Multimedia Systems - Sharifce.sharif.edu/courses/89-90/2/ce342-1/resources/root... · 2020. 9....

Multimedia Systems

Speech I

Mahdi Amiri

February 2011

Sharif University of Technology

Course Presentation

Multimedia Systems, Speech I

Sound

Sound is a sequence of waves of pressure which propagates through

compressible media such as air or water.

Digital Representation of

an Analog Signal

Sampling and Quantization

Parameters:

Sampling Rate (Samples per Second)

Quantization Levels (Bits per Sample)

This is a form of coding too:

Pulse-code modulation (PCM)

Basics


Pulse-code Modulation (PCM)Why Call it PCM?

4-bit PCM


Pulse-code Modulation (PCM)

How to choose proper…

Sampling Rate

8 Khz ?

Quantization Level

8 bit/sample ?

Bit per Second for 8000 Hz 8 bit PCM

64 kbit/s

Bit per Second (bit/s or bps)



Human Hearing Frequency Range

20 Hz to 20 kHz

Play with “Audacity”

tone generator to test

your hearing

Most people will find that their hearing is

most sensitive around 1-4 kHz and that it is less

sensitive at high and low frequencies.

Sampling Rate


Pulse-code Modulation (PCM)Hearing Range

Ferret

Porpoise

Check out

Freq. Alloc. Table



Human Vocal Range

Normal: 80 Hz to 1100 Hz

Charles Kellogg (14 KHz) (not verified)

Guinness Book of Records

Female: Georgia Brown

(Eight octaves, 25087Hz)

Male: Tim Storms

(Six octaves)

Sampling Rate



8,000 Hz - Telephone, adequate for human speech.

11,025 Hz – lower quality PCM (one quarter the sampling rate of audio CDs).

22,050 Hz – Radio.

32,000 Hz - miniDV digital video camcorder, DAT (LP mode).

44,100 Hz - Audio CD, also most commonly used with MPEG-1 audio (VCD, SVCD,

MP3) (Originally chosen by Sony, 1979).

48,000 Hz - Digital sound used for miniDV, digital TV, DVD, DAT, films and

professional audio.

96,000 or 192,000 Hz - DVD-Audio, some LPCM DVD tracks, BD-ROM (Blu-ray

Disc) audio tracks, and HD-DVD (High-Definition DVD) audio tracks.

2.8224 MHz - Super Audio CD (SACD), 1-bit sigma-delta modulation process known

as Direct Stream Digital (DSD), co-developed by Sony and Philips.

5.6448 MHz - Double-Rate DSD, 1-bit Direct Stream Digital at 2x the rate of the

SACD. Used in some professional DSD recorders (128 * 44100 Hz).

DXD - 24-bit sampled at 352.8 kHz, suited for editing, eq. with 8.4672 MHz 1-bit DSD

Common Sampling Rates



The trademark name used by Sony and Philips.

Uses pulse-density modulation encoding

Direct Stream Digital


Pulse-code Modulation (PCM)Quantization, Images



Simple and popular

midtread

odd number of

reconstruction levels

Uniform Quantizer, Midtread

Quantization error for bounded input



Simple and popular

midtrise

even number of

reconstruction levels

Uniform Quantizer, Midtrise



Want to prevent human ear fatigue by

minimizing quantization noise

Signal-to-Noise Ratio = 6.02*B dB

SNR is approximately 6 dB per bit.

16-bit => 96 dB

Above 36 dB is required

Quantization Levels


Pulse-code Modulation (PCM)6 dB per bit rule of thump

ˆe x xn n n

2 2e n

m mX x n X

Assumption: is uniform over ( , ]2 2

e n

The probability density function of e[n]

2

2

m

B

X

Average power of a process or signal:

2 2

x xx p x dx

x p x : Probability density function: Mean

2 2

1010logdB x eSNR B

: Variance

22

22 22 2

22 2

1

12 2 3

me e B

Xe p e de e de

2 2 2

10

2 2

10 10

10log 2 3

20log 2 10log 3

B

dB x m

x m

SNR B X

B X

2 2

106.02 10log 3dB x mSNR B B X


Pulse-code Modulation (PCM)6 dB per bit rule of thump

Example 1: SNR for Uniform Quantization of Uniformly-Distributed Input

2 2

106.02 10log 3dB x mSNR B B X

2 2 3x mX 6.02dBSNR B B

Example 2: SNR for Uniform Quantization of Sinusoidal Input

2 2 2x mX 6.02 1.76dBSNR B B

Example 3: SNR for Uniform Quantization of Gaussian Input

2 2 16x mX 6.02 7.27dBSNR B B



The average person cannot tell the difference between a bitrate above 192

kbit/s and the original CD/WAV.

Even if your headphones seal really well around your ears, they will

probably only give you about 20 to 25 dB insulation from the external sound

Good to Know

20 ~ 25 dB insulation

Noise level for 192 kbps audio is under -125 db and certainly inaudible


Pulse-code Modulation (PCM)Nonuniform Quantization

(a) Uniform and (b) non-uniform quantization Q(x) and quantization error q(x)

Typical speech signal and its histogram



Nonuniform quantizers: Difficult to make, Expensive.

Solution: Companding Uniform Q. Expanding

u-law, a-law


Pulse-code Modulation (PCM)u-law, a-law


Pulse-code Modulation (PCM)u-law, a-law

u-law

North America and Japan

a-law

Europe


Differential PCM (DPCM)Idea

Take advantage of data redundancy

[… 110 112 111 112 112 114 115 115 114 114… ] [… +2 -1 +1 0 +2 +1 0 -1 0 …]


Differential PCM (DPCM)Basic Scheme

1Delta Modulation (DM): i n ia x z

Problem?

General Predictive Coding


Differential PCM (DPCM)Better Structure


Thank You

1. http://ce.sharif.edu/~m_amiri/

2. http://www.dml.ir/

FIND OUT MORE AT...

Multimedia Systems

Speech I

Next Session: Speech II

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