diff --git a/doc/4-instrument/c64.md b/doc/4-instrument/c64.md
index a9bb080b6..0beb20d94 100644
--- a/doc/4-instrument/c64.md
+++ b/doc/4-instrument/c64.md
@@ -24,7 +24,7 @@ the C64 instrument editor consists of two tabs: "C64" to control various paramet
     - **low**: a low-pass filter. the lower the cutoff, the darker the sound.
     - **high**: a high-pass filter. higher cutoff values result in a less "bassy" sound.
     - **band**: a band-pass filter. cutoff determines which part of the sound is heard (from bass to treble).
-    - **ch3off**: mutes channel 3 when enabled. not sure why is this part of the chip's design, but it is.
+    - **ch3off**: mutes channel 3 when enabled. It was originally planned for usage with two registers where program could read current oscillator and envelope outputs, thus making vibrato and SFX generation easier. But who wanted to sacrifice one channel out of three! So aforementioned was just done in software, and the feature was never used.
     - multiple filter modes can be selected simultaneously. for example, selecting both "low" and "high" results in a bandstop (notch) filter.
 
 - **Absolute Cutoff Macro**: when enabled, the cutoff macro will go from 0 to 2047, and it will be absolute (in other words, control the cutoff directly rather than being relative).
@@ -57,4 +57,4 @@ the C64 instrument editor consists of two tabs: "C64" to control various paramet
 - **Sustain**: sets envelope sustain level.
   - if you modify sustain level when envelope is in sustain phase it immediately changes, although you can only go down. for example, 9-to-8 and 8-to-8 both work, but 8-to-9 immediately mutes the channel.
 - **Release**: sets envelope release speed.
-  - if you modify release speed when envelope is in release phase it immediately changes.
\ No newline at end of file
+  - if you modify release speed when envelope is in release phase it immediately changes.