Abstract: This paper makes improvement in G.728 algorithm aiming at reducing coding rate and presents an 8 Kbit/s speech coding algorithm whose delay is 2.5 ms.Adaptive codebook structure is introduced in the proposed algorithm，which is composed of the recent excitation.The algorithm uses double codebook structure which contains adaptive codebook and normalized fixed codebook.The exact value of gain is calculated and quantized.Fixed quantization is used in adaptive codebook and adaptive quantization is used in fixed codebook.For codebook searching，the algorithm detects backward pitch，and then searches the adaptive codebook subtly round the pitch period T.After searching 64 adaptive code words，8 gains，256 fixed code words and 8 gains，the best excitation is obtained.The algorithm uses 20 bit per framework.Testing with the average segment SNR and Perceptual Evaluation of Speech Quality（PESQ），the improved algorithm is close to G.728 in speech coding quality.

Key words: Adaptive codebook, Pitch detection, Low Delay Code Excited Linear Prediction（LD-CELP）, Perceptual Evaluation of Speech Quality（PESQ）

摘要： 以降低码率为目的对G.728算法进行改进，提出了一个延迟为2.5 ms的8 Kbit/s的语音编码算法。算法引入了由最近的历史激励构成的自适应码书和归一化的固定码书的双码书结构。计算增益真值并量化，增益量化时对自适应码书用固定量化，固定码书用自适应量化。码书搜索时先进行后向基音检测，在基音周期T附近对自适应码书进行精细搜索。搜索64个自适应码矢、256个固定码矢和各自8个增益值获得最佳激励，每帧耗费20 bit。用平均分段信噪比和感知语音质量评价（PESQ）测试，改进算法编码质量接近于G.728。

关键词: 自适应码书, 基音检测, 低延时码激励线性预测, 感知语音质量评价