Channels — five voices, and two of them share a table

4 Channels

Five voices, each with its own table, its own period and its own volume — except that on the original SCC, channels 4 and 5 read the same 32 bytes. One table, addressed once. Write to it and both voices change, because there was only ever one of them.

Edit ch4 below and watch ch5 redraw underneath your cursor. Then flip the chip to SCC+ and do it again.

What Konami did about it

You cannot give the two voices different instruments, but you can give them different periods — so the shared table gets used as one thick voice rather than two thin ones. Tune them an octave apart and the pair reads as a single bright lead; detune them a handful of periods and you get a chorus that costs no extra table. Both tricks are all over the Gradius 2 and Salamander soundtracks.

SCC vs SCC+

The K052539 in the Snatcher Sound Cartridge gives channel 5 a table of its own at 80–9F, moves the whole block from 9800h to B800h, and ignores the deformation register's bit 7. That is nearly the whole difference between the two chips: the oscillators, the volumes and the mixing are identical.