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Title: Non-Sequential Melody Generation

Paper content:
In this paper we present a method for algorithmic melody generation using a generative adversarial network without recurrent components.
Music generation has been successfully done using recurrent neural networks, where the model learns sequence information that can help create authentic sounding melodies.
Here, we use DCGAN architecture with dilated convolutions and towers to capture sequential information as spatial image information, and learn long-range dependencies in fixed-length melody forms such as Irish traditional reel.
Algorithmic music composition is almost as old as computers themselves, dating back to the 1957 "Illiac suite" (Hiller Jr & Isaacson, 1958) .
Since then, automated music composition evolved with technology, progressing from the first rule-and-randomness based methods to the sophisticated tools made possible by modern-day machine learning (see Fernández & Vico (2013) and Briot et al. (2017) for detailed surveys on history and state of the art of algorithmic music composition).
One of the first machine learning (ML) approaches to music generation was Conklin & Witten (1995) , who used the common notion of entropy as a measurement to build what they termed a multiple viewpoint system.
Standard feedforward neural networks have difficulties with sequence based information such as music.
Predicting the next note of a piece, when only based on the current note, does not account for long-range context or structure (such as key and musical sections) which help give coherence to compositions.
As music is traditionally represented as sequences of notes, recurrent neural networks are a natural tool for music (especially melody) generation, and multiple groups used RNNs fairly successfully for a variety of types of music.
Todd (1989) used a sequential model for composition in 1989, and Eck & Schmidhuber (2002) used the adapted LSTM structure to successfully generate music that had both short-term musical structure and contained the higher-level context and structure needed.
Subsequently, there have been a number of RNN-based melody generators (Simon & Oore, 2017; Lee et al., 2017; Eck & Lapalme, 2008; Sturm et al., 2016; Chen & Miikkulainen, 2001; Boulanger-Lewandowski et al., 2012; .
Other approaches such as MidiNet by Yang et al. (2017) , though not RNNs, also leveraged the sequential representation of music.
Using an RNN architecture provides a lot of flexibility when generating music, as an RNN has the ability to generate pieces of varying length.
However, in some styles of music this is not as desired.
This is true of traditional Irish music -and especially their jigs and reels.
These pieces have a more rigid format where the varying length can prevent capturing the interplay between the phrases of the piece.Finding jigs and reels to train on was made easy by an excellent database of Irish traditional melodies in ABC notation (a text based format), publicly available at TheSessionKeith.
Several RNN-based generators were trained on the melodies from TheSession, most notably Sturm et al. (Sturm et al., 2016; Sturm & Ben-Tal, 2018) , as well as Eck & Lapalme (2008) .
It is natural to view music, and in particular melodies, as sequential data.
However, to better represent long-term dependencies it can be useful to present music as a two-dimensional form, where related parts and occurrences of long patterns end up aligned.
This benefit is especially apparent in forms of music where a piece consists of a well-defined, fixed-length components, such as reels in Irish music.
These components are often variations on the same theme, with specific rules on where repeats vs. changes should be introduced.
Aligning them allows us to use vertical spatial proximity to capture these dependencies, while still representing the local structure in the sequence by horizontal proximity.
In this project, we leverage such two-dimensional representation of melodies for non-sequential melody generation.
We focus on melody generation using deep convolutional generative adversarial networks (DCGANs) without recurrent components for fixed-format music such as reels.
This approach is intended to capture higher-level structures in the pieces (like sections), and better mimic interplay between smaller parts (musical motifs).
More specifically, we use dilations of several semantically meaningful lengths (a bar or a phrase) to further capture the dependencies.
Dilated convolutions, introduced by Yu & Koltun (2015) , have been used in a number of applications over the last several years to capture long-range dependencies, notably in WaveNet (Oord et al., 2016) .
However, they are usually combined with some recurrent component even when used for a GAN-based generation such as in Zhang et al. (2019) or Liu & Yang (2019) .
Not all techniques applicable to images can be used for music, however: pooling isn't effective, as the average of two pitches can create notes which fall outside of the 12-semitone scale (which is the basis the major and minor scale as well as various modes).
This is reflected in the architecture of our discriminator, with dilations and towers as the main ingredients.
Converting sequential data into a format which implicitly encodes temporal information as spatial information is an effective way of generating samples of such data as whole pieces.
Here, we explored this approach for melody generation of fixed-length music forms, such as an Irish reel, using non-recurrent architecture for the discriminator CNN with towers and dilations, as well as a CNN for the GAN itself.
One advantage of this approach is that the model learns global and contextual information simultaneously, even with a small model.
LSTMs and other approaches need a much larger model to be able to learn both the contextual neighboring note sequences and global melody structure.
In future work, we would like to introduce boosting in order to capture the structure of the distribution more faithfully, and increase the range of pieces our model can generate.
Natural extensions of the model would be to introduce multiple channels to capture durations better (for example, as in Colombo et al. (2016) ), and add polyphony (ie, using some form of piano roll representation).
Another direction could be to experiment with higher-dimensional representation of the sequence data, to better capture several types of dependencies simultaneously.
Additionally, it would be interesting to apply it to other kinds of fixed-length sequential data with long-range patterns.

assistant: text: Representing melodies as images with semantic units aligned we can generate them using a DCGAN without any recurrent components.