When reading theoretical material about the DTD format I learn that the whole DTD document should be wrapped into one element like this:

<!DOCTYPE TVSCHEDULE [
<!ELEMENT .....
.....
]>

However, I cannot get Altova (http://www.altova.com/xmlspy) to see this kind of document as a well-formed DTD and I cannot validate an XML document against such DTD. It returns error "Character 'D' is gramatically unexpected in column 3 at line 1". How come?

When I chose a command to create a new DTD by Altova, it set up an empty sample of DTD where I could see what it considers a well-formed DTD. Namely it wants the document to start with an XML definition

<?xml version="1.0" encoding="UTF-8"?>

and then does not expect any "DOCTYPE" tag but simply the element/attribute definitions. But DTD even is not an XML document itself, is it!

Is it some kind of Altova error or have I misunderstood something?

>the whole DTD document should be wrapped into one element

It helps to try and use the correct terminology - it avoids a lot of misunderstandings. There's no such thing as a "DTD document", and the thing you are wrapping it in isn't an element.

A DTD that exists in a file of its own is called an "external DTD subset". This never contains the <!DOCTYPE ... > delimiters. The <!DOCTYPE ... > delimiters always appear in the instance document, immediately after the XML declaration if there is one. For example your instance document can be:

<!DOCTYPE doc SYSTEM "my.dtd">
<doc>Hello</doc>

which refers to an external DTD subset in file my.dtd that might contain the single line:

<!ELEMENT doc (#PCDATA)>

The syntax you showed is appropriate for an internal DTD subset (a DTD contained within the instance document that it describes, which might be:

<!DOCTYPE doc[
<!ELEMENT doc (#PCDATA)>
]>
<doc>Hello</doc>

I've left out the XML declaration <?xml version="1.0"?> in these examples because it's optional.

I'm not sure exactly what you did to get the error message from XML Spy.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

Thank you very much, mhkay, that makes perfect sense! :D It is, for example, similar to embedding or linking to a CSS or JavaScript code within a HTML document.

Still, what about an external DTD subset being an XML document? It's not correct, is it? So, even if the XML declaration does not give any errors, it should not be written in an external DTD subset file, should it?

Technically, an XML declaration can appear only at the start of the document entity (that is, the main XML document). However, a Text Declaration, which is for most purposes the same thing as an XML declaration, can appear at the start of any external entity, including the external DTD subset.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference