折叠腔内腔倍频激光器中基频与谐波相对相位对输出功率的影响

对折叠腔内倍频激光器基频与谐波的相对相位对倍频光输出功率的影响进行了理论分析,并在实验上采用全固体化LD泵浦Nd:YVO₄+KTP折叠腔内腔倍频激光器,利用光楔的色散作用调节腔内基频与谐波的相对相位,得到了倍频光输出随基频与谐波相对相位呈余弦函数平方的变化,验证了理论结果,为消除大功率激光器中相对相位的影响,提高输出功率提供了一种简单实用的方法.     

正弦相位调制双法布里—珀罗干涉术的实验研究

讨论了光纤传光、正弦相位调制的双法布里－珀罗干涉术实验结果。通过光强信号的傅里叶分析，证实基频幅值和相位均合谐振腔长度或程函变化信息，从而提出实现基频相位或幅值测量的时间间隔测量法或幅值整流基频幅值测量法。在已研制的实验装置上，测试两种方法的灵敏度阈，结果表明：作者提出的平行双通道结构和光纤传光的测试方法能补偿谐振腔温漂影响，简化信号处理过程，更适合实时测量。     

视听双任务条件下注意对视觉和听觉掩蔽效应的影响

在视听交互环境中,当视觉和听觉掩蔽刺激同时感知时,对视听觉的相互作用及掩蔽变化问题进行了实验研究。将视听交互场景按注意状态分类,实验测量了不同场景中视觉和听觉掩蔽阈值的变化,讨论了不同视觉和听觉掩蔽刺激的相互影响。实验结果发现对比单任务实验的阈值,不同视听交互环境中视觉和听觉的掩蔽阈值会产生相应变化。由实验结果可以推论视听交互环境中注意分配影响视觉和听觉掩蔽效应,并且视听掩蔽刺激的相互作用会进一步影响视觉和听觉唤起的注意。      

正弦相位调制双法布里－珀罗干涉术的实验研究

讨论了光纤传光、正弦相位调制的双法布里－珀罗（Ｆａｂｒｙ－Ｐｅｒｏｔ）干涉术实验结果．通过光强信号的傅里叶分析，证实基频幅值和相位均含谐振腔长度或程函变化信息，从而提出实现基频相位或幅值测量的时间间隔测量法或幅值整流基频幅值测量法．在已研制的实验装置上，测试两种方法的灵敏度阈，结果表明：作者提出的平行双通道结构和光纤传光的测试方法能补偿谐振腔温漂影响，简化信号处理过程，更适合实时测量．     

分段小信号近似方法研究准相位匹配倍频效率

从PPLN波导微结构出发分析准相位匹配倍频过程中的相位补偿机理,以极化畴为单元,通过耦合波方程计算极化畴内产生的二次谐波电场,通过电场相干叠加计算倍频效率。分别在小信号近似和基频光高消耗情况下计算准相位匹配倍频效率,结果显示：倍频效率小于5％时,两种情况下的计算结果吻合得很好;随着转换效率增大,小信号近似不再适用,需要在基频光高消耗情形下计算倍频效率。研究了基频光功率密度与准相位匹配倍频效率的关系,定性分析了极化周期存在误差时,误差对倍频效率的影响。     

视觉刺激条件下的听感差别阈限测量

视听交互的重要性日益突出,但视觉刺激对听觉感知的影响尚缺乏全面深入的研究。以视觉刺激下人耳对声音的主观听感差别阈限变化为研究对象,在主观听觉实验中施加颜色、质量、亮度、运动状态四个不同属性视觉刺激,同时测量纯音信号的响度、主观音长和音高的听感差别阈限。通过与无视觉刺激下相应差别阈限的比较,分析不同视觉条件对响度感知、主观音长感知、音高感知能力的影响。实验数据显示,施加视觉刺激后主观听觉感知的差别阈限值增大,主观音长、音高和响度的差别阈限值平均分别提高了45.1%,14.8%和12.3%。进一步分析的结果表明,施加视觉刺激后基本的听觉感知能力呈下降趋势。同一视觉属性的不同水平视觉条件对听觉感知的影响程度不同,主观听感的变化呈现出一定的规律性,即视觉刺激越舒适,听感的差别阈限变化越小。      

