Frank
I was trying to analyse these changes and must admit I'm runing short on
ideas. Well, I don't know the melody which could clarify a little.
I was trying to interpret it in terms of Bb major, because the Dm7 G7 could
mean a iiim7-VI7 passage, which isn't uncommon, but I got stuck on the Ab7.
It would be a bVII7 but although this is a diatonic mode of the natural
scale, it seems a little far fetched.
But if you think of this four bars as C major, then Ab7 can be the result of
applying the tritone sub principle to the Dm7, yelding basically 3 bars of
Dm7 followed by a bar of G7.
Is this too simple? (most things in life are...)
Pedro