Consider how loudness is calculated for live / non-file-based encodes

[tdaede]

The text for computing loudness values requires it to be calculated for the entire stream. This means it cannot be provided for live encodes.

We could potentially add additional ways to compute it over a window, or per-"program".

[tdaede]

We have to include integrated_loudness right now, so we need to specify some value to use here if loudness cannot be computed.

[tdaede]

Another option would be to make integrated_loudness option or reserve a value for live use cases.

[tdaede]

A third option would be to specify a way that it can be computed (or targeted) for live streams.

[sunghee-hwang]

How about allowing to set integrated_loudness = 0x7FFF to indicate that no loudness information for live use cases is provided?
It is only allowed for live use cases to set such the value.

For V2, we can continue to find a way that it can be computed (or targeted) for live streams.

[tdaede]

We will not add the sentinel value, but rather loosen our restrictions for loudness calculation for live in the future.