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  • 1. Introduction Psycho-Acoustics

2. Definition Psycho-acoustics is the study of the perception of sound. How humans hear How humans can separate different sounds. Wiki Psychoacoustics is the study of subjective human perception of sounds Alternatively it can be described as the study of the psychological correlates of the physical parameters of acoustics 3. Computer Vs Human Brain Human brain does not work exactly like a computer. A computer records a series of samples that form the sound. Human ear sends information to the brain based on the spectrum of the sound. Studying the key differences aids us with digital audio compression. 4. Limits of Perception Sounds in the range of 20Hz to 20,000Hz are typically audible to the human ear. As age increases the range decreases, most adults unable to hear about 16,000Hz. Study has shown that although sounds below 20Hz cannot be heard, these sounds do have affects on the listeners. This is called the Hypersonic effect, and studies are on going to show brain activity when these sounds are present. 5. Hypersonic effect Does the hypersonic effect occur via the "ordinary" route of sound travelling through the air passage in the ear, or in some other way? A peer-reviewed study in 2006 seemed to confirm the second of these options, by testing the different effect of high-frequency components when presented via loudspeakers or via headphones The hypersonic effect did not occur when the high- frequency components were presented via earphones. ref T. Oohashi, et al, Inaudible high-frequency sounds affect brain activity: Hypersonic effect. Journal of Neurophysiology, 83(6):35483558, 2000. T. Oohashi, et al, The role of biological system other than auditory air-conduction in the emergence of the hypersonic effect. Brain Research, 1073:339347, February 2006. 6. Why study? The study of human sound perception is highly important in the development of audio codecs Lossy audio data compression is based in the principle that some audio information is not perceivable and is therefore discardable Psychoacoustics is a complex problem that involves physiological and psychological factors. 7. ImageSound Compression Techniques A codec is a device or program capable of encoding and/or decoding a digital data stream or signal (Audio or Video) The word codec may be a combination of any of the following: 'compressor-decompressor', 'coder-decoder', or 'compression/decompression algorithm In a distributed environment or hand held device based environment image and sound files remain a major bottleneck within systems Much of the slowness of a website's downloading is due to the presence of non-optimized sound and graphics 8. Categories Compression solutions today are more portable due to the change from proprietary high end solutions to accepted and implemented international standards. Two categories of data compression algorithm can be distinguished: lossless and 'lossy' 9. Lossy techniques Lossy techniques cause imagesound quality degradation in each compression/decompression step Careful consideration of the human perception ensures that the degradation is often unrecognisable, though this depends on the selected compression ratio. In general, lossy techniques provide far greater compression ratios than lossless techniques 10. Lossless coding techniques Lossless coding guaranties that the decompressed image is absolutely identical to the soundimage before compression. This is an important requirement for some application domains e.g. medialscientific applications, where not only high quality is in demand, but unaltered archiving is a legal requirement Lossless techniques can also used for the compression of other data types where loss of information is not acceptable e.g. text documents and program executables. 11. Codecs and Compression quality Lossy codecs Many of the more popular codecs in the software world are lossy Lossless codecs There are also many lossless codecs which are typically used for archiving data in a compressed form while retaining all of the information present in the original stream If preserving the original quality of the stream is more important than eliminating the correspondingly larger data sizes, lossless codecs are preferred Especially if the data is to undergo further processing (for example editing) in which case the repeated application of processing (encoding and decoding) on lossy codecs will degrade the quality of the resulting data such that it is readily identifiable (visually, audibly or both 12. Codecs and Compression quality Codecs are often designed to emphasise certain aspects of the media to be encoded For example, a digital video (using a DV codec) of a sports event, such as baseball or soccer, needs to encode motion well but not necessarily exact colours, while a video of an art exhibit needs to perform well encoding colour and surface texture. E.g audio codecs for cell phones need to be very low latency between a word being spoken and that word being heard; while audio codecs for recording or broadcast can use high-latency audio compression techniques to achieve higher fidelity at a lower bit-rate 13. Psycho-acoustics and digital audio processing Psycho-acoustics effects are important in digital audio processing: Haas effect Masking effect Missing fundaments/Phantom effect 14. Haas Effect A psycho-acoustic effect related to the ability of listeners to accurately localize sounds coming from around them. When two sounds originate from two sources at different distances from the listener, humans localize the sound source based upon the first arriving sound, if the arrival times of the sounds are within ~30 ms. A phenomenon that might be described as "involuntary sensory inhibition" in that one's perception of later arrivals is suppressed 15. Applications of Haas Effect Our well-being and insanity. Sound wave bounce off of all walls, if we could hear each individual sound wave we would hear the same thing over and over. Echo's/reverbs are caused when the sound is delayed by more then ~30ms 3D sound Haas effect can be used with other effects to create 3D sound from only two speakers. Duplicating sound and delaying by 30ms causes a more surrounded sound. Utilized by surround sound technologies such as Dolby Digital 16. Applications of Haas Effect Public Address Systems Most commonly used application for the Haas Effect Used to ensure that the perceived location or direction of the original signal remains unchanged. Large audiences may demand multiple loud speakers that need to be placed away from the stage and closer to the ear. Sound must be delayed to these speakers so that the sound from the stage is heard first. This serves to ensure that the sound is perceived as coming from the point of origin rather than from a loudspeaker that may be physically nearer the listener 17. Example of Haas Effect 18. Masking Effect When two pure tones are close in frequency and are largely different in amplitude, the louder one drowns out the weaker one. Simultaneous masking Temporal masking 19. Masking Effect Simultaneous masking: When close tones are produced at the same time humans have difficulty perceiving each as a unique or separate tone A perceptual phenomenon that relates to unconscious filtering Temporal masking: When both tones are produced with a small difference in time humans have trouble hearing both So if a loud sound and a quiet sound are played simultaneously, you cannot hear the quiet sound This distance, or threshold, turns out to be 5ms appox when working with pure tones, though it varies up and down in accordance with different audio passages 20. Applications of Masking Effect An important concept for audio compression Frequency components can be discarded or compressed more when a sound is masked. A fundamental element of data transfer over networks 21. Sound illusions The way pitch is computed from tones with multiple harmonics can be used to construct a number of illusions Continuously Ascending and Descending Series of Pitches Missing fundamentals 22. Missing Fundamentals Effect A sound is said to have a missing fundamental, suppressed fundamental, or phantom fundamental when its overtones suggest a fundamental frequency but the sound lacks a component at the fundamental frequency itself For some people the brain perceives the pitch of a tone not only by its fundamental frequency, but also by the ratio of the higher harmonics. Thus, we may perceive the same pitch (perhaps with a different timbre) even if the fundamental frequency is missing from a tone Thus a sound with a missing fundamental refers to a sound that due to its overtones suggest a fundamental frequency but the sound lacks a component at the fundamental frequency itself. Thus we perceive the same pitch even if the fundamental frequency is missing or encapable of being produced by the speaker. Note: It is possible that one may not experience this effect. 23. Applications of Missing Fundamentals Used to create an illusion of bass. By processing certain overtones selectively a rich bass effect can be created using small bookshelf speakers. The processed bass overtones compel the brain to replace the missing fundamental bass signals creating the illusion of bass. 24. Conclusion Many different audio fields are interested in Psycho-acoustics effects: Home theatre (designs, components, etc) Audio compression Speaker design (loud and bookshelf speakers) Public Address Systems And many more

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