Dynamic range is the range of sound intensity a mic can provide to the recording device. A small dynamic range means a limited range of amplitude levels relative to the noise floor. For an empty concert hall, the nosie floor is around 50 dB SPL. Noise floor is the point at which the softest sound can be registered as a useable signal. Any sound below the noise floor cannot be heard.
Frequency response measures how the mic translate SPL (Sound Pressure Level) into audio signal at different frequencies. An ideal frequency response is flat meaning the mic can capture sound with different frequencies into equal amplitude level. Some mic are designed to respond to certain frequencies based on their needs.
Omnidirectional mic responds to sound pressure from all angles. Condenser mic are typically omnidirectional. A directional mic responds to sound pressire from a particular angle. Cardoid is the most common response pattern. It is named as the pattern is heart shaped. Both dynamic and condenser mic exhibit this pattern. Hypercardoid is more directional. Another name is called mini-shotgun. It is used if it needs to keep a distance from the source. Supercardoid, or shotgun, is highly directional.
Monday, April 29, 2013
Sunday, April 28, 2013
Microphones
Any device that converts one form of energy into another is called a transducer. A microphone is a transducer and so does a loud speaker.
According to the theory of electro-magnetic induction, a metal suspended in a flux field of magnet will produce a current of certain direction and magnitude within the metal.
The most commonly microphone used is dynamic microphone. They are extremelyt durable and less expensive. They are commonly used in live performance and concerts. The mic is constructed based on a diaphragm connected to a coil of metal floating in the flux planes of a magnet. When the diaphargm vibrates to the sound pressure, the coil moves and sending an electrical current through the coil connected to an output line.
Dynamic mic contains rather heavy magnets which makes it durable. However, weight of component also limits its frequency response. High frequencies require a diaphragm to move very quickly but the response of the heavy component is slower, thus antenuattimng higher frequencies.
Condenser mic, on the other hand, is not based on magnetic but can generate a voltage. The voltage however has no power behind it. The design is based on the movement of electrons and the open-air capacitor. Behind the diaphragm, there is a conductive back plane separated by a small pocket of air. This forms a capacitor. A current is sending through the plane. When the diaphragm vibrate, closing up and opening up the gap between the diaphragm and the plane, it varies the amount of current through, thus generating a signal. Condensor mic requires an external power source, known as phantom power (48 volts). The power can be supplied by battery. Condenser mic is delicate and can be damaged when falls.
Ribbon mic are least used but in radio broadcast. It uses same principle as the dynamic mic, wherein a thin ribbon of corrugated aluminium is located between 2 strong magnets. It generate a current but typically not strong enough. Instead of using phantom power, ribbon mic contains a built-in transformer to boost the level up. It is like a mic with a signla booster (pre-amp) built in. Ribbon mic are famous to have a warm sound, which lends well with voice. The microphone is fragile and heavy. They are also very expensive.
According to the theory of electro-magnetic induction, a metal suspended in a flux field of magnet will produce a current of certain direction and magnitude within the metal.
The most commonly microphone used is dynamic microphone. They are extremelyt durable and less expensive. They are commonly used in live performance and concerts. The mic is constructed based on a diaphragm connected to a coil of metal floating in the flux planes of a magnet. When the diaphargm vibrates to the sound pressure, the coil moves and sending an electrical current through the coil connected to an output line.
Dynamic mic contains rather heavy magnets which makes it durable. However, weight of component also limits its frequency response. High frequencies require a diaphragm to move very quickly but the response of the heavy component is slower, thus antenuattimng higher frequencies.
Condenser mic, on the other hand, is not based on magnetic but can generate a voltage. The voltage however has no power behind it. The design is based on the movement of electrons and the open-air capacitor. Behind the diaphragm, there is a conductive back plane separated by a small pocket of air. This forms a capacitor. A current is sending through the plane. When the diaphragm vibrate, closing up and opening up the gap between the diaphragm and the plane, it varies the amount of current through, thus generating a signal. Condensor mic requires an external power source, known as phantom power (48 volts). The power can be supplied by battery. Condenser mic is delicate and can be damaged when falls.
Ribbon mic are least used but in radio broadcast. It uses same principle as the dynamic mic, wherein a thin ribbon of corrugated aluminium is located between 2 strong magnets. It generate a current but typically not strong enough. Instead of using phantom power, ribbon mic contains a built-in transformer to boost the level up. It is like a mic with a signla booster (pre-amp) built in. Ribbon mic are famous to have a warm sound, which lends well with voice. The microphone is fragile and heavy. They are also very expensive.
Wave
Sound move in the form of longitudinal wave. Analysis of this waveform is complex. A simpler visualization is to use a transverse wave. When throwing a rock into water, it creates ripples. When looked at from above, the ripples propagate outwards in the form of longitudinal wave. When looked at form the side (like a cross section), we see the transverse waveform. The upper part of a transverse wave represents the greatest point of compression, while the lowest point represents the rarefaction. The mid point is the position of molecule which it is not vibrating, is called the standard reference level.
