DIGITAL IMPLEMENTATION OF ARTIFICIAL REVERBERATION ALGORITHMS Irina DORNEAN, Marina ŢOPA, Botond Sandor KIREI Faculty of Electronics, Telecommunications and Information Technology, Technical University of Cluj-Napoca Str. G. Bariţiu nr. 26-28,400020 Cluj-Napoca, Romania, Tel: +40264401470; Fax: +40264591340 Irina.Dornean@bel.utcluj.ro Erwin.Szopos@bel.utcluj.ro Abstract The paper presents the digital implementation of signal processing algorithms that simulate natural concert hall reverberation. It deals with the complete artificial reverberators proposed by Schroeder, Moorer and Gardner. For each reverberation algorithm the implementation was done in two steps: 1. the algorithm is checked using a model in Matlab Simulink; 2. the Verilog code is written and tested. The results prove that the Verilog design is feasible and can be further developed for acoustic improvements of rooms. Keywords: artificial reverberators, FPGA 1 INTRODUCTION Natural reverberation is the combined effect of multiple sound reflections within a room [1], [4]. It might be described by the impulse response of the room, obtained as room’s time response to a very short burst. A typical impulse response in a reverberating room is shown in Figure 1. Figure 1 The impulse response of a room The reverberation time is the amount of time it takes for the sound to die away to 60 dB of its original value. The first reflection time is the amount of time separating the direct sound from the first reflection. Natural reverberation typically follows a quasi- exponential curve that reaches a peak and decays more or less slowly. Artificial reverberation is generated, manipulated and combined in order to get a stereo high quality sound recording that looks natural. This effect spatializes sounds, thus leading to the illusion of sounds emerging from imaginary environments. From a signal processing point of view, an artificial reverberator is a filter with an impulse response that resembles the impulse response of a concert hall, cathedral, etc. [2]. The digital approach of the reverberation phenomena plays the most important role in the audio signal processors domain [5]. The scope of the digital reverberators is to delay certain frequencies, which will make the sounds reach the listener in different moments of time, adding life and sense of space. 2 ARTIFICIAL REVERBERATORS The modern technologies bring several solutions for the reverberation problem. Nowadays, the new computers and digital signal processors allow a complicated digital simulation of a natural reverberation. Schroeder (I) reverberator The first digital solution of the reverberation was founded by Manfred Schroeder in 1961 and it is based on the comb filter (Figure 2). It consists of a delay cell with an output redirected to the input. Out 1 Integer Delay -1 Z Gain -K- Add In 1 Figure 2 Comb filter Schroeder noticed that the structure of a comb filter can be modified to get an all-pass filter as shown in Figure 3. Such a filter has a smooth frequency response. Out 1 Integer Delay -1 Z Gain2 -K - Gain1 - K- Add2 Add1 In 1 Figure 3 All-Pass filter Schroeder proposed a reverberator composed of parallel comb filters, series connected with all pass filters, as depicted in Figure 4.