Audio Directional Loudspeaker Signal Processing System Design And Testing

Directional loudspeaker is a revolutionary acoustic device generating audible sound with high directivity which is based on non-linear interaction of ultrasound in the air. A DSP (Digital Signal Processing) platform, where modulation algorithms run in real-time, is created in this thesis, and the best algorithm is defined to produce audible sound with low distortion and high directivity tested and verified.The DSP platform, based on ADSP-Blackfin533, is designed to modulate the carrier with audible sound in real time. While the measurment instrument, which ranks SPL (Sound Pressure Level), THD (Total Harmonic Distortion) and directivity of the loudspeaker, is set up in PC. Parameters setting and modulation algorithm are discussed innovatively. The main content of this thesis is as following:1. Researches on directional audio are stated, while non-linear interaction of ultrasounds and parametric array are illustrated. Above are theoretical basis of modulation algorithms.2. DSP module is established according to the requirement of whole directional loudspeaker. The signal sampling, signal output, memory, power and reset are designed. And the layout of PCB (Printed Circuit Board) is accomplished conforming to signal integrity and electromagnetic compatibility.3. Digital interval pulse modulation is proposed to load switch signal with low frequency to amplifier, and the square wave is analyzed in time domain and frequency domain:high harmonics are reduced and efficiency is enhanced.4. Several parameters such as the carrier frequency and modulation depth are determined, the maximum of SPL is limited and the optimal distance for testing is given.5. Truncated DSB (Double Sideband) modulation, square rooting truncated DSB, approximating square rooting, SSB (Single Sideband) are compared to approach the best algorithm in respect of SPL and THD. Approximating square rooting has the lowest THD in the former three algorithms, and SSB has the least harmornics as analyzing. Finally, high directivity is achieved in the directional loudspeaker in this thesis: The directional angle of -3dB is only 3°, and the one of -15dB is 30°.