A sinusoidal wave represents simple harmonic motion (SHM). A sine waveform result from mass vibration is the simplest and most economical way because it only contains a single frequency and has no harmonic content. Otherwise, the wave is called a complex periodic waveform. A waveform without pattern is called a random waveforms.
Frequency = speed of sound/wave length
The range of frequency human can hear is between 20 to 20kHz. Sound below the lower limit of hearing is called subsonic, whereas above the limit is called ultrasonic. Cat can hear between 45Hz to 85kHz. Bat and dolphin can hear up to 120kHz.
Music occupies about 1/4 of the range of hearing. The fundamental tone in music is that which you hear most prominently when an instrument is played. It occupies about 50% of the total sound heard. Some example of the frequency range of musical instruments:
violin = 200Hz to 3.5kHz
viola = 124Hz to 1kHz
Cello = 63Hz to 630Jz
Double Bass = 40Hz to 200Hz
Guitar = 80Hz to 630Hz
Piano = 28Hz to 4.1kHz
When hearing a periodic wave, we are actually hearing a complex averaging of the waveform's peak to peak values. The root mean square (RMS) ks the average level of a waveform over time. For a sine wave, RMS = 0.707*peak values.
Unlike frequency, amplitude cannot be measured without a reference value. Decibel is a logarithmic unit representing a ratio. Intensity level of a sound is measured as the energy transmitted per unit time and area of a sound wave. The greater the amplitude of a vibration, the greater the energy transmitted.
I = P/S of which P = Power (energy) and S = area covered
The loudest sound one hear is about 1 W/m2, which is a trillion times more energy than the softest sound (1*10-12 W/m2). These values are very awkward to use and so decibels are used. Another reason is that we hear sound intensity logarithmically.
Decibel is one tenth of a Bel (derived from Alexander Graham Bell). Bel is a ration of 10 to 1 between 2 numbers. The amount of energy between 1 Bel and 2 Bel is 10 times. The standard ratio of hearing is 0 dB SPL (Sound Pressure Level). 10 dB SPL is 10 times louder than 0 dB SPL. 20 dB is 100 times louder than 0 dB.
When we walk away from a sound, the loudness decrease following the inverse square law.
When 2 sounds of difference frequencies (e.g. 100 Hz and 105 Hz) are produced at the same time, they produce a pulsation effect, call beats. The number of beats = f1 - f2. When the difference between 2 frequencies is greater than 30 Hz to 40 H, the beat phenomenons ceases to exists. In its place is the existence of the simultaneous sounding 2 frequencies known as interval in music.
A sinusoidal wave represents simple harmonic motion (SHM). A sine waveform result from mass vibration is the simplest and most economical way because it only contains a single frequency and has no harmonic content. Otherwise, the wave is called a complex periodic waveform. A waveform without pattern is called a random waveforms.
Frequency = speed of sound/wave length
The range of frequency human can hear is between 20 to 20kHz. Sound below the lower limit of hearing is called subsonic, whereas above the limit is called ultrasonic. Cat can hear between 45Hz to 85kHz. Bat and dolphin can hear up to 120kHz.
Music occupies about 1/4 of the range of hearing. The fundamental tone in music is that which you hear most prominently when an instrument is played. It occupies about 50% of the total sound heard. Some example of the frequency range of musical instruments:
violin = 200Hz to 3.5kHz
viola = 124Hz to 1kHz
Cello = 63Hz to 630Jz
Double Bass = 40Hz to 200Hz
Guitar = 80Hz to 630Hz
Piano = 28Hz to 4.1kHz
When hearing a periodic wave, we are actually hearing a complex averaging of the waveform's peak to peak values. The root mean square (RMS) ks the average level of a waveform over time. For a sine wave, RMS = 0.707*peak values.
Unlike frequency, amplitude cannot be measured without a reference value. Decibel is a logarithmic unit representing a ratio. Intensity level of a sound is measured as the energy transmitted per unit time and area of a sound wave. The greater the amplitude of a vibration, the greater the energy transmitted.
I = P/S of which P = Power (energy) and S = area covered
The loudest sound one hear is about 1 W/m2, which is a trillion times more energy than the softest sound (1*10-12 W/m2). These values are very awkward to use and so decibels are used. Another reason is that we hear sound intensity logarithmically.
Decibel is one tenth of a Bel (derived from Alexander Graham Bell). Bel is a ration of 10 to 1 between 2 numbers. The amount of energy between 1 Bel and 2 Bel is 10 times. The standard ratio of hearing is 0 dB SPL (Sound Pressure Level). 10 dB SPL is 10 times louder than 0 dB SPL. 20 dB is 100 times louder than 0 dB.
