Method of signal shredding

Details Method of signal shredding Method of signal shredding

2000-01-12

2004-09-28

Ramakrishnaiah, Melur (Department: 2747)

Electrical audio signal processing systems and devices

Sound effects

C381S058000, C704S266000, C084S608000

Reexamination Certificate

active

06798886

ABSTRACT:

FIELD OF THE INVENTIONS
The present inventions relate to signal and waveform processing and analysis. It further relates to the identification and separation of more simple signals contained in a complex signal and the modification of the identified signals.
BACKGROUND OF THE INVENTION
Audio signals, especially those relating to musical instruments or human voices, have a characteristic harmonic content that defines how the signal sounds. It is customary to refer to the harmonic as harmonic partials. The signal consists of a fundamental frequency (first harmonic f
1
), which is typically the lowest frequency (or partial) contained in a periodic signal, and higher-ranking frequencies (partials) that are mathematically related to the fundamental frequency, known as harmonics. Thus, when the partial has a mathematical relationship to the fundamental, they are just referred to as harmonics. The harmonics are typically integer multiples of the fundamental frequency, but may have other relationships dependant upon the source.
The modern equal-tempered scale (or Western musical scale) is a method by which a musical scale is adjusted to consist of 12 equally spaced semitone intervals per octave. This scale is the culmination of research and development of musical scales and musical instruments going back to the ancient Greeks and even earlier. The frequency of any given half-step is the frequency of its predecessor multiplied by the 12th root of 2=1.0594631. This generates a scale where the frequencies of all octave intervals are in the ratio 1:2. These octaves are the only consonant intervals; all other intervals are dissonant.
The scale's inherent compromises allow a piano, for example, to play in all keys. To the human ear, however, instruments such as the piano accurately tuned to the tempered scale sound quite flat in the upper register, so the tuning of some instruments is “stretched,” meaning the tuning contains deviations from pitches mandated by simple mathematical formulas. These deviations may be either slightly sharp or slightly flat to the notes mandated by simple mathematical formulas. In stretched tunings, mathematical relationships between notes and harmonics still exist, but they are more complex. Listening tests show that stretched tuning and stretched harmonic rankings are unequivocally preferred over unstretched. The relationships between and among the harmonic frequencies generated by many classes of oscillating/vibrating devices, including musical instruments, can be modeled by a function
f
n
=f
1
×G
(
n
)
where f
n
is the frequency of the n
th
harmonic, f
1
is the fundamental frequency, known as the 1st harmonic, and n is a positive integer which represents the harmonic ranking number. Examples of such functions are
f
n
=f
1
×n
  a)
f
n
=f
1
×n
×(S)
log
2
n
  b)
f
n
=f
1
×n
×[
1
+(
n
2
−1)&bgr;]
1/2
  c)
where S and &bgr; are constants which depend on the instrument or on the string of multiple-stringed devices, and sometimes on the frequency register of the note being played. The n ×f
1
×(S)
log
2
n
is a good model of harmonic frequencies because it can be set to approximate natural sharping in broad resonance bands, and, more importantly, it is the one model which simulates consonant harmonics, e.g., harmonic
1
with harmonic
2
,
2
with
4
,
3
with
4
,
4
with
5
,
4
with
8
,
6
with
8
,
8
with
10
,
9
with
12
, etc. When used to generate harmonics, those harmonics will reinforce and ring even more than natural harmonics do.
Each harmonic has an amplitude and phase relationship to the fundamental frequency that identifies and characterizes the perceived sound. When multiple signals are mixed together and recorded, the characteristics of each signal are predominantly retained (superimposed), giving the appearance of a choppy and erratic waveform. This is exactly what occurs when a song is created in its final form, such as that on a compact disk, cassette tape, or phonograph recording. The harmonic characteristics can be used to extract the signals from the mixed, and hence more complex, audio signal. This may be required in situations where only a final mixture of a recording exists, or, for example, a live recording may have been made where all instruments are being played at the same time.
Musical pitch corresponds to the perceived frequency that the human recognizes and is measured in cycles per second. It is almost always the fundamental or lowest frequency in a periodic signal. A musical note produced by an instrument has a mixture of harmonics at various amplitudes and phase relationships to one another. The harmonics of the signal give the strongest indication of what the signal sounds like to a human, or its timbre. Timbre is defined as “The quality of sound that distinguishes one voice or musical instrument from another”. The American National Standards Institute defines timbre as “that attribute of auditory sensation in terms of which a listener can judge two sounds similarly presented and having the same loudness and pitch are dissimilar.”
Instruments and voices also have characteristic resonance bands, which shape the frequency response of the instrument. The resonance bands are fixed in frequency and can be thought of as a further modification of the harmonic content. Thus, they do have an impact on the harmonic content of the instrument, and consequently aid in establishing the characteristic sound of the instrument. The resonance bands can also aid in identifying the instrument. An example diagram is shown in
FIG. 1
for a violin. Note the peaks show the mechanical resonances of the instrument. The key difference is that the harmonics are always relative to the fundamental frequency (i.e. moving linearly in frequency in response to the played fundamental), whereas the resonance bands are fixed in frequency. Other factors, such as harmonic content during the attack portion of a note and harmonic content during the decay portion of the note, give important perceptual keys to the human ear. During the sustaining portion of sounds, harmonic content plays a large impact on the perceived subjective quality.
Each harmonic in a note, including the fundamental, also has an attack and decay characteristic that defines the note's timbre in time. Since the relative levels of the harmonics may change during the note, the timbre may also change during the note. In instruments that are plucked or struck (such as pianos and guitars), higher order harmonics decay at a faster rate than the lower order harmonics. The string relies entirely on this initial energy input to sustain the note. For example, a guitar player picks or plucks a guitar string, which produces the sound by the emission of energy from the string at a frequency related to the length and tension of the string. In the case of the guitar, the energy of the harmonics has its largest amount of energy at the initial portion of the note and then decay. Instruments that are continually exercised, including wind and bowed instruments (such as flute or violin), harmonics are continually generated. This is because the source is continually creating a movement of the string or breath of a wind player. For example, a flute player must continue to blow across the mouthpiece in order to produce a sound. Thus, each oscillation cycle puts additional energy into the mouthpiece, which continually forces the oscillatory resonance to sound and subsequently continues to produce the note. The higher order harmonics are thus present throughout most or all of the sustain portion of the note. An example of a flute and piano are shown in
FIGS. 2A and 2B
respectfully.
As an example, an acoustic guitar consists of 6 strings attached at one end to a resonating cavity (called the body) via an apparatus called a bridge. The bridge serves the purpose of firmly holding the strings to the body at a distance that allows the strings to be plucked and played. The bo

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