There is no clear ‘end’ to an analog signal. A digital signal ends when the data runs out. An analog signal could become quiet for a while, then resume — and this could be planned. There can also be small amounts of residual analog noise — forcing the player to attempt discriminating between states. 

Even before we had a significant number of digital systems, that first track on Pink Floyd’s Dark Side of the Moon album was very frustrating when played on automatic switching systems. The beginning of that track gradually emerged from record surface or tape residual noise. Even without any auto switching glitches, if you had the system Volume set to play the very beginning of the track, you were soon racing to reduce the Volume when the main contents suddenly arrived.

Thanks Buzz. Maybe my interpretation of ‘auto-detect’ feature on the Line In RCA ports on Port is incorrect.

Maybe I incorrectly assumed the Port ‘auto-detecting’ feature is detecting a signal from the line out built-in amp in TT.

Where it seems its actually ‘auto-detecting’ impedance, ie its physically connected to something other than fresh air.

No major issue, just unnecessary WiFi bandwidth used when TT not being used, unless I remember to press ‘Stop’ in the Sonos App after using TT.

There is a physical switch on the Line-In jack.

OK, I have figured this out now, every day is a school day.

The physical RCA ports on the Port are ‘auto-detecting’, they detect a physical connection, and make the line-in source available in Sonos App, even if there is no electrical signal.

There is also an ‘Autoplay’ feature, that is disabled by default, that detects a signal. It seems the ‘Autoplay’ option also provides auto stop functionality that I originally wrongly assumed was part of the auto-detect function.

Now the Autoplay is enabled, when I turn my TT OFF, the network stream does stop, sometimes takes a few seconds before Sonos stops. I have tested with a few albums, working as expected.

I don’t (yet) have Dark Side of the Moon to fully test.