However, comparing with translation from normal perspective, the additive characteristic of subtitling is sometimes underestimated and acquires a relatively low status. For some scholars, subtitling is just a supplement to other communicative ways on the screen (such as images and sounds) which possesses no life of its own. Subtitling also plays a role of “synchronous semiotic channel” which means that subtitling demands timing uniformity between the source text utterances and the subtitles. The synchronicity refers to the general agreement among the subtitles, the spoken source language dialogue and the corresponding image. Moreover, subtitling is part of a transient text. Just as Ceron says, “Each subtitle is a clean slate, as it erases totally what came before” (Ceron 2001: 174). One piece of subtitle can only appear on the screen for limited time, normally, for a few second. “Due to the limited storage capacities of human brains, the viewers are merely capable of keeping in mind a fleeting memory of the previously presented subtitles” (Ceron 2001: 175). If we don’t care about backtracking function, the piece of subtitle will never appear on the screen again. If the viewer misses this piece of subtitle, he will just have to move on. Furthermore, subtitling can be part of a polysemiotic text. A scene on the screen not only includes subtitle but also image and sound, and they can appear on the screen simultaneously.

3.2.2 The constraints of subtitling
There is no doubt that every kind of translation has certain rules and is restricted by these rules. So is subtitle translation. In the following part, the paper tries to deal with the main constraints of subtitling.
From the point of Gottlieb, he pides subtitling constraints into two categories: formal constraints and textual constraints. He thinks that “The former refer to the factors of space and time, whereas the latter concern the imposition of visual context on the subtitles” (Gottlieb 1992: 161).
Considering the features of subtitling, space and time are indeed of great importance. Space constraint “stems from the size of the screen and the minimum letter-size supposedly legible to the average viewers”(Gottlieb 1992: 167) to a large extent. When we see movies or TV series, subtitle just accounts for a part. Generally speaking, subtitles are at the bottom of the screen with one or two lines. “Subtitles should not draw much attention to themselves and thus they may not consume over two twelfths of the screen”(Diaz-Cintas & Ramael 2007: 57). The subtitles are not intended to disturb the image. As far as Chinese subtitling is concerned, subtitles in Chinese generally occupy less space than subtitles in English if English movies or TV series do have subtitles. A scholar from China on subtitling once says that, he prefers one line to double lines in one scene in Chinese.
As for time constraint, Gottlieb holds that watchers should be given “five to six seconds to read a two-liner (of some 60 to 70 characters)”. However, another scholar thinks, “the most ideal duration of a full single-line subtitle is three and a half seconds, and around one quarter of a second should be inserted between two consecutive subtitles in order to evade the effect of subtitle’s overlay” (Karamitroglou 2001: 190). Different scholars may hold different views about the specific display time of subtitle.
As for the textual constraint of subtitling Gettlieb mentioned above, there are two factors that make sense. “First, the cueing (time) and positioning (space) of the subtitles must correspond with the static and dynamic visual characteristics, namely, the composition of the picture and montage of the film. Besides, the wording of the subtitles should maximally reflect the style, speech tempo and the syntax of key elements in the dialogue, whatever the nature of the material may be”(Gottlieb 1992: 164). When translating, the translator should take into consideration whether the subtitle suits the background image. Only if the subtitle and the scene fits well, the viewers can enjoy movies or TV series. Besides, whatever the original text may be, the translator should translate the style and features of it. For instance, there are a lot of actors in a TV series, and each character has its own features. Some of the features are shown through lines, and thus the translated lines should also occupy these features so as to make the character vivid and impressed in both original culture and target culture. 《疯狂动物城》从接受理论角度研究动画电影字幕翻译(5):http://www.751com.cn/yingyu/lunwen_18840.html