When we walk away from a sound, the loudness decrease following the inverse square law.
When 2 sounds of difference frequencies (e.g. 100 Hz and 105 Hz) are produced at the same time, they produce a pulsation effect, call beats. The number of beats = f1 - f2. When the difference between 2 frequencies is greater than 30 Hz to 40 H, the beat phenomenons ceases to exists. In its place is the existence of the simultaneous sounding 2 frequencies known as interval in music.
Sound
Sound is an aural pecrception of vibration. There are 2 types of sound. Noise is sound that is not organized or harmonized. Music is organized and intentional.
A sound is produced when an object is set in motion by conversion of mechanical energy into acoustic energy. The acoustic energy is in a form of pressure waves in the medium (e.g. surrounding air). The disturbances in the air are known as compressions and rarefactions. These forms of compression and rarefaction occurs around the source and move away in all directions. As a result, the wave propagate outwards. The air molecules does not move with the wave, thet just dislodged from their current locations. The form of acoustical energy transmission is respresented by a longitudinal waveform.
The scientific study of sound perception is called psychoacoustics. It is not concerned with how sounds produce a particular emotional or cognitive response, which is in the area of psychology. Psychological perception of sound is on 2 categories - pitch and loudness. This is equivalent to 2 properties of sound - frequency and amplitude. Frequency measures the rate of repetition and amplitude measures the strength of air pressure produced.
The psychological measurement of the magnitude of sound include its frequency, pressure, harmonics, duration and surface properties within the sound space.
A sound is produced when an object is set in motion by conversion of mechanical energy into acoustic energy. The acoustic energy is in a form of pressure waves in the medium (e.g. surrounding air). The disturbances in the air are known as compressions and rarefactions. These forms of compression and rarefaction occurs around the source and move away in all directions. As a result, the wave propagate outwards. The air molecules does not move with the wave, thet just dislodged from their current locations. The form of acoustical energy transmission is respresented by a longitudinal waveform.
The scientific study of sound perception is called psychoacoustics. It is not concerned with how sounds produce a particular emotional or cognitive response, which is in the area of psychology. Psychological perception of sound is on 2 categories - pitch and loudness. This is equivalent to 2 properties of sound - frequency and amplitude. Frequency measures the rate of repetition and amplitude measures the strength of air pressure produced.
The psychological measurement of the magnitude of sound include its frequency, pressure, harmonics, duration and surface properties within the sound space.
Saturday, April 13, 2013
Signals
SIGABRT - sent by abort() to its calling process. The process terminates and generates a core dump. assert() call abort() when the condition fails.
SIGALRM - sent by alarm() and setitimer() when the period has lapsed to the calling process.
SIGBUS - rasied by kernel when the process incurs a hardware fault other than memory protection, usually a irrecoverable errors such as unaligned memory access.
SIGCHLD - sent to the parent process when a process ends. Parent process issues a wait().
SIGCONT - sent to the process that resumed from stop. Usually caught by terminal or editor use to refresh screen.
SIGFPE - cover not just floating point exception but all arithmetic exception
SIGHUP - kernel sends to the session leader when the terminal disconnects. The kernel also send to all foreground processes when the session leader terminates. The default action is to terminate. This signal means the user has logged out. Daemon overloads this signal to instruct them to reload its configuration. As daemon has no control terminal, it should never receive this signal from other sources.
SIGILL - sent when process execute an illegal instruction. Process can catch this signal but the behaviour is undefined.
SIGINT - sent to all foreground processes when user presses the interrupt key (CTL-C). This allow the processes to clean up before terminating.
SIGIO - BSD style asynchronous I/O event
SIGKILL - sent from the kill() system call. It cannot be caught or ignored.
SIGPIPE - If a process write to a queue but the reader has terminated, kernel raised this signal.
SIGPROF - raised by setitimer() with the ITIMER_PROF flag when the profile timer expires.
SIGPWR - system dependent. A UPS monitoring process sends this signal to init when the the battery level is low to allow the system to shut down orderly.
SIGQUIT - sent to all foreground processes when user presses the quit key (CTL-\)
SIGSEGV - sent when process access an invalid memory address (segmentation violation)
SIGSTOP - sent by kill() system call. This cannot be caught or ignored. The process is unconditionally stopped.
SIGSYS - process executes an illegal system call. For example, code compiled with newer version of OS runs on an older version.
SIGTERM - sent by kill(). Allows a process to catch it to initiate an oerderly termination.
SIGTRAP - sent when process cross a breakpoint, generally caught by debugger and ignored by most other processes.
SIGTSTP - sent by kernel to foreground process when user press suspend key (CTL-Z)
SIGTTIN/SIGTTOU - sent to a background process when it attempts to read from/write to control terminal.