基于样本熵的听觉神经锁相机理的实验分析

锁相是指系统的响应与周期性刺激的特定相位同步的物理现象. 听觉神经的锁相对揭示人的听觉认知基本的神经机理及改善听觉感知有重要意义. 然而, 现有研究主要集中于心理物理方法和幅度谱分析, 不能有效区分包络响应和时域细节结构响应, 不能直观反映神经锁相. 本文主要利用拔靴法和离散傅里叶变换, 提出了基于样本熵的时域细节结构频率跟随响应(temporal-fine-structure-related frequency following response, FFR_T)的神经锁相值(phase locking value, PLV)计算方法, 用于分析神经物理实验数据. 两个脑电实验结果表明: FFR_T的PLV样本熵显著大于包络相关频率跟随响应(envelope-related frequency following response, FFR_E)的PLV, 且二者正交独立, 新方法能有效地分别反映听觉系统对包络和时间细节结构的锁相机理; 基频处的响应主要来源于FFR_E的锁相; 基频处, 不可分辨谐波成分包络的锁相能力优于对可分辨谐波; 基频缺失时, 畸变产物是不同的听觉神经通路的FFR_E的混合; 谐波处, FFR_E 集中于低频, FFR_T则集中于中、高频; 听觉神经元锁相能力与声源的频率可分辨性相关. FFR_T的PLV方法克服了现有FFR分析的局限性, 可用于深入研究听觉神经机理.     

连续话语中双音节韵律词的重音感知

对于从微软亚洲研究院的汉语语音语料库中获得的300个语句中的1,898个双音节韵律词进行了重音感知实验,实验结果表明,连续话语中双音节词的重音感知特点与孤立词的重音感知特点有所不同,它受到词所在的韵律边界的显著影响。在感知实验中,词内两音节的重音得分之差与它们的高音点音高差和时长差都表现出正相关,但与高音点音高差的相关强于与时长差的相关。高音点音高差和时长差在非停顿前不相关,在停顿前为较弱的正相关。实验结果还表明,音节的重音感知受到调型的显著影响。     

维吾尔语焦点的韵律实现及感知

通过严格控制的语音实验,研究了维吾尔语陈述句中焦点对音高和时长的调节作用。实验设计了两个目标句,请发音人根据上下文自然地强调句中相应的词,随后还考察了焦点的感知问题。结果表明:(1)以句末焦点为基线,维吾尔语焦点的韵律编码方式类似于北京话和英语中的"三区段"调节模式,表现为焦点词音高升高、音域扩大和焦点后音高骤降(音域变窄),而焦点前音高变化不大;(2)焦点词和焦点前的词时长都有延长,而焦点后的词没有明显变化;(3)对焦点感知的正确率平均可达90%左右,表明焦点的韵律编码方式是有效的感知线索;(4)感知实验及语调分析还显示,维吾尔语"中性焦点"语调特征与英语和汉语不同,它接近句首焦点而不是句末焦点。另外,论文特别讨论了"焦点后音高骤降"在中国语言中的分布及来源问题。      

基于噪声追踪的二值时频掩蔽到浮值掩蔽的泛化算法

虽然浮值掩蔽比二值掩蔽有更好的语音分离效果,但是由于理想浮值掩蔽难以直接估计,现有的语音分离系统通常以理想二值掩蔽估计作为计算目标。我们提出了一个二值掩蔽到浮值掩蔽的泛化算法。由于实现浮值掩蔽估计的关键在于噪声能量追踪,我们首先采用指数分布刻画以混合谱和噪声能量以混合能量及二值掩蔽为观测的条件分布。其次,采用高斯马尔柯夫条件随机场刻画噪声估计在连续几帧内的关联。最后,采用马尔柯夫链-蒙特卡洛计算噪声能量最小均方误差估计并进一步计算浮值掩蔽。实验表明,相比于基于二值掩蔽估计的常规算法,我们所提出的算法在信噪比增益和客观感知质量两方面都有显著提高。      

Pitch perception of concurrent harmonic tones with overlapping spectra

Fundamental frequency difference limens (F0DLs) were measured for a target harmonic complex tone with nominal fundamental frequency (F0) of 200 Hz, in the presence and absence of a harmonic masker with overlapping spectrum. The F0 of the masker was 0, ± 3, or ± 6 semitones relative to 200 Hz. The stimuli were bandpass filtered into three regions: 0-1000 Hz (low, L), 1600-2400 Hz (medium, M), and 2800-3600 Hz (high, H), and a background noise was used to mask combination tones and to limit the audibility of components falling on the filter skirts. The components of the target or masker started either in cosine or random phase. Generally, the effect of F0 difference between target and masker was small. For the target alone, F0DLs were larger for random than cosine phase for region H. For the target plus masker, F0DLs were larger when the target had random phase than cosine phase for regions M and H. F0DLs increased with increasing center frequency of the bandpass filter. Modeling using excitation patterns and "summary autocorrelation" and "stabilized auditory image" models suggested that use of temporal fine structure information can account for the small F0DLs obtained when harmonics are barely, if at all, resolved.     

