Chet, you got it pretty much right.

 

JT6M was originally conceived to be a meteor-scatter mode optimized for 6 meters -- and indeed it does work very well when meteor pings are longer in duration (as they typically are on 6 meters when compared to 2 meters and above). However, the fact remains that meteor pings can be extremely short (100 ms or so) even on 6 meters. Pings this short are oftentimes too short to provide time enough for a full instance of the longest MS messages (both calls, or worst-case, both calls plus reports). The tradeoff between the two modes is that FSK441 is less sensitive than JT6M because of its higher bandwidth requirements, but it requires much less time to send a complete message. JT6M is more sensitive, bit is often too slow for very short duration meteor pings.

 

People use both FSK441 and JT6M on 6 meters, depending on how the variables ine up. On 2 meters and above, meteor pings are almost always very short, and JT6M is little used.

 

Yes, you can tell by ear which mode is being used. Switching modes unilaterally in the middle of a QSO, however, can lead to confusion, especially if operating in "random" mode (i.e., not coordinated on PJ) . If operating using PJ, it's a better idea to announce to the other station that you are switching modes and start the QSO attempt over from scratch. This is typically the case anyway, because if you're making progress with the Q using one mode, the practice is usually to let that mode play out to completion.

 

Bill W5WVO