Tempo change
I struggled with this too, then discovered how to do it. When you are in song mode, you simply write those measures that you want to be at different tempos as different songs and then chain them together. All of this is done internally in the DR-770.
Let's say I have a song in 4/4 that I want to be at 130 bpm from measures 1-20, then I want it to go down to 115 from 21-30, and then to 140 from 31-40, and then return to 130 from 40 to the end. Thus, I want to have 4 different tempo changes in this song, or rather 4 areas in the song that have different tempos (tempi?). I can't actively change the tempo while the song is playing because the device simply doesn't work that way, but I CAN write each section of different tempo as a new song. So, for measures 1-20, I'll select SONG, STEP TIME, set the TEMPO for 130, and select CHAIN = ON. This last step will ensure that after the last measure of Song 01 finishes, it will continue on directly to Song 02. So... write measures 1-20 into Song 01. Start all over again and do the same for measures 21-30, this time setting the TEMPO for 115. This will be Song 02. Repeat. Works like a charm!
Hope this helps.
Bruce in Korea