3-D Spatialization and Localizationand Simulated Surround Sound with

Headphones

Lucas O’NeilBrendan Cassidy

Overview

• 3D with headphones– HRTF Model– Convolution– 360 + elevation Panning

• Upmixing– Pro Logic– Delays– Filters– Sub– Autopanning

• Downmixing with HRTF

Mathematical HRTF Model

• We tell 3D directionality through 3 cues other than just ITD and IID

• Pinna Reflections

• Shoulder and Torso Reflections

• Head Shadow and ITD

• Can model using filters/delays.

• Shoulder/Torso reflection simulated by echo:

• Pinna reflections via a tapped delay line

• Head shadow diffracts the sound wave.• Simulated in digital domain by 1st order IIR filter:

• ITD due to separation obtained by allpass filter with group delay:

• Input azimuth and elevation angle.

• Delay by shoulder echo.

• Add delay line due to pinna reflections.

• Filter through Head Shadow and ITD filters.

• spatialization~!

Convolving with HRIR

• HRIR = Head Related Impulse Response

• Measured with KEMAR dummy (MIT)

• Convolve audio with impulse response corresponding to appropriate angle

360 Corkscrew Panning

• Demo to shown point source spatialization.

• Pick rotation frequency for azimuth and elevation.

• Breaks up signal into blocks and performs HRTF with different angles on each block to simulate 360 rotation around head and elevation from -90 to +90

• Done with both mathematical model and convolution techniques.

5.1 Surround Sound

Upmixing to 5.1 Surround

• Investigated Dolby Pro Logic decoder.

• Initially used gains/phase shift matrix to split up stereo signal

• Tweaked further adding delays to center and surround channels

• Pro Logic II has 3 modes of operation:– Movie (not used in this project)– ‘Pro Logic’– Music

• Surround channel uses 7kHz LPF in Pro Logic mode

• Surround channel uses Shelving Filter in music mode– Used 4kz cutoff for shelving.

• Surround channel has 20ms delay in Pro Logic mode, but not music mode

Subwoofer Simulation

• 5 channel surround was losing some low frequency due to cross talk corellation and phase cancellation in surround channels.

• Solved by cloning low frequencies of signal (using 300Hz LPF), then mixing them back in the stereo channel after downmixing the 5 channels

Autopanning

• Pro Logic mode uses autopanning to detect directionality and adjust 5 speaker mix.

• Preserves RMS energy in signal.

Sub-band Autopanning

• Scope of project did not allow for implementation

• Surround upmixer breaks signals into bands and pans those bands to the appropriate location

• Can detect different instruments in music (like frequency keying in DAW software) and localize each instrument.

Dolby Pro Logic Decoder

Our Implementation of Upmixer

Downmixing with HRTF

• Resultant stereo sound file has convincing spatialization effects.

• Pro Logic mode autopanning implemented without sub-band separation tends to have vocals that jump back and forth across the left and right channels.

• Music mode sounded better (for music).

Questions?