Pitch discrimination of diotic and dichotic tone complexes: harmonic resolvability or harmonic number?

Three experiments investigated the relationship between harmonic number, harmonic resolvability, and the perception of harmonic complexes. Complexes with successive equal-amplitude sine- or random-phase harmonic components of a 100- or 200-Hz fundamental frequency (f0) were presented dichotically, with even and odd components to opposite ears, or diotically, with all harmonics presented to both ears. Experiment 1 measured performance in discriminating a 3.5%-5% frequency difference between a component of a harmonic complex and a pure tone in isolation. Listeners achieved at least 75% correct for approximately the first 10 and 20 individual harmonics in the diotic and dichotic conditions, respectively, verifying that only processes before the binaural combination of information limit frequency selectivity. Experiment 2 measured fundamental frequency difference limens (f0 DLs) as a function of the average lowest harmonic number. Similar results at both f0's provide further evidence that harmonic number, not absolute frequency, underlies the order-of-magnitude increase observed in f0 DLs when only harmonics above about the 10th are presented. Similar results under diotic and dichotic conditions indicate that the auditory system, in performing f0 discrimination, is unable to utilize the additional peripherally resolved harmonics in the dichotic case. In experiment 3, dichotic complexes containing harmonics below the 12th, or only above the 15th, elicited pitches of the f0 and twice the f0, respectively. Together, experiments 2 and 3 suggest that harmonic number, regardless of peripheral resolvability, governs the transition between two different pitch percepts, one based on the frequencies of individual resolved harmonics and the other based on the periodicity of the temporal envelope.     

The relationship between frequency selectivity and pitch discrimination: effects of stimulus level

Three experiments tested the hypothesis that fundamental frequency (fo) discrimination depends on the resolvability of harmonics within a tone complex. Fundamental frequency difference limens (fo DLs) were measured for random-phase harmonic complexes with eight fo's between 75 and 400 Hz, bandpass filtered between 1.5 and 3.5 kHz, and presented at 12.5-dB/component average sensation level in threshold equalizing noise with levels of 10, 40, and 65 dB SPL per equivalent rectangular auditory filter bandwidth. With increasing level, the transition from large (poor) to small (good) fo DLs shifted to a higher fo. This shift corresponded to a decrease in harmonic resolvability, as estimated in the same listeners with excitation patterns derived from measures of auditory filter shape and with a more direct measure that involved hearing out individual harmonics. The results are consistent with the idea that resolved harmonics are necessary for good fo discrimination. Additionally, fo DLs for high fo's increased with stimulus level in the same way as pure-tone frequency DLs, suggesting that for this frequency range, the frequencies of harmonics are more poorly encoded at higher levels, even when harmonics are well resolved.     

Detection and F0 discrimination of harmonic complex tones in the presence of competing tones or noise

Normal-hearing listeners' ability to "hear out" the pitch of a target harmonic complex tone (HCT) was tested with simultaneous HCT or noise maskers, all bandpass-filtered into the same spectral region (1200-3600 Hz). Target-to-masker ratios (TMRs) necessary to discriminate fixed fundamental-frequency (F0) differences were measured for target F0s between 100 and 400 Hz. At high F0s (400 Hz), asynchronous gating of masker and signal, presenting the masker in a different F0 range, and reducing the F0 rove of the masker, all resulted in improved performance. At the low F0s (100 Hz), none of these manipulations improved performance significantly. The findings are generally consistent with the idea that the ability to segregate sounds based on cues such as F0 differences and onset/offset asynchronies can be strongly limited by peripheral harmonic resolvability. However, some cases were observed where perceptual segregation appeared possible, even when no peripherally resolved harmonics were present in the mixture of target and masker. A final experiment, comparing TMRs necessary for detection and F0 discrimination, showed that F0 discrimination of the target was possible with noise maskers at only a few decibels above detection threshold, whereas similar performance with HCT maskers was only possible 15-25 dB above detection threshold.     

