## 1 Introduction  
Holographic interferometry began to be actively used in research in various fields of physics for several decades. This approach was applied to, among other problems, studies of the mechanical properties of membranes and solids [1; 2; 3], processes occurring in plasma [4; 5; 6], gas flows [7], shock waves [8; 9] and many other physical phenomena [10; 11; 12]. Besides this, holographic interferometry was used for the non-destructive testing of a wide range of industrial products [13; 14]. The rapid progress in digital matrix photodetectors and their widespread use significantly affected further development of holographic methods.  
Digitalization allowed for the significant simplification of hologram recording procedures and enabled automatic data processing. However the transition from high-resolution photographic films or plates to digital matrices imposed significant restrictions on the minimal resolvable width of interference fringes, and necessitated changes in experimental methodology, which was previously based on the simultaneous optical reconstruction of two object waves existing at different time moments but recorded on the same frame [10]. An adaptation of interference methods to digital recording triggered the development of a number of numerical algorithms optimized for the analysis of digital holograms, in particular, those of phase-image deconvolution [15; 16; 17], filtering [18; 19; 20; 21] the automatic segmentation of specific parts of object waves on recorded phase distributions [22; 23], as well as tomographic algorithms [24; 25]. The development of high-speed digital cameras equipped with fast, global shutters allowed eliminating the need to probe rapid dynamic processes using pulsed lasers [26]. At the same time, despite much more frequent use of digital holography, especially in the field of microscopy and biology [27; 28; 29; 30; 31], holographic interferometry with traditional recording on holographic plates and films is still in demand in a number of applications [32; 33; 34; 35; 36]. Among these applications is the recording of elastic waves of different types in solids, in particular, shock and strain solitary waves (solitons) in solid waveguides. Linear elastic waves are widely used for the non-destructive analysis of internal structure and integrity of constructional elements [37]. Nonlinear elastic waves can be applied for the investigation of the nonlinearly elastic and viscoelastic properties of materials [38; 39; 40].  
In this paper we report a comparative analysis of experimental results on monitoring the strain soliton formation from shock waves in a solid waveguide by means of holographic interferometry and digital holography. Specific advantages and disadvantages of the two techniques are discussed in view of the investigation of this particular process, characterized by significant difference in the parameters of these two types of elastic waves. The two, basically different, pump-probe approaches applied in the holographic monitoring of fast processes (at the nanosecond time scale) are validated and analyzed. The utilization of both techniques for the investigation of the same process demonstrated similar results; in particular, close values of soliton parameters and the evolution dynamics of fast-decaying, short-wavelength, oscillating components. Specific features of the procedure for stitching together several frames obtained by both digital holography and classical holographic interferometry are discussed. Certain specifics of spatial resolution, achieved using these two techniques, are analyzed as well. In general, we demonstrate that, in spite of the relatively complex experimental implementation classical holographic interferometry has certain advantages over easily-automated digital holography.