A detection algorithm of pitch modulation information hiding in G.729A low bit-rate speech codec is proposed.The analysis of speech stream showed that pith modulation information hiding would change the pitch symbiotic characteristics of adjacent speech frames.We designed a model to quantify these pitch symbiotic characteristics for steganalysis.However

the dimension of quantitative feature vector of pitch symbiotic characteristics was too high

so PCA (Principal Component Analysis) was employed to reduce the dimension of the feature vector.Finally

we built a pitch modulation information hiding detector based on the dimension reduced feature vector and SVM (Support Vector Machine) classifier.Experiments on different speech datasets show that the proposed steganalysis algorithm is very effective: the accuracy is more than 96% when speech length equals to or is greater than 2s.So this paper gives an effective method for compression domain information hiding detection.