低比特率语音编解码器中的隐写术集成与应用

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本文研究的主题是"将隐写技术集成到低比特率语音编解码器中"，发表在《IEEE Transactions on Information Forensics and Security》第七卷第六期，2012年12月。随着VoIP（Voice over Internet Protocol）和移动通信的普及，低比特率语音编码因其在音频通信中的广泛应用，使得在这些流中嵌入隐藏信息（即隐写术）具有广泛的实用价值。作者们提出了一种创新的算法，旨在将隐写术无缝整合进低比特率语音编码过程，特别针对VoIP通话。该方法的重点在于在进行语音编码的同时，利用语音信号的特性进行数据嵌入，如在声学参数的预测过程中，如音高周期（pitch period）。这样做的目的是保持隐藏信息与语音编码之间的同步，确保信息隐藏不会显著影响语音质量，同时尽量减少对标准低比特率语音编码流程的影响，避免额外的延迟。通过数据嵌入，该算法能够在保证语音质量的同时，有效地进行秘密通信。然而，为了防止被隐写分析检测到，算法的设计需要在不影响语音自然度的前提下实现高效的信息隐藏。实验结果显示，采用该算法，可以在保持语音通信流畅性的同时，实现较高的数据嵌入率，这对于需要在不引起怀疑的情况下传递秘密信息的应用场景尤其有价值。总结来说，这篇研究论文的核心贡献在于开发了一种兼顾隐写安全性和编码效率的技术，它在低比特率语音编码的常规流程中实现了隐写术的集成，这不仅拓宽了隐写技术在实际通信中的应用领域，也展示了在信息技术安全领域的创新思考。

HUANG et al.: STEGANOGRAPHY INTEGRATION INTO A LOW-BIT RATE SPEECH CODEC 1867

the ﬁrst two subframes and the other for the last two. The open

loop pitch period e sti m at ion,

, is computed using the per-

ceptually weighted speech

. A cro s s corre lati on criter ion ,

namely

, calculated by using the maxim ization method

[13], is used to determine the pitch period, as shown in (1).

(1)

The index

which maximizes the cross correlation, ,

is selected as the open loop pitch estim a tio n f or th e a pp ropriate

two subf rames. Wh ile searching for the best index, preference is

given to smaller pitch periods to avoid choosing pitch m ultiples.

Maximums of

are searched for beginning with .

For every maximum

found, its value is compared to the

best previous maximum found ,

. The following pseudo

code shows how it works:

then (if

then (select , )

)

else (if

then (select , )

)

Using the pitch period estimation,

, a closed loop pitch

predictor is com pu ted. The pitch predictor in G.723.1 is a

ﬁfth order pitch predictor. The p itch predicti on contribu tion

is treated as a convention al adaptive codebo ok contrib ution.

For subframes 0 and 2, the closed loop pitch lag is selected

from around the appropriate open loop pitch lag in the range of

. For subfram es 1 and 3, the closed loop pitch lag is coded

differentially usin g 2 bits and may differ from the previous

subframe lag only by

,0, or [10].

IV. P

ITCH PERIOD PREDICTION-BASED STEGANOGRAPHY

ALGORITHM

A. Embeddin g Algorithm

In the process of G.723.1 encoding, th e open-loop pitch

estimation is cond ucted ﬁrst, followed by closed-loop pitch

prediction. The open-loop p itch estimation computes the

open-loop pitch period

of a fram e of speech signal

. For each fram e, two p itch periods

are computed by using the ﬁrst two subframes and the last

two subframes, respectively. The method for computing the

open-loop pitch period is described below.

First, a cross correlation criterion

is computed by using

(1), and then it searches for t he open-loop pitch following the

procedures below [13]:

1) Suppose

, , ;

2) Using (1), compute

.If

(2)

and

(3)

then

,and .

3) Set

,if , return to 2), otherwise stop.

Having obtained the pitch period

of a frame of speech

signal

, search for the closed-loop pitch

period and embed information.

The closed-loop pitch period of a subframe is deﬁned by

, 1, 2, 3, and its open-loop pitch period is ,

, 1, representing the open-loop pitch periods of the ﬁrst two

subframes and the last two subframes, respectively. Adjusting

yields

(4)

The

closed-loop pitch period

is assigned a value close

o the op en- loo p pitch period

.The values for odd

subframes and for even subframes are obtained from different

rangesasshownin(5).

(5)

The minimum value of

is 17, and its maximum is 143. The

number of

is equal to the number of elements in ,denoting

. represents the th element in ,

The pitch prediction contribution is treated as a con vent ion a l

adaptive codebook contr ibution. For subframes 0 and 2, the

closed loop pitch lag is selected around the appropriate open

loop pitch lag in the range

and coded using 7 bits. For sub-

frames1and3,theclosedlooppitch lag is coded differentially

using 2 bits and may differ from the previous subframe lag only

,0, or [13]. The quantized and d ecoded pitch lag

values are referred to as

from this point on. The pitch pre-

dictor gains are vector quan tized using two co debo oks with 85

or 170 entries for the high bit rate and 170 entries f or the low

bit rate. The 170 entry codebookisthesameforbothrates.For

the high rate, if

is less than 58 for s ubfram e s 0 and 1 or if

is less than 58 for subframes 2 and 3, then the 85 entry code-

book is used for the pitch gain quantization. Other w ise, the pitch

gain is quantized using the 170 entry cod eboo k. We studied the

pitch distribution probabilities of closed-loo p pitch perio d of un-

touched G.723.1 VoIP speeches, and Fig. 1 shows the pitch dis-

tribution probability results for fo ur types of untouched G.723.1

VoIP speeches, each with 250 samples.

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