==== TTML_subtitling_descriptor

Defined by DVB in <<ETSI-303-560>>.

[source,xml]
----
<TTML_subtitling_descriptor
    ISO_639_language_code="char3, required"
    subtitle_purpose="uint6, required"
    TTS_suitability="uint2, required"
    qualifier="uint32, optional"
    service_name="string, optional"
    reserved_zero_future_count="uint8, default=0">

  <!-- one per dvb_ttml_profile value -->
  <dvb_ttml_profile value="uint8, required"/>

  <!-- one per font_id value -->
  <font_id value="uint7, required"/>

</TTML_subtitling_descriptor>
----
