School of Electrical Engineering and Telecommunications, The University of New South Wales, Sydney, Australia ; Cochlear Limited, Sydney, Australia.
PLoS One. 2013 Nov 28;8(11):e82263. doi: 10.1371/journal.pone.0082263. eCollection 2013.
Nucleus cochlear implant systems incorporate a fast-acting front-end automatic gain control (AGC), sometimes called a compression limiter. The objective of the present study was to determine the effect of replacing the front-end compression limiter with a newly proposed envelope profile limiter. A secondary objective was to investigate the effect of AGC speed on cochlear implant speech intelligibility. The envelope profile limiter was located after the filter bank and reduced the gain when the largest of the filter bank envelopes exceeded the compression threshold. The compression threshold was set equal to the saturation level of the loudness growth function (i.e. the envelope level that mapped to the maximum comfortable current level), ensuring that no envelope clipping occurred. To preserve the spectral profile, the same gain was applied to all channels. Experiment 1 compared sentence recognition with the front-end limiter and with the envelope profile limiter, each with two release times (75 and 625 ms). Six implant recipients were tested in quiet and in four-talker babble noise, at a high presentation level of 89 dB SPL. Overall, release time had a larger effect than the AGC type. With both AGC types, speech intelligibility was lower for the 75 ms release time than for the 625 ms release time. With the shorter release time, the envelope profile limiter provided higher group mean scores than the front-end limiter in quiet, but there was no significant difference in noise. Experiment 2 measured sentence recognition in noise as a function of presentation level, from 55 to 89 dB SPL. The envelope profile limiter with 625 ms release time yielded better scores than the front-end limiter with 75 ms release time. A take-home study showed no clear pattern of preferences. It is concluded that the envelope profile limiter is a feasible alternative to a front-end compression limiter.
耳蜗植入系统的核包含一个快速作用的前端自动增益控制（AGC），有时也称为压缩限制器。本研究的目的是确定用新提出的包络轮廓限制器替代前端压缩限制器的效果。次要目的是研究 AGC 速度对人工耳蜗语音可懂度的影响。包络轮廓限制器位于滤波器组之后，当滤波器组的最大包络超过压缩阈值时，会降低增益。压缩阈值设置为响度增长函数的饱和水平（即映射到最大舒适电流水平的包络水平），以确保不会发生包络削波。为了保持频谱轮廓，相同的增益应用于所有通道。实验 1 比较了前端限制器和包络轮廓限制器的句子识别，每个限制器都有两个释放时间（75ms 和 625ms）。在安静环境中和四人噪声环境中，6 名植入者在 89dB SPL 的高呈现水平下进行了测试。总体而言，释放时间的影响大于 AGC 类型。对于两种 AGC 类型，75ms 释放时间的言语可懂度都低于 625ms 释放时间。在较短的释放时间下，包络轮廓限制器在安静环境中的群体平均得分高于前端限制器，但在噪声中没有显著差异。实验 2 测量了噪声中句子识别的功能作为呈现水平的函数，从 55dB 到 89dB SPL。625ms 释放时间的包络轮廓限制器产生的分数优于 75ms 释放时间的前端限制器。一项家庭研究表明，没有明显的偏好模式。结论是，包络轮廓限制器是前端压缩限制器的可行替代方案。
