# A Real-Time Sound Field Rendering Processor
## 4 Performance Estimation
### Performance of the Prototype Machine
#### 4.1.2 Sample Rate of the Output Sound  
Although the prototype machine carries out sound rendering at run-time, the output sound quality is worse than that output by the offline computer-based software simulation. In the software simulation, incident data are read from a sound wave file directly at each time step, and the rendering results are written into another sound wave file after the computation is finished. The output sound  
Figure 11: Impulse response of the defined room.  
Figure 10: Computation time by using the OpenMP in software simulation.  
wave, therefore, has same sample rate and bitrate as the incidence. However, in the prototype machine, the sample rate of the output sound is calculated through Equation (14).  
\[f_{sampe}=\frac{f_{clk}}{M} \tag{14}\]  
\(f_{clk}\) is the system clock frequency, and \(M\) denotes the number of grids processed by a PE. In the current prototype machine, since each PE performs sound rendering in a sound space with 4 \(\times\) 4 \(\times\) 4 grids, and the system clock frequency is 200 MHz, the sample rate of the output sound is about 3.125 MHz (200 MHz/64). Compared with the rendering system with the time-sharing architecture, in which the sample rate of the output sound is about 12.5 kHz [19], the prototype achieves about 256 times gain in sample rate of the output sound. In the rendering system with the time-sharing architecture, only a computing unit is applied to calculate the sound pressure grid by grid. Hence, as the number of grids is increased, the computation time at a time step will become longer, which results in a low sample rate of the output sound. However, the proposed rendering processor is the hybrid architecture, in which the top level is the parallel architecture, and the PE is based on the time-sharing architecture. Therefore, the computation is speeded up, and high sample rate is obtained in the output sound.  
On the other hand, the incidence is input through an A/D converter and the rendered results are output through a D/A converter in the prototype machine. Because the A/D converter is 14-bit, each incident datum is 14-bit, but it is 16-bit in the software simulation. This long data width of the incidence results in high accuracy in computation in the software simulation. Furthermore, each PE performs sound rendering in a sound space with 4 \(\times\) 4 \(\times\) 4 grids, if the computation at a grid is completed at one cycle through a pipelining technique, calculation at a time step takes 64 cycles. In other words, the incidence will be input in the system every 64 cycles from the A/D converter. This may result in the information loss in the incident wave sound, and reduce the computation accuracy. Another factor to affect the output sound quality comes from the electronic noise of the A/D and D/A boards. When the prototype machine has no input at the A/D converter, the electronic noise can be heard at the output of the D/A converter. Compared with professional audio devices, current A/D and D/A converters provides worse performance in suppressing electronic noise.