Presentation on theme: "Psycho-acoustics and MP3 audio encoding"— Presentation transcript:
1 Psycho-acoustics and MP3 audio encoding Physics of Music PHY103Would be good to add examples of sizes of different filespitch examples are repetitive
2 MP3MPEG is moving pictures experts group. set up by ISO (international standards organization) every few years issues a standard MPEG1 (1992), MPEG2(1994)..MP3 stands for MPEG audio layer IIILonger history – age of photo-video compression – in part started with audio compression experiments in the late ’80s
3 Auditory CodingTime frequency decomposition – divide the signal into pieces, obtain the spectrum of each pieceUse psycho-acoustic masking model to determine what information to keepStore the information in the most compact way possible – minimize the bitrate and maximize the audible auditory contentSystem of synchronization
4 Encoding and Decoding Encoding: Auditory signal (from a recording) is coded into an mp3 file containing carefully stored spectral informationDecoding:mp3 file is turned back into an auditory file that can be output to your speakersStreaming:This can be done in real time even if you don’t have the entire file
5 Lossy vs Lossless compression Compression: Store in a very compact format, more compact than the original audio fileLossless compression means no information is removedMP3 is a lossy type of compression. Information is lost during compression. Only inaudible information should be removed. Topic of current research on whether expert listeners can hear differences and how much is enough ...MP3 achieves a 10:1 compression ratio!This enables bit-streaming, makes storing audio very compact
6 Adding noiseRather than removing information MP3 adds noise. This is done by describing the signal with degraded digital precision.If you fail to digitize something sufficiently accurately, this is equivalent to adding noiseThe added noise should be inaudible it is below the mask threshold
7 Easy chops:Don’t bother storing information outside the range of hearing (outside <40Hz, >15kHz)Stereo info not stored for low frequencies
8 Bad ways to compress an audio file Reduce the total number of bits per sample (e.g. 32 bit to 16 or 16 to 8 bit) this gives you a factor of 2 in compression. However you get a noisier signalReduce the sampling rate (44kHz to 22kHz or 22kHz to 10kHz). Total loss of all high frequency information. Again only a gain of a factor of 2 in size. Equivalent to a high pass filter.A factor 10:1 in compression cannot be achieved using linear compression schemes
9 MaskingIf a dominant tone is present then noise can be added at frequencies next to it and this noise will not be heard. Less precision is required to store nearby frequencies.13dBcritical band
10 Definition of maskingThe process by which the threshold of audibility for one sound is raised by the presence of another (masking) soundThe amount by which the threshold is raised by the masker (in dB).
11 Critical Bands by Masking ASAdemo2 Tone in presence of broad band noiseThe critical band width at 2000Hz is about 280Hz so you can hear more steps when the noise bandwidth is reduced below this width.
12 critical band critical band A sine (signal) in the presence of noise that has a band width (in frequency) centered around the signal.critical bandPast a particular frequency width the masking doesn’t increase.The wider the noise bandwidth the more the signal (sine wave) is masked.critical band
13 Critical Bands by Loudness comparison A noise band of 1000Hz center frequency.The total power is kept constant but the width of the band increased.When the band is wider than the critical band the noise sounds louder.ASA demo 3
14 Critical band width as a function of frequency Size of critical band is typically one tenth of the frequency
15 Critical band conceptOnly a narrow band of frequencies surrounding the tone – those within the critical band contribute to masking of the toneWhen the noise just masks the tone, the power of the tone divided by the power of the noise inside the band is a constant.
16 The nature of the auditory filter The auditory filter is not necessarily square – actually it is more like a triangle shapeCritical band width is sometimes referred to as ERB (equivalent rectangular bandwidth)Shape difficult to measure in psychoacoustic experiments because of side band listening affects some innovative experiments (notched filtered noise + signal) designed to measure the actual shape of the filter).
17 Physiological reasons for the masking Basal membrane? The critical bandwidths at different frequencies correspond to fixed distances along the basal membrane.However the masking could be a result of feedback in the neuron firing instead. Negative reinforcement or suppression of signals. Or swamping of signals.
18 Temporal effects - non-simultaneous masking The peak ratio of the masker is important -- that means its variations in volume as a function of time compared to its rms value. Short loud peaks don’t necessarily contribute to the masking as much as a continuous noise.Both forward and backward masking - masking can occur if a loud masker is played just after the signal!Masking decays to 0 after ms
19 Physiological explanations for temporal masking Basal membrane is ringing preventing detection in that region for a particular timeNeurons take a while to recover - neural fatigue
20 Comodulation masking release A masked signal if comodulated with frequencies outside the critical band can be detected below the masking thresholdIn the same way that the overtones/spectrum is used to identify a sound. Sounds outside the critical band, since they are modulated the same as the signal, are used to pull it out (detect it) from more than one critical band region.
21 Perception of loudness Just noticeable difference JND in Sound IntensityA useful general reference is that the just noticeable difference in sound intensity for the human ear is about 1 decibel.JND = 1 decibelIn fact, the use of the factor of 10 in the definition of the decibel is to create a unit which is about the least detectable change in sound intensity.
22 JND as a function of loudness There are some variations. The JND is about 1 dB for soft sounds around dB at low and midrange freqencies. It may drop to 1/3 to 1/2 a decibel for loud sounds.Caution must be used in applying the "one decibel" criterion. It presumes that you are increasing the same sound by one decibel.
23 Loudness and the Critical Band When two sounds of equal loudness when sounded separately are close together in pitch, their combined loudness when sounded together will be only slightly louder than one of them alone. They may be said to be in the same critical band where they are competing for the same nerve endings on the basilar membrane of the inner ear. According to the place theory of pitch perception, sounds of a given frequency will excite the nerve cells of the organ of Corti only at a specific place. The available receptors show saturation effects which lead to the general rule of thumb for loudness by limiting the increase in neural response.
24 Outside the critical band If the two sounds are widely separated in pitch, the perceived loudness of the combined tones will be considerably greater because they do not overlap on the basilar membrane and compete for the same hair cells.
26 Pitch depends on partial pitches Butler 3.5b second of each pair has partials 10% sharp. Perceived pitch change depends on frequency
27 MP3 schematicInput: 16 bit at 44kHz sampling is 768kbit/s
28 Filter bank: band pass filter into 32 sub-bands each centered at a different frequency MDCT: Modified Discrete Cosine Transform– each sub-band is divided into time windows.Windows overlap to get rid of a problem called aliasing (high frequencies are confused with low ones). Overlap needed for MDCT
29 13 dB miracleif the signal is 13 dB louder than then noise then the noise can’t be heard (within a band).Each sub-band is quantized differently depending upon the masking threshold estimated in that band
30 FFT is used to compute the masking thresh-holds
31 Pushing MP3 to its limits -uncompressed-over compressed mp3Above compressing to 60kbpsUsing home.c4.scale.AIFF show mp3 options DEMO with Adobe to experiment
32 Limits of MP3Above ~80kbps (kilo bits per second) and 22kHz sampling I find I get reasonable sound.Compressing beyond this can do pretty weird things – I found that noise sounded weird and lack of high frequencies led to lost brilliance in timbre - also attacks suffered pitch and timbre changes