Pitch discrimination interference: the role of pitch pulse asynchrony

Gockel, Carlyon, and Plack [J. Acoust. Soc. Am. 116, 1092-1104 (2004)] showed that discrimination of the fundamental frequency (F0) of a target tone containing only unresolved harmonics was impaired when an interfering complex tone with fixed F0 was added to the target, but filtered into a lower frequency region. This pitch discrimination interference (PDI) was greater when the interferer contained resolved harmonics than when it contained only unresolved harmonics. Here, it is examined whether this occurred because, when the interferer contained unresolved harmonics, "pitch pulse asynchrony (PPA)" between the target and interferer provided a cue that enhanced performance; this was possible in the earlier experiment because both target and interferer had components added in sine phase. In experiment 1, it was shown that subjects were moderately sensitive to the direction of PPA across frequency regions. In experiments 2 and 3, PPA cues were eliminated by adding the components of the target only, or of both target and interferer, in random phase. For both experiments, an interferer containing resolved harmonics produced more PDI than an interferer containing unresolved harmonics. These results show that PDI is smaller for an interferer with unresolved harmonics even when cues related to PPA are eliminated.     

Pitch discrimination and phase sensitivity in young and elderly subjects and its relationship to frequency selectivity.

Frequency difference limens for pure tones (DLFs) and for complex tones (DLCs) were measured for four groups of subjects: young normal hearing, young hearing impaired, elderly with near-normal hearing, and elderly hearing impaired. The auditory filters of the subjects had been measured in earlier experiments using the notched-noise method, for center frequencies (fc) of 100, 200, 400, and 800 Hz. The DLFs for both impaired groups were higher than for the young normal group at all fc's (50-4000 Hz). The DLFs at a given fc were generally only weakly correlated with the sharpness of the auditory filter at that fc, and some subjects with broad filters had near-normal DLFs at low frequencies. Some subjects in the elderly normal group had very large DLFs at low frequencies in spite of near-normal auditory filters. These results suggest a partial dissociation of frequency selectivity and frequency discrimination of pure tones. The DLCs for the two impaired groups were higher than those for the young normal group at all fundamental frequencies (fo) tested (50, 100, 200, and 400 Hz); the DLCs for the elderly normal group were intermediate. At fo = 50 Hz, DLCs for a complex tone containing only low harmonics (1-5) were markedly higher than for complex tones containing higher harmonics, for all subject groups, suggesting that pitch was conveyed largely by the higher, unresolved harmonics. For the elderly impaired group, and some subjects in the elderly normal group, DLCs were larger for a complex tone with lower harmonics (1-12) than for tones without lower harmonics (4-12 and 6-12) for fo's up to 200 Hz. Some elderly normal subjects had markedly larger-than-normal DLCs in spite of near-normal auditory filters. The DLCs tended to be larger for complexes with components added in alternating sine/cosine phase than for complexes with components added in cosine phase. Phase effects were significant for all groups, but were small for the young normal group. The results are not consistent with place-based models of the pitch perception of complex tones; rather, they suggest that pitch is at least partly determined by temporal mechanisms.     

Evidence against an effect of grouping by spectral regularity on the perception of virtual pitch

Two experiments investigated the role of the regularity of the frequency spacing of harmonics, as a separate factor from harmonicity, on the perception of the virtual pitch of a harmonic series. The first experiment compared the shifts produced by mistuning the 3rd, 4th, and 5th harmonics in the pitch of two harmonic series: the odd-H and the all-H tones. The odd-H tone contained odd harmonics 1 to 11, plus the 4th harmonic; the all-H tone contained harmonics 1 to 12. Both tones had a fundamental frequency of 155 Hz. Pitch shifts produced by mistuning the 3rd harmonic, but not the 4th and 5th harmonics, were found to be significantly larger for the odd-H tone than for the all-H tone. This finding was consistent with the idea that grouping by spectral regularity affects pitch perception since an odd harmonic made a larger contribution than an adjacent even harmonic to the pitch of the odd-H tone. However, an alternative explanation was that the 3rd mistuned harmonic produced larger pitch shifts within the odd-H tone than the 4th mistuned harmonic because of differences in the partial masking of these harmonics by adjacent harmonics. The second experiment tested these explanations by measuring pitch shifts for a modified all-H tone in which each mistuned odd harmonic was tested in the presence of the 4th harmonic, but in the absence of its other even-numbered neighbor. The results showed that, for all mistuned harmonics, pitch shifts for the modified all-H tone were not significantly different from those for the odd-H tone. These findings suggest that the harmonic relations among frequency components, rather than the regularity of their frequency spacing, is the primary factor for the perception of the virtual pitch of complex sounds.     

Accuracy of pitch matching for pure tones and for complex tones with overlapping or nonoverlapping harmonics.

