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EECS > W6 (Representing Audio) > Flashcards

W6 (Representing Audio) Flashcards

1
Q

How many parts does Digital Data Media have?

A
  1. Representing numbers, Text, Audio, Images, Video
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2
Q

What is audio?

A

is a sound wave

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3
Q

what is microphone?

A

is a transducer, converting audio (sound waves) into an electric signal; sometimes immediately digitizing it.

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4
Q

What is transducer?

A

A device that converts variations in a physical quantity such as pressure or brightness into electrical signals or vice versa.

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5
Q

what is speaker?

A

is similar to microphone, electric voltage/ current to sound waves

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6
Q
A

sound waves (analog) –> electrical signals (digital) –> sound waves(analog)

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7
Q

what are the ways of creating Digital Audio?

A

1) sampling 2)synthesis

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8
Q

Note

A

Each digitized sample of audio is assigned a value that corresponds to the amplitude of the analog wave

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9
Q

explain and describe sine wave as function of time

A

f(t) = a sin(2 pi number f (t -

Amplitude: related to intensity; (a)
Frequency: related to perceived pitch; (f)
Phase: can depend on relative location of the sound
t is time

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10
Q

what is sampling?

A

process by which an analog signal is measured or reconstructed, often a millions of time per second in order to convert the analog signal into the digital

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11
Q

Digital synthesis

A

sound is created by manipulating numbers, converting those numbers to an electrical current and amplifying result
Higher sample rate, higher bite rate=higher quality and large file
(https://www.youtube.com/watch?v=ccvjMZpK_ls)

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12
Q

Sound pressure level (dB-SPL)

A

is a relative logarithmic unit relative to the 20 mpa value above

Without using dB, the numbers
vary by a factor of 100000 or more!

dB-SPL = 20 log10 (sound ∕ threshold)

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13
Q

Hearing Frequency Limits?

A

16 Hz – 20 kHz

Upper boundary is variable (!) and steadily decreases (!) with age
High frequencies usually change the timbre (coloration) of sound

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14
Q

Nyquist-Shannon

A

Sample interval half period to be able to catch peaks and valleys: i.e., sampling-rate (frequency) 2f

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15
Q

Bit depth

A
  • is sample is recorded using the fixed number of bits
  • Bit Depth in the Practice <8 bits Not used for sound*; Can be used to record physical processes like blood pressure, heartbeat, motion due to walking or running 8 bits Common in telephony Quantization noise can be perceptible sometimes 16 bits Most high-quality sound (CDs, DAT, MP3, movie audio tracks) 24+ bits Even higher quality/dynamic range (DVD-Audio, DTS) Often used before or during sound processing/editing (mastering)
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16
Q

threshold of hearing?

A

RMS sound pressure of 20 μPa
(0.98 pW ∕ m2 at 1 atm, and 25°C)

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17
Q

sampling rate VS bit rate (similarities, differences and connection)

A
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18
Q

how is the speed of the large file and inaccurate?

A

Large file- too fast and inaccurate- too slow

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19
Q

How often (frequent) should we sample?

A

A sample should be taken at least as often as the process is expected to change. Examine all factors that are expected to cause change, and identify the one that changes most frequently. Sampling must occur at least as often as the most frequently changing factor in the process.
( not sure about it check it)

20
Q

what is the bit depth? what does higher bit depth mean?

A

The Bit Depth of an image refers to how much computer memory is devoted to each pixel in that image. One bit in computer memory represents a single switch that can be in the on or off position.

Higher bit depth means a more accurate sample.

21
Q

difference between bit depth and bit rate? What are the example of them ?

A

The number of bits used to describe each sample is called the bit depth or word length

The number of bits transmitted per second is the bit rate.

22
Q

What does <8 bits use for?

A

Not used for sound/ can be used for record physical processes like blood pressure, heartbeat, motion due to walking or running

23
Q

What does 8 bits use for?

A

common for telephony
quantization noise can be perceptible sometimes

24
Q

What does 16 bits use for?

A

Most high quality sound (CDs, DAT, MP3, movie audio tracks)

25
Q

What does 24+ Bits use for?

A

even higher quality/ dynamic range (DVD-Audio, DTS)
often used before during around processing/ editing (mastering)

26
Q

how does a bit rate relate to the bit depth and sampling rate?

A

Combining bit depth and sampling rate gives a bit rate

Higher bit rate= better quality= larger file size

27
Q
A

Nyquist-Shannon theorem recommends sampling rate
bozorg tar mosavie 2 * max frequency for accurate reproduction

28
Q

What is CD- quality audio (industry standard)

A

16 bits/bit depth (per channel, stereo has 2 channels)
44.1KHZ sampling rate
1.4 Mbits/second bit rate
16*44.1k *2= 1.4 M bit/sec

29
Q

when did CD release?

A

1982

30
Q

when did digital audio develop?

A

during the 70s

31
Q

what was the problem of the CD?

A

Storage of DATA

32
Q

How much was the CD data capacity?

A

-cd data capacity (74 min audio format mode): Aprox 780 MB
-hard disks at the time: aprox 1o MB

33
Q

what was the practical way to store massive amount data ?

A

video tape

IT was easier to tweak digital audio parameters than to modify an existing TV standard

34
Q

What are the Audio compression techniques. explain?

A

1) Lossless
2) Lossy

Lossless is similar to text compression techniques
Lossy 1) remove imperceptible sounds (using psychoacoustic models)
2) reduce bit rate (less accurate reproduction of the original)

35
Q

What is the example of Lossy audio compression technique?

A

too close to a louder sound either in frequency or in time

36
Q

which of the audio compression technique cause less accurate reproduction?

A

Lossy

37
Q

What is codecs? And how many example code we have ?

A

1) it is a compression and decompression algorithms for audio (and video)
2) portmanteau (word combination) of Coder and Decoder or compressor and decompressor

We have FLAC/ MP3/ AAC

38
Q

What is the FLAC

A

1) It is the free lossless audio code
2)lossless, using a combination of run-length and huffman encoding
3) compression ratio is around 62%
4) typically used for archiving high quality audio

39
Q

What is the compression ratio of FLAC?

A

Around 62%

40
Q

What does FLAC codecs use for?

A

used for archiving high quality audio

41
Q

what is MP3?

A

1) moving pic experts group, audio layer 3)
2) lossy, with a compression ratio around 13%
3) uses psychoacoustics, huffman encoding, and lower bit rates
4) popular on most mobile devices

42
Q

which codecs is popular on mobiles?

A

MP3

43
Q

How much is the compression ratio of MP3?

A

13%

44
Q

What does AAc use for?

A

1) advanced audio coding
2) Lossy, with a compression ratio around 14%
3)uses psychoacoustics, huffman encoding, and lower bit rates
4)better perceived quality than MP3
5) popular on apple devices and most modern smart phones

45
Q

which codecs does use on apple devices and most modern smart phone?

A

AAC

46
Q

How much is the compression ratio of AAC ?

A

around 14%

EECS (6 decks)

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