# A Review of Time-Scale Modification of Music Signals +
## 6 TSM Based on Harmonic-Percussive Separation  
### The Procedure  
As we have seen in the previous sections, percussive sound components and transients in the input signal cause issues with WSOLA and PV-TSM. On the other hand, OLA is capable of modifying percussive and transient sounds rather well, while causing phase jump artifacts in harmonic signals.  
Figure 10: Illustration of a smeared transient as a result of applying PV-TSM. The input signal \(x\) has been stretched by a factor of \(\alpha=1.8\) to yield the output signal \(y\).  
The idea of a recent TSM procedure proposed in [8] is to combine PV-TSM and OLA in order to modify both harmonic and percussive sound components with high quality. The procedure's principle is visualized in Figure 11. The shown input signal consists of a tone played on a violin (harmonic) superimposed with a single click of castanets halfway through the signal (percussive). The first step is to decompose the input signal into two sound components: a _harmonic component_ and a _percussive component_. This is done by using a _harmonic-percussive separation_ (HPS) technique. A simple and effective method for this task was proposed by [PERSON] in [23], which we review in Section 6.2. As we can see in Figure 11, the harmonic component captures the violin's sound while the percussive component contains the castanets' click. A crucial observation is that the harmonic component usually does not contain percussive residues and vice versa. After having decomposed the input signal, the idea of [8] is to apply PV-TSM with identity phase locking (see Section 5.6) to the harmonic component and OLA with a very short frame length (see Section 3.3) to the percussive component. By treating the two components independently with the two different TSM procedures, their respective characteristics can be preserved to a high degree. Finally, the superposition of the two modified signal components forms the output of the procedure. A conceptually similar strategy for TSM has been proposed by [PERSON] and [PERSON] in [24] where they use a _sines+transients+noise_ signal model [25; 26; 27; 28] to parameterize a given signal's sinusoidal, transient, and noise-like sound components. The estimated parameters are then modified in order to synthesize a time-scaled output signal. However, their procedure relies on an explicit transient detection in order to estimate appropriate parameters for the transient sound components.  
In contrast to other transient preserving TSM procedures such as [13; 22; 24; 29], the approach based on HPS has the advantage that an explicit detection and preservation of transients is not necessary. Figure 12 shows the same input signal \(x\) as in Figure 10. Here, the signal is stretched by a factor of \(\alpha=1.8\) using TSM based on HPS. Unlike Figure 10, the transient from the input signal \(x\) is also clearly visible in the output signal \(y\). When looking closely, one can see that the transient is actually doubled by OLA (Section 4.2). However, due to the short frame length used in the procedure, the transient repeats so quickly that the doubling is not noticeable when listening to the output signal.  
Figure 11: Principle of TSM based on harmonic-percussive separation.  
The TSM procedure based on HPS was therefore capable of preserving the transient without an explicit transient detection. Since both the explicit detection and preservation of transients are non-trivial and error-prone tasks, the achieved implicit transient preservation yields a more robust TSM procedure. The TSM approach based on HPS is therefore a good example of how the strengths of different TSM procedures can be combined in a beneficial way.