I am developing a loudness meter conforming to the EBU R128 recommendation. Currently, it works with stereo files but not with mono files.
In the ITU Recommendation, where the metering algorithms are defined, weightings for the individual audio channels are given (table 3, page 6). From that, I assumed that the weighting for a mono channel is 1.0 (0 dB).
As you can see in figure 1 on page 3, after applying k-filter, mean square and channel weighting to each individual channel, the resulting values of each individual channel are summed to one value. Logic dictates that when having two channels of the same audio signal, the value will be higher than when just having one.
As a consequence, a mono file will result in short-term loudness values, that are 3 LU lower in comparison to a stereo file with same content (on both L and R channels). This is clearly wrong, because established meters like "Klangfreund LUFS Meter" and "MeldaProductions MLoudness Analyzer" will output the same short-term loudness with both files.
How am I interpreting the algorithm definition wrong? How to deal with mono signals?
Side note: It seems that FFmpeg once had a very similar issue, but I could not find the solution to it.