>>973
I think it's still the same thing. While it usually adds an explanatory tone, I've seen it/heard of it used just to add extra emotion. Though, I'm only at an intermediate level with my Japanese so I may be wrong myself (if anyone else wants to come in and correct me!).
I just thought about looking it up in "A Dictionary of Japanese Grammar" series and heres part of the entry of it.
3. [Subject] のだ is also used when no information is shared by the speaker and the hearer and the speaker is not explaining or asking for an explanation about anything. In this case, the speaker is talking as if some information were shared with the hearer and the effects of this are, for example,
(a) to involve the hearer in the affairs he is talking about (See (3) and (4) below.)
and, or
(b) to impose his idea upon the hearer or, at least, to emphasize his idea emotively. (See (5) below.)
Examples:
(3)今日フットボールがあるんですが一緒に行きませんか。
(There is a football game today, wouldn't you like to go with me?)
(4)先生、困っているんです。助けてください。
(Sensei, I'm in trouble. Please help me.)
(5)日本語の文法は難しいですがおもしろいんですよ。
(Japanese grammar is difficult, but it's interesting, you know.)
The parts prior to that section are just explaining のです used in its explanatory tone and the fact that it can be shortened to のだ or んだ.
As I said before I may be incorrect myself and there may be another grammar point I don't know about yet, but every time I've seen んだ (and equivalents) used, it's always been this same form.
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