Perceptually Transparent Information Hiding in G.729 Bitstream
∗
Lihua Liu, Mingyu Li, Qiong Li and Yan Liang
Harbin Institute of Technology, Harbin, China
{mingyu.li, qiong.li, yan.liang}@ict.hit.edu.cn
Abstract
In this paper a information hiding algorithm applied to
compressed speech bitstream is proposed. Covering codes
are used to hide information in the least important bits in
G.729 frames. The embedding rate is fairly high compared
with the state of art algorithms, while the experiment results
show that the distortion of the speech signal is impercepti-
ble.
1. Introduction
Information hiding is the art and science of hiding mes-
sage in a cover object such as a digital image, a digital
video, or a speech bitstream. In the last decades, a lot of
methods and algorithm applied to digital image[6][7] and
audio[5] emerged. But, in the literature, there are only
a few information hiding methods robust to or based on
parametric and hybrid speech coding, like CELP, which is
a type of mainstream speech coding method. Some clas-
sical information hiding algorithms for audio signal, such
as spread spectrum watermarking, transform-based water-
marking, etc. are not resilient against parametric-based
speech coding[4][12][3].
G.729 is one of the most widely used hybrid speech
coding especially in VoIP systems, and it is also a well
studied typical CELP-based speech coding algorithm. In
[10], the LSP (line spectrum pair) parameters is modified
to embed information. Celik proposed a watermarking al-
gorithm applying QIM to the average pitch period of each
segment[3]. While it is robust to some CELP-based speech
coding, the embedding capacity is as low as 3 bits/sec.
In[13], a ABS-based information hiding approach is applied
to G.729 by modification of the index of adaptive and fixed
∗
This work is supported by the National Natural Science Foundation of
China (Project Number: 60671064,60703011), the Foundation for the Au-
thor of National Excellent Doctoral Dissertation of China (Project Num-
ber: FANEDD-200238), the Program for New Century Excellent Talents
in University (Project Number:NCET-04-0330), the Chinese national 863
Program (Project Number:2007AA01Z458), and The Research Fund for
the Dectoral Program of Higher Education (RFDP20070213047).
codebook. The capacity is higher, but the imperceptibility
of this method is not very good, and, as the paper suggests,
the cover speech should be carefully selected to achieve op-
timal information hiding results.
In this paper, a information hiding algorithm for G.729
bitstream is proposed. Three least significant bits of each
speech frame are modified to embedding secret message
with linear covering codes. The experiment result indicates
that the method is perceptually transparent while the pay-
load is relatively high.
The details of the algorithm are described in Section 2.
In Section 3, experimental results are presented and dis-
cussed. Conclusion and further discussion are made in Sec-
tion 4.
2. The Proposed Method
The bits in a hybrid speech coding bitstream are percep-
tually unequally important. This non-uniform bit sensitivity
has been exploited to create Unequal Error Protection ap-
proach and partial encryption schemes [11]. Experiments
show that a few bits can be substituted without introducing
perceptual distortion. In this paper, these bits are used to
embed secret messages with covering codes.
2.1. The Least Significant Bits in G.729
The frame length of G.729 is 10ms, and each frame is
compressed into 80 bits. The bit allocation is shown in Ta-
ble 1.
International Conference on Intelligent Information Hiding and Multimedia Signal Processing
978-0-7695-3278-3/08 $25.00 © 2008 IEEE
DOI 10.1109/IIH-MSP.2008.297
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