The technology of C3D HRTF Positional Audio

The technology of C3D is licensed from CRL (Central Research Lab.) using an audio filter called Head Related Transfer Functions (HRTFs). The basic concept of C3D : Since we can hear sound three dimensionally in the real world only using two ears, it must be possible to regenerate the same sound effect from two loud speakers.

What is HRTF ?

HRTF (Head Related Transfer Functions) is a set of audio filters which are varying locations of sound effects (spatial hearing cues) in three-dimension measured from listener's eardrum.

Using this technology and special digital signal processing to re-create spatial hearing cues makes our ears to hear a realistic and three-dimensional sounds coming from a pairs of loud speakers or headphones.

There are several listening cues that allows us to hear sounds three-dimensionally :

(I). Spatial hearing : Primary 3D-cues

1. IAD

The head shadowing effect creates differences in the amplitudes of the sound signals arriving at each ear from the source. The effects of diffraction are most noticeable in the range between about 700 Hz to 8 KHz, where the A and S functions periodically converge and diverge gently. This Inter-aural Amplitude difference (IAD) is one of the primary 3D sound cues.

2. ITD

In addition to the IAD, there will be a time-of-arrival difference between the left and right ears, unless the sound source is in one of the pole positions (i.e. directly in front, behind, above and below). This is known as the Inter-aural Time Delay (ITD).

3. Pinna effects

It has been supposed by several researchers, that the convolutions of the pinna create the spectral features which constitute the 'height' cues. In practical experiments by Gardner, in which different parts of the pinna were occluded, and then the ability of a number of subjects to identify sound source positions at different heights was tested, it was shown that the different features all contributed by different amounts. For example, if the fossa is excluded, then height localization capability is impaired, but not totally extinguished. It would be reasonable to conclude that it is the combined effect of the pinna convolutions which create the various localization cues, and it is not valid - or logical - to attempt to assign particular spatial capabilities with individual physical features.

(II). Spatial hearing : Secondary 3D-cues (shoulder & local reflections)

In addition to the 'primary' 3D sound cues (IAD, ITD and pinna effects), there are several additional cues which do contribute to the localization capability; these will be referred to here as 'secondary' cue, and include should/torso reflections, local room reflections, and psychological cues.

1. Shoulder / Torso reflections

The presence of a torso attached to an artificial head has the effect of increasing the pressure in the vicinity of the ear up to frequencies of around 2 kHz. The effect is greater for frontal sources than lateral sources. In the experience, the presence of the torso does not appear to contribute much to spatial accuracy. However, the shoulder are located very close to the ears, and their effect is greater, this time, in respect of lateral sounds. If one listens to an artificial head first without - and then with - shoulder fitments, then it is clear that the shoulders do contribute to spatial effects in certain positions. The shoulders provide a strong reflection from lateral sources, with a short path-length of around 10 cm between direct sound and reflection. The effects are most important for side-positioned sources, especially for "height" effects, where the shoulders tend to mask sources which move below about 30

degrees depression.

2. Local, Room reflections

In simulations, it is clear that the incorporation of first-ordr simulated room reflections can help in the creation of sound images which have a "solid" nature. However, the effects - if accurately simulated - are relatively slight. Experience has shown that it is primarily the quality of the HRTFs themselves which determine the quality and solidity of the sound image. The furter addition of second-order reflections does not help significantly, because in reality, there is a great number of reflections in the average room. A method which does help to recreate the acoustic experience of a room, however, is to use approximate simulations of lateral reverb, using either 2 or 4 laterally placed "virtual" sources at, say, +-70 degrees and 80 degrees azimuth.

- The quality of the sound image relates to the HRTFs used.
- The quality of the room image relates to addition of reflections and reverb.

3. Psychological cues

There are clearly psychological cues present in everyday life which work together with the audio cues to tell us about the world around us. For example, if you hear the sound of a helicopter flying, you expect it to be up in the air, and not downwards. If a dog were to bark nearby, you would expect it to be downwards.

How to listen to C3D sound correctly and properly?

1. Use headphones to have much better effect

When you use headphones in listening, there will less interference such as outside voices or room reflections comparing to using speakers.

2. Choose correct output devices

Choose the correct output devices in the options of demo program in accordance with what listening devices you want to listen to. Because listening through speakers must be proceeded by crosstalk cancellation, if you choose the wrong output devices, there won’t be any 3D positional audio effect..

3. Location of speakers

If you listen through speakers, please do not inverse the left and right speakers. They must be in equal distance from the listener. That means the listener,the left, and the right speaker must be in the topmost of a right triangle. The point of the listener is called “sweet spot”. In addition, the height of the listener’s ears must be equal to that of the speakers.

4. Turn surround sound functions off

When the surround sound effect is enabled,it will cause confusion with C3D sound, and make positional sound effect invalid.