ieee paper template in a4 (v1) - web viewsome features with their detailed specifications have been...

Version: 9 September 2014

111Equation Chapter 1 Section 1FAQ: DSSS-

based Audio Watermarkby ITB Watermark Team

I. INTRODUCTION

This document is intended for explaining features that might be beneficial for DSSS-based Audio Watermark

(DSSS-AWM). Some features with their detailed specifications have been implemented in Matlab, yet some others

are for discussion only. The following chapter is divided based on the topics of our interest in AWM, such as PN

code generator, Error Correcting Code (ECC) implementation, interference reduction by High Pass Filter (HPF),

simple power control-based watermark gain, phase modulation, and synchronization.

II. DISCUSSION

A. DSSS MODULATION

The notion of DSSS to spread the spectrum of information suits to the requirement of watermarking, which is robust to

signal processing attack. By spreading the watermark signal as wide as the hosting audio’s spectrum, the watermark

signal can not be easily removed from the hosting audio signal . Pseudo Noise (PN) Code which has wider

spectrum relative to the watermark signal is multiplied with the watermark signal . The multiplication in time

domain corresponds to convolution in frequency domain. The transmitted baseband signal will have wide spectrum

similar to the PN Code’s spectrum. It is obvious now that in time domain PN Code pulse has much narrower width

than the watermark signal pulse. The ratio between the bandwidth of the watermark signal after spreading and

before spreading is called processing gain , which is usually an integer number. Therefore, the DSSS system

merely depends on how good the PN Code is. Some parameters to assess about the PN Code are discussed in the PN

Code Generator section. The block diagram of DSSS modulation is shown in Fig. 1.

1


Fig. 1. DSSS-AWM Modulation

The bandwidth of the transmitted baseband signal is determined by the chip rate as in 2. For passband modulation

by M-PSK, and the bandwidth of transmitted passband signal is shown in 3. Here, bandwidth is

defined as the width of the main spectral lobe as seen in Fig. 2. An illustration of DSSS modulation is given in Fig. 3 where

is pulse chip duration and is pulse symbol duration.

22\* MERGEFORMAT ()

33\* MERGEFORMAT ()

Fig. 2. Signal Pairs in Time and Frequency Domain

2

Polar Non Return to Zero

Encoder

BinaryData Sequence

:

10110110...kb

( )b t

PN-Codegenerator

( )m t

( )i t

( )y t

( )Tc t

Gain


Fig. 3. DSSS Modulation Ilustration

The transmitted baseband signal should be imperceptible when is added to the hosting audio signal . Thus,

changing the gain of the transmitted baseband signal is necessary. It is known that the gain should be about 0.5% of

the dynamic range of the signal. Let assume that the loudest level of a sound card is 1 V and the rms noise floor is 0.05 V.

Then, the dynamic range becomes and the watermark gain is chosen to be or

. In the case of interactive Automatic Content Recognition (ACR), the noise floor is obtained from the sound

card of televisions.

If the static gain adjustment is the only way to control the imperceptibility, the watermark signal might not be acquired

at the receiver since it will produce considerably low correlation output. The reason is due to the condition of transmission

channel (air) and the quality of the recording device (the 2nd screen). The typical condition is that the recorded watermark

signal is buried below the noise floor of the recording device. For example, the audience are not able to hear the watermark

signal comes from the television’s speaker (,which is good), while the recording device is not able to decode the watermark

signal (,which is bad). Thus, this hardware limitation of the receiver is the challenge that goes along with the low

watermark gain (0.5% of the dynamic range of the signal). Moreover, the imperceptibility of the watermark signal is

subjected to both human perception and recording device capability. The former issue, human perception, belongs to

psychoacoustic model topic.

The obvious idea to overcome the above problem is to make the gain of the watermark signal adaptive. It means that during

the silent frames of the hosting audio , the gain of the watermark signal is made low affirming the 0.5% of the dynamic

range of the signal requirement. While the hosting audio’s amplitude is higher than a certain level, the gain of the

3


watermark signal is made high to an acceptable level. This adaptive gain adjustment will boost the average power ratio

between hosting audio and watermark signal (average SNR). This adaptive gain control is discussed in Simple Power

Control-based Watermark Gain section.

B. DSSS DEMODULATION

Having assumed that the unit gain is used and the hosting audio is set to all zero, transmitted baseband mixed audio

signal equals to transmitted baseband signal as in 4. At the receiver, the received signal is the attenuated

version of added by noise . For simplicity, the channel attenuation and the noise are set both zero so that the

received signal satisfies 5. The block diagram of DSSS demodulation is shown in Fig. 4. Perfect synchronization

between PN Code used at transmitter and at receiver satisfies 6 and multiplication between received signal

and at the receiver yields 7. Simple majority polling is sequentially done as represented by integration within

one bit duration to decide whether bit one or bit zero is received. So far the procedure has solved an inverse problem,

namely to get back from .

