I have a question about why there is such a big difference in volume levels with this configuration:

• Sky decoder connected to Samsung Smart TV via HDMI
• TV connected via optical digital out to D to A converter box (TV output is set to PCM)
• Analog cable from D to A to old home theatre system with analogue inputs

Sound level from home theatre system when watching Sky is very good.  Watching any app on the TV (eg NetFlix, Youtube) the volume level is very low even with home theatre turned to max volume.  How is it that there is such a massive difference in sound when they are both sending audio via optical digital out on the TV?  Is there some kind of volume level information in the PCM output from the TV that might differ depending on the source?

PS. TV has no analogue audio out and D to A converter has no volume control.  Home theatre system has no digital input.  No antenna on TV to test volume levels of broadcast TV.  Setting TV to use internal speakers, the volume levels for both Sky and apps are similar.