The discrimination of the fundamental frequency (fo) of pairs of complex tones with no common harmonics is worse than the discrimination of fo for tones with all harmonics in common. These experiments were conducted to assess whether this effect is a result of pitch shifts between pairs of tones without common harmonics or whether it reflects influences of spectral differences (timbre) on the accuracy of pitch perception. In experiment 1, pitch matches were obtained between sounds drawn from the following types: (1) pure tones (P) with frequencies 100, 200, or 400 Hz; (2) a multiple-component complex tone, designated A, with harmonics 3, 4, 8, 9, 10, 14, 15, and fo = 100, 200, or 400 Hz; (3) A multiple-component complex tone, designated B, with harmonics 5, 6, 7, 11, 12, 13, 16, and with fo = 100, 200 or 400 Hz. The following matches were made; A vs A, B vs B, A vs P, B vs P and P vs P. Pitch shifts were found between the pure tones and the complex tones (A vs P and B vs P), but not between the A and B tones (A vs B). However, the variability of the A vs B matches was significantly greater than that of the A vs A or B vs B matches. Also, the variability of the A vs P and B vs P matches was greater than that for the A vs B matches. In a second experiment, frequency difference limens (DLCs) were measured for the A vs A, B vs B, and A vs B pairs of sounds. The DLCs were larger for the A vs B pair than for A vs A or B vs B. The results suggest that the poor frequency discrimination of tones with no common harmonics does not result from pitch shifts between the tones. Rather, it seems that spectral differences between tones interfere with judgements of their relative pitch.     

Difference limens for phase in normal and hearing-impaired subjects

These experiments measure the ability to detect a change in the relative phase of a single component in a harmonic complex tone. Complex tones containing the first 20 harmonics of 50, 100, or 200 Hz, all at equal amplitude, were used. All of the harmonics except one started in cosine phase. The remaining harmonic started in cosine phase, but was shifted in phase half-way through either the first or the second of the two stimuli comprising a trial. The subject had to identify the stimulus containing the phase-shifted component. For normally hearing subjects tested at a level of 70 dB SPL per component, thresholds for detecting the phase shift [i.e., phase difference limens (DLs)] were smallest (2 degrees-4 degrees) for harmonics above the eighth and for the lowest fundamental frequency (F0). Changes in phase were not detectable for harmonic numbers below three or four at the lowest F0 and below 5-13 at the highest F0. The DLs increased slightly for the highest harmonics in the complexes. The DLs increased markedly with decreasing level, except for the highest harmonic, where only a small effect of level was found. Subjects reported that the phase-shifted harmonic appeared to "pop out" and was heard with a pure-tone quality. A pitch-matching experiment demonstrated that the pitch of this tone corresponded to the frequency of the phase-shifted component. For the highest harmonic, the phase shift was associated with a downward shift of the edge pitch heard in the reference (all cosine phase) stimulus. When the phases of the components in the reference stimulus were randomized, phase DLs were much higher (and often impossible to measure), the pop-out phenomenon was not observed, and no edge pitch was heard. Subjects with unilateral cochlear hearing impairment generally showed poorer phase sensitivity in their impaired than in their normal ears, when the two ears were compared at equal sound-pressure levels. However, at comparable sensation levels, the impaired ears sometimes showed lower phase DLs. The results are explained by considering the waveforms that would occur at the outputs of the auditory filters in response to these stimuli.     

Asymmetry of masking between complex tones and noise: the role of temporal structure and peripheral compression

Thresholds for the detection of harmonic complex tones in noise were measured as a function of masker level. The rms level of the masker ranged from 40 to 70 dB SPL in 10-dB steps. The tones had a fundamental frequency (F0) of 62.5 or 250 Hz, and components were added in either cosine or random phase. The complex tones and the noise were bandpass filtered into the same frequency region, from the tenth harmonic up to 5 kHz. In a different condition, the roles of masker and signal were reversed, keeping all other parameters the same; subjects had to detect the noise in the presence of a harmonic tone masker. In both conditions, the masker was either gated synchronously with the 700-ms signal, or it started 400 ms before and stopped 200 ms after the signal. The results showed a large asymmetry in the effectiveness of masking between the tones and noise. Even though signal and masker had the same bandwidth, the noise was a more effective masker than the complex tone. The degree of asymmetry depended on F0, component phase, and the level of the masker. The maximum difference between masked thresholds for tone and noise was about 28 dB; this occurred when the F0 was 62.5 Hz, the components were in cosine phase, and the masker level was 70 dB SPL. In most conditions, the growth-of-masking functions had slopes close to 1 (on a dB versus dB scale). However, for the cosine-phase tone masker with an F0 of 62.5 Hz, a 10-dB increase in masker level led to an increase in masked threshold of the noise of only 3.7 dB, on average. We suggest that the results for this condition are strongly affected by the active mechanism in the cochlea.