44\* MERGEFORMAT ()

55\* MERGEFORMAT ()

66\* MERGEFORMAT ()

77\* MERGEFORMAT ()

4


Fig. 4. DSSS-AWM Demodulation

DSSS demodulation is a procedure to despread the signal by correlating the received signal with the PN code at the

receiver . This process heavily depends on the synchronisation between the received signal and the PN code at

the transmitter . Phase error as much as will reduce the processing gain by a factor of 2. Delay-locked Loop

(DLL) might be used for phase tracking and will be discussed in Synchronization section.

Fig. 5. DSSS Demodulation Ilustration

C. FEATURES

Spek simulasi disini.

5

c cN T

cRcR

cRcR

sRsR

( )r t

( )Rc t

( ) ( ) ( )Ra t r t c t

PN-Codegenerator

0

bT

dtDecision/Threshold

Device

u

BinaryData Sequence

ˆ :

10110110...kb

( )a t

( )Rc t( )n t

( )r t

Attenuation

( )y t


TABLE I. Spesifikasi Simulasi di Matlab

II.C.1 PN CODE GENERATOR

Max sequence length: period makin ≫ corelasi ≫. Contoh gamba berbagai perioda

Fig. 6. An Example of Maximal-length PN Sequence Generator with 7-bit Periodicity

II.C.2 ERROR CORRECTING CODE

BCH: tentang correcting code dibatasi oleh bit rate spesifikasi: Gambar frame by frame

II.C.3 INTERFERENCE REDUCTION BY HIGH PASS FILTER (HPF)

Not Yet

II.C.4 SIMPLE POWER CONTROL-BASED WATERMARK GAIN

Not Yet

II.C.5 PHASE MODULATION

Although the phase modulation is not mandatory in DSSS-based AWM, it is still useful to increase bit rate of watermark

information. For example, by using QPSK instead of BPSK, the bit rate is doubled without any BER loss. However, the

other M-PSK might cause BER loss with -higher bit rates.

Another advantage of using phase modulation after spreading the watermark information is to distribute signal’s power

more evenly. It is expected that the one half of the power is distributed to negative frequency components and the other half

to positive frequency components following 8. This makes the DSSS more robust to signal processing attack.

88\* MERGEFORMAT ()

6

1 2 3

flip flop

0s 1s2s 3s

Outputsequence

Modulo-2 Adder

clock


The price of deploying the phase modulation feature to DSSS is complexity increase. The simplest case problem is how to

synchronize the carrier signal at the receiver to the carrier signal at the transmitter as seen in Fig. 7, i.e. to estimate .

The more complex problem is for the case of M-PSK, e.g., what if there is phase imbalance as much as between the in-

phase and quadrature component of IQ modulation as shown in Fig. 8.

Fig. 7. Carrier Synchronization

Fig. 8. Phase Imbalance between In-Phase and Quadrature Component

As seen in Fig. 7, let consider the first case when there is phase error of the generated carrier signal at the receiver and,

for simplicity, the unit gain is used and the hosting audio , the attenuation, and the noise are set to all zero. The

received signal becomes 9 and the output of LPF is 10. It is clearly seen that the demodulated signal has

independent variable, namely as phase error. Depending on the value of , the demodulated signal and modulating

signal are said to be (attenuated and) in phase, or (attenuated and) antiphase, or even orthogonal to each other. It is

expected that Phase Locked Loop (PLL) can solve the carrier synchronization problem.

99\* MERGEFORMAT ()

7

cos( )ot

2

sin( )ot ( ) cos( ) sin( )i o q ox t m t m t S/P

binary NRZ stream

im

qm

( )m t ( )x t

( )i t

( )y t LPF ˆ ( )m t

cos( )ot cos( )ot

Gain

( )r t

Attenuation

( )n t


1010\* MERGEFORMAT ()

II.C.6 SYNCHRONIZATION

Coarse+fine

8


III. NOTATIONS

Binary Non-Return-to-Zero watermark signal

Pseudo Noise (PN) code sequence at transmitter

Transmitted baseband watermark signal (similar to )

In-phase component of transmitted baseband watermark signal

Quadrature component of transmitted baseband watermark signal

Gain of the watermark signal

Transmitted passband watermark signal

Hosting audio signal

Transmitted mixed audio signal

Received audio signal

BPSK-demodulated signal

Pseudo Noise (PN) code sequence at receiver

DSSS-demodulated signal

Carrier frequency in rad/s

Sampling frequency

The number of bits of the ECC input

The number of bits of the ECC output

Cut-off frequency of LPF or HPF

Bit rate

Pulse bit duration

Symbol rate

Pulse symbol duration

Chip rate

Chip duration

9


IV. MATLAB SCRIPT

10


11


12


13


14

ieee paper template in a4 (v1) - web viewsome features with their detailed specifications have been...

Documents