fix missing STL25.01 in GSI block when converting TTML to STL

stl.go

@@ -378,10 +378,14 @@ func newGSIBlock(s Subtitles) (g *gsiBlock) {
 			g.creationDate = *s.Metadata.STLCreationDate
 		}
 		g.countryOfOrigin = s.Metadata.STLCountryOfOrigin
-		g.displayStandardCode = s.Metadata.STLDisplayStandardCode
+		if s.Metadata.STLDisplayStandardCode != "" {
+			g.displayStandardCode = s.Metadata.STLDisplayStandardCode
+		}
 		g.editorContactDetails = s.Metadata.STLEditorContactDetails
 		g.editorName = s.Metadata.STLEditorName
-		g.framerate = s.Metadata.Framerate
+		if s.Metadata.Framerate > 0 {
+			g.framerate = s.Metadata.Framerate
+		}
 		if v, ok := stlLanguageMapping.GetInverse(s.Metadata.Language); ok {
 			g.languageCode = v.(string)
 		}

stl_test.go

@@ -96,3 +96,42 @@ func TestIgnoreTimecodeStartOfProgramme(t *testing.T) {
 	firstStart := 99 * time.Second
 	assert.Equal(t, firstStart, s.Items[0].StartAt, "first start at 0")
 }
+
+func TestTTMLToSTLGSIBlock(t *testing.T) {
+	// Test that TTML to STL conversion includes correct disk format code and display standard code
+	// This verifies the fix for the bug where "STL25.01" was missing from the GSI block
+	// when converting from TTML files that have no framerate metadata
+
+	// Open TTML file
+	s, err := astisub.OpenFile("./testdata/example-in.ttml")
+	assert.NoError(t, err)
+	assert.NotNil(t, s)
+
+	// TTML files typically don't have STL-specific metadata
+	assert.Empty(t, s.Metadata.STLDisplayStandardCode, "TTML should not have STL-specific display standard code")
+
+	// Write to STL
+	w := &bytes.Buffer{}
+	err = s.WriteToSTL(w)
+	assert.NoError(t, err)
+
+	stlData := w.Bytes()
+	assert.True(t, len(stlData) >= 1024, "STL file should have at least GSI block (1024 bytes)")
+
+	// Check GSI block header
+	// Bytes 0-2: Code page number (should be "850")
+	codePageNumber := string(stlData[0:3])
+	assert.Equal(t, "850", codePageNumber, "Code page number should be '850'")
+
+	// Bytes 3-10: Disk format code (should be "STL25.01" for 25fps, not spaces)
+	diskFormatCode := string(stlData[3:11])
+	assert.NotEqual(t, "        ", diskFormatCode,
+		"Disk format code should not be empty spaces (this was the bug)")
+	assert.True(t, diskFormatCode == "STL25.01" || diskFormatCode == "STL30.01",
+		"Disk format code should be 'STL25.01' or 'STL30.01', got '%s'", diskFormatCode)
+
+	// Byte 11: Display standard code (should be "1" for Level 1 Teletext by default)
+	displayStandardCode := string(stlData[11:12])
+	assert.True(t, displayStandardCode == "0" || displayStandardCode == "1" || displayStandardCode == "2",
+		"Display standard code should be '0', '1', or '2', got '%s'", displayStandardCode)
+}