2. Audio / Sound INTRODUCTION TO MULTIMEDIA SYSTEMS Lect. No 3: AUDIO TECHNOLOGY

3. What is sound? Sound is a physical phenomenon caused by vibration of material (ex.: violin). As the matter vibrates, pressure variations are created in the air around it. The pressure waves propagate in the air. When a wave reaches the human eardrums, a sound is heard. Ear: receive 1-D waves. Cochlea: convert to frequency dependent nerve firings, sent to the brain. Cochlea: قوقعة الأذن

4. The wave form occurs repeatedly at regular interval or periods. A sound with a recognizable periodicity is called music. Non-periodic sounds called noises. period amplitude time Air pressure A sound frequency is the reciprocal value of its period. The frequency represents the number of periods per seconds and is measured in hertz (Hz). A kHz describes 1000 oscillations per second or 1000 Hz.

5. Frequency Ranges The frequency range is divided into: Infrasonic: 0 to 20 Hz Audio-sonic: 20Hz to 20 kHz (Human hearing frequency) Ultrasonic: 20kHz to 1 GHz Hypersonic: 1GHz to 10 THz In multimedia we are concerned with sounds in the audiosonic range.

6. Frequency-Change of pitch طبقة الصوت

7. Amplitude The amplitude of the sound is the displacement of the air pressure from its quiescent state, which humans perceive subjectively as loudness or volume. Sound pressure levels are measured in (db). OR The amplitude of a sound is a measuring unit used to deviate the pressure wave from its main value. 0 db - no sound 20 db - rustling of paper 20 db - rustling خشخشة of paper 35 db - quiet home 70 db - noisy street 130 db – pain threshold Amplitude-determines loudness of sound loudness of sound

8. Amplitude-changevolume Amplitude-change volume

9. Audio Representation on Computers A computer measures the amplitude of the waveform at regular time intervals It then generates a series of sampling values. The mechanism that converts an audio signal into digital samples is the analog-to-digital converter (ADC). A digital-to-analog converter (DAC) is used to achieve the opposite conversion. Audio Sampling 1. 1.Determine number of samples per second; 2. 2.At each time interval determine the amplitude; 3. 3.Stored the sample rate and the individual amplitudes. Sampling Rate: The number of samples per second. The CD standard rate of 44100 Hz means that the wave form is sampled 44100 times per second.

10. Quantization: is a value of the sample. The resolution of a sample value depends on the number of bits used in measuring the height of the waveform. The sampled waveform with a 3 bits quantization results in only eight possible values: 0.75, 0.50, 0.25, 0.00, - 0.25, - 0.50, -0.75 and -1. An 8 bit quantization yields 256 possible values, 16 bit result in over 65536 values. Quantization introduces noise The lower quantization, the lower quality of the sound.

11. File size versus quality 1: Sampling at higher rates more accurately captures the high frequency content. 2: Audio resolution determines the accuracy with which a sound can be digitized. 3: Using more bits yields a recording that sounds more like its original. 4: High sample rate with high resolution = large files. Here are the formulas for determining the size (in bytes) of a digital recording: For monophonic recording: Sampling rate * duration of recording in second * bit resolution / 8 For stereo recording: Sampling rate * duration of recording in second * bit resolution / 8 * 2 Thus the formula for a 10 second recording at 22.05 KHz, 8 bit resolution would be: 22050 * 10 * 8/8 * 1 = 220, 500 bytes 22050 * 10 * 8/8 * 1 = 220, 500 bytes A ten second stereo recording at 44.1 KHz, 16 bit resolution would be : 44100 * 10 * 16 / 8 * 2 = 1,764, 000 bytes 44100 * 10 * 16 / 8 * 2 = 1,764, 000 bytes 3D Sound Projection: 3D Sound Projection: The shortest path between sound source and the auditor is called the direct sound path. All other sound paths are reflected which means they are temporarily delayed before they reach the auditor's ear.

12. MIDI ( Musical Instrument Digital Interface )versus digital audio MIDI ( Musical Instrument Digital Interface ) versus digital audio The MIDI is a small piece that plugs directly into the computer’s serial port and allows the transmission of music signals. Note that: MIDI does not produce sound, only produce the parameters that are needed to be sent to the device that translates those numbers into sound. Physical Specification: 1: 5 pin DIN 1: 5 pin DIN a - pin 2: ground a - pin 2: ground b - pins 4 and 5: data b - pins 4 and 5: data c - pins 1 and 3: unconnected c - pins 1 and 3: unconnected 2: Shielded twisted pair of 50 feet max length 2: Shielded twisted pair of 50 feet max length Electrical Specification: 1: Asynchronous غير متزامنserial interface. 1: Asynchronous غير متزامنserial interface. 2: 8 data bits, 1 start bit, and 1 stop bit 2: 8 data bits, 1 start bit, and 1 stop bit 3: Logic 0 is current ON 3: Logic 0 is current ON 4: Rise and fall time <= 2 msec 4: Rise and fall time <= 2 msec Data format has instrument specification, notion of beginning and end of note, frequency and sound volume. This data grouped into MIDI messages that specify a musical event.

13. A message contains 1 to 2 or 3 bytes. 1: First byte is status byte used to transmit message to a specific channel. 1: First byte is status byte used to transmit message to a specific channel. 2: Remaining bytes are data bytes: 2: Remaining bytes are data bytes: The number of data bytes is dependent on status byte. The number of data bytes is dependent on status byte. MIDI standard specifies 16 channels and identifies 128 instruments. For example, 0 is for piano, 40 for violin, 73 for the flute, etc. Q1: Does one channel correspond to one instrument? If an instrument is defined as a MIDI device, then typically yes, one channel will send information to one of the instruments in the MIDI chain. If, on the other hand, instrument is defined as the sound patch or voice being played (i.e. piano, tuba, violin), then yes, one MIDI channel carries voice message information for a single patch only. Q2: Can a user define a new instrument? No. MIDI contains only control information. The instrument heard depends entirely on the MIDI device used to decode the stream. Q3: Can physical modeling be included? No. physical modeling cannot be represented using MIDI protocol.

14. Time Voltage Q4: What goes through the MIDI cable? lohilohilohilohilohilohilovoltage: 1: Timed pulses of electricity – 31250 per second 11010001101010001100 1 bits: 2: MIDI Data Encoding 11010001101010001100 1 bits: byte stop bitstart bit

15. Q4: Talk about the MIDI messages ? MIDI messages are divided into two different types: 1: Channel Message: Channel messages go only to specified devices. A - Channel voice messages that describe music by defining pitch, A - Channel voice messages that describe music by defining pitch, amplitude, duration and other sound qualities. amplitude, duration and other sound qualities. B - Channel mode messages determine the way that a receiving MIDI B - Channel mode messages determine the way that a receiving MIDI device responds to channel voice messages. device responds to channel voice messages. 2: System Message: System message go to all devices in a MIDI system because no channel number are specified. A - System real time messages are short and simple consisting of only A - System real time messages are short and simple consisting of only one byte. These message synchronize the timing of MIDI devices in one byte. These message synchronize the timing of MIDI devices in performance; therefore, it is important that they be sent at precisely performance; therefore, it is important that they be sent at precisely the time they are required. the time they are required. B - System common message are commands that are used to play a B - System common message are commands that are used to play a song. These messages enable you to select a song, find a common song. These messages enable you to select a song, find a common starting place in the song and tune all the synthesizers. starting place in the song and tune all the synthesizers. C - System exclusive message allow MIDI manufactures to create C - System exclusive message allow MIDI manufactures to create customized MIDI message to send between their MIDI devices. customized MIDI message to send between their MIDI devices.

16. MIDI versus digital audio In contrast to MIDI data, digital audio data are the actual representation of sound, stored in the form of thousands of individual samples (i.e. MIDI data are device dependent; digital audio are not). MIDI data has several advantage: 1. MIDI data are much more compact than digital audio files, and the size of the MIDI file is completely independent on playback quality. In general MIDI files will be 200 to 1000 times smaller than digital audio files. 2. Because MIDI files are small, they don’t take up as much RAM, disk space, and CPU resources. 3. MIDI data are completely editable. Now for disadvantage: 1. Because MIDI data do not represent sound but musical instruments, you can certain that playback will be accurate only if the MIDI playback device is identical to the device used for production. 2. MIDI can not easily be used to playback spoken dialog. MIDI Devices: 1. Sound generator: The principal purpose of the generator is to produce an audio signal that becomes sound when fed into a loudspeaker. 2. Microprocessor: The microprocessor communicates with the keyboard to know what notes the musician is playing, and with control panel to know what commands the musician wants o send to the microprocessor. 3. Keyboard: The sound intensity of a tone depends on the speed and acceleration of the key pressure. 4: Control panel: The control panel controls those functions that are not directly concerned with notes and duration. 5. Memory: Memory is used to store patches for sound generator and setting on the control panel.

17. Coding Methods achieve minimal data rate for a given quality.