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CS4237B Datasheet(PDF) 69 Page - Cirrus Logic |
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CS4237B Datasheet(HTML) 69 Page - Cirrus Logic |
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69 / 114 page  Hearing Basics It has long been known that the hearing system uses several methods to determine from which direction a particular sound is coming. Since hu- man hearing is binaural (two ears), these methods include relative phase shift for low fre- quency sounds, relative intensity for sounds in the voice range, and relative time of arrival for sounds having fast rise times and high frequency components. The outer ear plays a significant role in the de- termination of direction. Due to the complex nature of the ear’s shape, sound is subject to re- flection, reinforcement, and cancellation at various frequencies. Effectively, the human hear- ing system functions as a multiple filter, emphasizing some frequencies, attenuating oth- ers, and letting some get through with no change. The response changes with both azimuth and elevation, and together with the binaural ca- pabilities helps determine whether a sound is coming from up, down, left, right, ahead, or be- hind. The frequency response of microphones is not dependent on azimuth in the same way as the ear. Omni-directional microphones exhibit flat response in all directions. Cardioid microphones exhibit flat response to sounds coming from the front and sides and are dead at the rear. As no microphone behaves like the human ear, the sounds picked up by a microphone are accurate as far as the microphone is concerned but are not the same as the sounds impinging on the human eardrum under similar circumstances. When the sound is reproduced by speakers, the situation is further altered by speaker location. If sounds which originally came from one side or the other are reproduced by speakers which are frontally located, these side sounds are heard with the incorrect spectral response. The same is true for frontal sounds which are coming from side-mounted loudspeakers. The result is spacial distortion of the sound field which prevents the user from hearing what was originally performed with the proper spatial cues. The SRS 3D Stereo Process The Crystal SRS DSP, illustrated in Figure 7, processes the signal in such a manner that the spacial cues lost in the record/playback process are restored. Since the human hearing system is involved and is actually part of the loop, its transfer function is made part of the system transfer function. At the same time, SRS 3D Ste- reo processing avoids an objectionable buildup of frequencies of increased phase sensitivity and is effective over a wide area so that the listener is not restricted to a favorable listening position (sweet spot) between two speakers. In the stereophonic signal, frontal sounds pro- duce equal amplitudes in the left and right channels and are therefore present in the "sum" or L+R signal. Ambient sounds, which include reflected and side sounds, produce a complex sound field and do not appear equally in the left and right channels. They are therefore present in the "difference" or L-R signal. Although these two signals are normally heard as a composite signal, it is possible to separate and process them independently and then remix them into a new composite signal which contains the required spatial cues that the stereo recording and play- back processes do not provide. The directional cues are mostly contained in the difference sig- nals, so these can be processed, (L-R)p, to bring the missing directional cues back to their normal levels. The processed difference signal can then be increased in amplitude, using SPC3-0, in or- der to increase apparent image width. SRS Space Control The SRS Space adjustment, SPC3-0 in C2, con- trols the amount of processed difference signal, (L-R)p, that is added to the final left and right digital signals going to the DACs. The difference DS213PP4 CS4237B 69 |
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