare the early styles of western classical music to early idol music. I mean, how do the styles of the ars antiqua and ars nova eras apply to any idol music? It was around this period in history when concepts of form/structure in music started developing properly, but the musical styles themselves are too vastly different to anything you find in idol music that it can't be compared. So, the idol music "ars antiqua"/"ars nova" era will have to be their own style, separate to anything from medieval music.
The whole idol craze thing came about in the 1970s, with Yamaguchi Momoe being among the most popular idols. Take a listen to this song of hers from 1976 -
And another from 1977 -
And here's a song from 1977 by another 70s idol, Sakurada Junko. I should warn you that the audio is so badly out of sync that it might make you sick -
Well, I guess you could call this style the ars antiqua of idol music, though it probably isn't a good idea to assign a label to idol music this early, because the "idol musical style" hadn't been properly established at this point. So, it's probably a better idea to look a little further forward in idol history before calling something the ars antiqua style. The 70s idol music I linked to could therefore be considered the early medieval era of idol music (era before ars antiqua).
Common features of this era (particularly late 70s) seem to be simple monophonic lines over a drum accompaniment and other parts (such as strings) providing chordal support. There also tends to be use of instrumental refrains.
Let's jump forward to the 1980s, which is considered the "golden age of idols", because this is when idols started dominating the popular music scene. So, this is probably a good era to call the ars antiqua of idol music. Here are some idols songs from across the whole decade, starting with a song by Kyoko Koizumi in 1982 -
Next, here's a song by Matsuda Seiko from 1983 -
Here's a song by her from 1985 -
And here's a song from Akina Nakamori from 1986 (actual song doesn't start until about 3:08) -
Here's a song by Shizuka Kudou from 1988 -
Finally, here's another Matsuda Seiko song, this time from 1989 -
I don't know about you, but this is sounding a lot more like "idol music" to me.
What I gather from all that (and other idol songs that I haven't linked - the above are just a few examples), is the 80s could be divided as ars antiqua - ars nova - ars subtilior in the same way mid-late medieval music is divided. Early 80s idol music could be considered the ars antiqua era of idol music. The early 80s has features similar to that of 70s idol music, including the accompaniment style which is still very simplistic, but there tends to be more call and response between parts and more variety with the instrumentation as well. There also tends to be more going on within the texture, rather than just a voice singing over a drum beat and chordal support. Regardless of how similar it is to late 70s idol music, it's still stepping towards the "idol music style" which truly became established later on in the 80s. So, we can call the early 80s the ars antiqua of idol music because the concepts are starting to evolve here.
The mid 80s is the ars nova era. The ars nova era of medieval music was a big development because it was here when a new notational system developed, which resulted in specific/unambiguous notation of pitches and rhythms for the first (you could write something in one city and take it somewhere else and it could be played by performers using nothing else but the manuscript). Why am I telling you this? Because it signifies a big development in musical history, much like the mid 80s had it's own big development in idol music. The difference between early 80s and mid 80s idol music is clear - mid 80s songs are structured differently featuring more variety in a song and the harmony tends to be more interesting. There's more rhythmic interest found in songs from this era too (compared to earlier songs).
The late 80s can be considered the ars subtilior era. This expanded upon the traditions of ars nova in medieval music and can be seen to do so in idol music also. Listening to late 80s idol songs, we can hear that the features of mid 80s songs is still there, but a lot more "sophisticated"/developed; the use of harmony is awesome in these songs and a lot more interesting than that used in much earlier songs. Creative use of rhythm continues to be used here also, with use of triplets, subtle cross-rhythms, syncopation and such. Incidentally, music of the medieval ars subtilios also tended to be more rhythmically complex than previous eras. Though obviously the idol song rhythms aren't anywhere near as complex as those seen in the classical ars subtilior era...
An idol group called Onyanko Club showed up during this decade as well. Their style fits in with what I've already mentioned. This song from 1986 is a good example -
The popularity of idols died away in the early 90s, but started regaining popularity very late in the decade (/early 2000s).
Idol music of the early 90s is basically a transition period from ars subtilior into the early renaissance). Here's an early 90s example from Matsuda Seiko (1993) -
One of the changes during the transition from ars subtilior to early renaissance was less emphasis on rhythm, going for a more flowing style. Rhythmically, early 90s idol music was also less complex and more flowing, so this is yet another development that is similar between idol music development and western classical music.
The idol group Nezumikko Club from the early 90s also fits in with this style -
Vocal harmonies in idol pop music (with a main vocalist and backing vocals harmonising) become more prominent later on in the decade. This could be considered the renaissance era of idol music, because the renaissance era of western music made much use of polyphonic vocal writing and also featured voices harmonising in 3rds or other intervals. Of course, earlier idol music made use of harmonies, but it didn't seem to be done to the extent of late 90s/early 2000s. The rise of use of harmonies in this way is obviously due to the fact that in idol groups there are more singers, giving the songwriter more to work with.
But before we go into late 90s, let's look at what happened in the mid 90s.
Here's a mid 90s song by Kudou Shizuka (1996) -
Note the use of vocal harmonies, particularly in the chorus. This seems to be a fairly common feature of idol music of this time. Best way to explain what happened between early 90s and late 90s in idol music is that it started transitioning from the ars subtilior era of idol music to the renaissance era of idol music with the use of harmonies and more contrapuntal movement between vocal parts.
Now let's look at early Morning Musume. This was their first single, in early 1998 -
Now this is the idol renaissance! Dem harmonies and dat call and response between vocal parts. Also use of some contrapuntal vocal writing in the chorus.