SIGURG - kernel sends to process when an out-of-band data arrived at a socket
SIGURS1/2 - used solely by user processes. Common use is to instruct daemon to change behaviour
SIGVTALRM - raised by setitimer() when timer created with ITIMER_VIRTUAL flag expires
SIGWINCH - sent by kernel to all foreground processes when the terminal window size changes
SIGXCPU/SIGXFSZ - riased by kernel when the CPU and file size limit reached.
SIGALRM - sent by alarm() and setitimer() when the period has lapsed to the calling process.
SIGBUS - rasied by kernel when the process incurs a hardware fault other than memory protection, usually a irrecoverable errors such as unaligned memory access.
SIGCHLD - sent to the parent process when a process ends. Parent process issues a wait().
SIGCONT - sent to the process that resumed from stop. Usually caught by terminal or editor use to refresh screen.
SIGFPE - cover not just floating point exception but all arithmetic exception
SIGHUP - kernel sends to the session leader when the terminal disconnects. The kernel also send to all foreground processes when the session leader terminates. The default action is to terminate. This signal means the user has logged out. Daemon overloads this signal to instruct them to reload its configuration. As daemon has no control terminal, it should never receive this signal from other sources.
SIGILL - sent when process execute an illegal instruction. Process can catch this signal but the behaviour is undefined.
SIGINT - sent to all foreground processes when user presses the interrupt key (CTL-C). This allow the processes to clean up before terminating.
SIGIO - BSD style asynchronous I/O event
SIGKILL - sent from the kill() system call. It cannot be caught or ignored.
SIGPIPE - If a process write to a queue but the reader has terminated, kernel raised this signal.
SIGPROF - raised by setitimer() with the ITIMER_PROF flag when the profile timer expires.
SIGPWR - system dependent. A UPS monitoring process sends this signal to init when the the battery level is low to allow the system to shut down orderly.
SIGQUIT - sent to all foreground processes when user presses the quit key (CTL-\)
SIGSEGV - sent when process access an invalid memory address (segmentation violation)
SIGSTOP - sent by kill() system call. This cannot be caught or ignored. The process is unconditionally stopped.
SIGSYS - process executes an illegal system call. For example, code compiled with newer version of OS runs on an older version.
SIGTERM - sent by kill(). Allows a process to catch it to initiate an oerderly termination.
SIGTRAP - sent when process cross a breakpoint, generally caught by debugger and ignored by most other processes.
SIGTSTP - sent by kernel to foreground process when user press suspend key (CTL-Z)
SIGTTIN/SIGTTOU - sent to a background process when it attempts to read from/write to control terminal.
SIGURG - kernel sends to process when an out-of-band data arrived at a socket
SIGURS1/2 - used solely by user processes. Common use is to instruct daemon to change behaviour
SIGVTALRM - raised by setitimer() when timer created with ITIMER_VIRTUAL flag expires
SIGWINCH - sent by kernel to all foreground processes when the terminal window size changes
SIGXCPU/SIGXFSZ - riased by kernel when the CPU and file size limit reached.
Signal Handling
Ignore
No action is taken. Two signals cannot be ignored - SIGKILL and SIGSTOP to allow SA to be able to kill or stop all processes. Otherwise, there will be processes that is unstoppable.
Catch and handle
The kernel suspend the execution of the process's current code path and jump to the signal handler registered. Execution will continue once the handler ends. SIGINT and SIGTERM are 2 commonly caught signal. SIGINT allows the shell process to return to the prompt. SIGTERM allow the process to clean up for a orderly terminating.
Perform default action
Take the defaul action usually means terminating the process
No action is taken. Two signals cannot be ignored - SIGKILL and SIGSTOP to allow SA to be able to kill or stop all processes. Otherwise, there will be processes that is unstoppable.
Catch and handle
The kernel suspend the execution of the process's current code path and jump to the signal handler registered. Execution will continue once the handler ends. SIGINT and SIGTERM are 2 commonly caught signal. SIGINT allows the shell process to return to the prompt. SIGTERM allow the process to clean up for a orderly terminating.
Perform default action
Take the defaul action usually means terminating the process
Anonymous Memory Mapping
Large memory allocation request will not be satisfied using heap. Kernel allocates an anonymous memory mapping for this type of request. Anonymous memory mapping is like file-based memory mapping except it is not backed by any file, thus the name. It is just a large piece of zero-filled memory area (in multiple of page size) ready for use.
Anonymous memory mapping uses mmapp call with special flag MAP_ANONYMOUS. The fd parameter is ignored. In BSD without the flag, anonymous memory mapping is implemented by mapping /dev/null with copy-on-write pages.
Anonymous memory mapping uses mmapp call with special flag MAP_ANONYMOUS. The fd parameter is ignored. In BSD without the flag, anonymous memory mapping is implemented by mapping /dev/null with copy-on-write pages.
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