Shouted speech or screaming signals have been studied mostly through spectral representation such as melcepstral coefficients. Intuitive evidence that the characteristics of the excitation source may vary in the case of shouted speech has drawn little attention yet. In this paper we examine how the characteristics of both components of speech production mechanism, especially the glottal excitation source, are modified during the production of shout signals. Shouted and normal speech signals are examined along with the corresponding Electro-glotto-graph (EGG) signals. Distinguishing features like the dominant frequency and the strength of excitation are explored, along with the instantaneous fundamental frequency. These features are computed using linear prediction analysis and zero frequency filtering of the speech signal. Efficacy of these features in discriminating between shouted and normal speech is tested in five different vowel contexts.