The grestest example of this style is without a doubt Yume no Naka from their first album -
DEM harmonies. DAT contrapuntal vocal writing. DAT call and response.
I'm not saying it's the best song on the album, I'm just saying it's the best example of this style.
This style continued on into their second album -
And, to a certain extent, on their third album -
(whoever mixed that song needs to be shot)
They toned it down a bit on this album, but it's still there.
Of course, solo artists tended to not have the same use of harmonies and counterpoint due to the fact there's only one singer... But there was still good use of harmonies. For example, a 1999 song from Amuro Namie -
Mainly though, it was morning musume doing all these harmonies and stuff.
By the time the 4th Morning Musume album came about, the style of the first 3 albums was almost completely gone; limited use of harmonies and pretty much no counterpoint in the voices. This album marks the start of the transition away from the "renaissance era" and into the baroque era.
Impossible to speak of the 4th album without linking this song:
The stylistic difference between the first 3 albums and the fourth album is disturbing. It's like Tsunku said "you know what, it's time for something different" and overnight completely changed style. The music focuses more on straightforward melodies and rhythms, with more focus on the solo line (which is often emphasised with many vocalists singing in unison) instead of textural interest between voice parts as in earlier songs. The songs here have a certain energy to them which wasn't present in most earlier songs. There is still use of harmonies found in this era too, of course, but it's nowhere near the level of the first album.
Interestingly, during the transition from renaissance to baroque in western classical music there was attempt to revive the dramatic forms of Ancient Greece. More importantly, the revival of the musical device of monody (solo line with accompaniment) - which is an extreme contrast to the polyphonic style of renaissance. So it was during this era when the shift from polyphony (with voices treated equally) to monody (with the solo melody and bass line being the imporant parts, and any inner parts merely provided harmony). This is basically what we're seeing in the 4th album; the shift from a contrapuntal style to this new style which focuses on a solo melody. Sure, the first 3 albums weren't exactly "true" renaissance polyphony, but there was more focus on equality between the different interweaving vocal lines than you hear in the new style.
So, the 4th album begins the transition into the baroque era of idol music.
This style continued on into the 5th album (2002) -
And the 6th album (2004) -
...and the 7th (2006) -
Well, they certainly dragged out the transition.
This style stayed around for ages. I guess they were happy with it and didn't see any point moving forward. There are slight differences of course between the 4th album and 7th album, but it's still the same style. It wasn't just limited to Morning Musume either, as solo artists such as Hamasaki Ayumi had songs in this style.
I think we finally start hitting high baroque in the 8th Morning Musume album (2007), due to the rise of excessive amounts of sequential development and the simpler melodic patterns built from shorter ideas and built upon, as demonstrated in the first two tracks in particular. And, of course, more of dem fifths.
Other idol groups finally started showing up en masse around this time too (such as C-ute and AKB48). Of course, the style is similar to what we hear in the 8th morning musume album.
So, the rise of the spam of idol groups brings about the high baroque era of idol music...
Moving forward to 2009, we see the release of Morning Musume's 9th album. Still sticking to the style of 8.
AKB48 was also sticking to the style (2010) -
More groups started coming out left right and centre around this time. Groups like SKE48, NMB48, Momoiro Clover Z, Dream5 and so on. The list goes on forever.
Thus, this brings up to our current era - an era of spam and copy+paste. So many groups out there now and they all seem to spam out music all the time. This is definitely the emergence of the late baroque era of idol music for a couple of reasons:
1) Spamming. So many groups and idol songs these days. Late Baroque composers were known for their prolific writing, as they were expected to have at least one new piece ready to perform every single week. So many groups, just like there were so many composers back in those days (Telemann, Bach, Vivaldi, Rameau, Handel, Scarlatti, etc) and they were all spamming music all the time.
2) Musical style. Well, more precisely, the abuse of the circle of fifths (as seen in late baroque music - see: vivaldi in particular). The abuse of another chord progression in particular, which wasn't really used in baroque music, but it still counts as a point here because it involves harmonic repetition. This style of idol music, like the one before it, uses a lot of sequential development too.
Probably sounds like I'm flaming late baroque music. They did often use the same progressions and frequently used the circle of fifths, but at least it wasn't over the top. Also, late baroque uses epic counterpoint, which is missing in the spam of late baroque idol music.
Fifths in particular has been spammed to a disturbing new level these past few years.
Just look at this madness!
(2011)
(2012)
(2013)
(2012)
(2011)
(2012)
(2013)
(2013)
(2011)
(2011)
(2010)
(2013)
(2013)
(2012)
(2010)
(2011)
(2012)
(2013)
(2010)
(2013)
(2013)
Sweet Jesus, so many and I'm only just getting started! There's just no end to it! I'll better leave it at that or this will go on forever...
You know, fifths have actually been used in idol music since the dawn of time, it's just that recently it's being spammed to hell and back. For example, you can go back all the way to 1973 to find an idol song with dem fifths:
For a bonus, here's a compilation of fifths from late baroque and classical music:
There's another progression (and variations of it) spammed a lot as well... Not quite as much as the fifths, I don't think, but there are a lot. Here are just a few of them:
(2011)
(2012)
(2012)
(2013)
(2013)
So, what's next for idol music? Well, after late baroque would be the galante style. The style is similar to baroque, but with less emphasis on counterpoint and more on simpler melody-driven structures. With this style, idol music should be completely melody-driven and simple, completely based on rhythmic and melodic patterns... This is an example of galante music:
As you can hear, less emphasis on spamming dem fifths and more emphasis on melodic phrasing. Maybe that's where idol music goes next - it'll stop spamming fifths and having more developed melodies or something. Who knows?
