The top end of Medium Wave is useless for anything national, unless you use megga high power. (Luxemburg) One or Two megawatts at one point, and directed.
There are several transmitters on 1215, and it sounds total rubbish here in Lincolnshire, as it always was.
The only true way to synchronise the audio would be to pick up an analogue signal from a satellite and ensure it was NOT digital in any way.
We have 2kw on 1242 just for Lincolnshire. I can almost see the site from where I live. The Modulation is low and flat so its not a very nice listen.
The lower end (648) and below, has superior ground wave properties, so goes further for way less power, however, if you do use high power there, its sure to fill in any gaps.
David (G1ZQC).