Echo · Frame Advance

Echo mixes a delayed copy of the sound back into the output, so a song sounds as if it plays in a room or a cave.

Games

Every claim about this mechanic, by game (2)

The Legend of Zelda: A Link to the Past 1991	The songs set the echo themselves, through four commands in their track data. $F5 sets which voices echo and the left and right echo volumes; $F6 sets the flag that disables echo writes; $F7 sets the echo delay, the feedback, and one of four 8-tap filters; $F8 slides the two echo volumes to new values over a given number of ticks. A change of delay first zeroes both echo volumes, the feedback, and the echo voices, disables echo writes, and then writes the new delay. The echo buffer starts at $D000 − $800 × the delay and is $800 bytes long per step of delay, so it ends at $D000, where the songs' pointer table begins (computed). The sound program starts with a delay of 1, 512 samples, and a main volume of $60 on both sides. The first filter is the identity filter, which leaves the echo unchanged.	Code
Donkey Kong Country 1994	In the first US release, every song gets the same echo buffer and delay: when it restarts, the driver sets the sound chip's echo delay to 4, which makes an 8,192-byte buffer (computed), and its buffer to start at `$DF00`, and clears the 8,192 bytes from there (computed). It also sets echo volume, feedback, and every voice's echo bit to 0, and starts the song with echo writing off. A song turns echo on with its own commands: `$15` gives the feedback and the echo volume and turns writing on, `$16` and `$17` set echo per voice, and `$18` the filter.	Code