VSU: Describe various clock periods in terms of sample clock period

When calculating these numbers, I took the frequencies listed in the hw docs and calculated their periods in cycles, rounded to whole numbers. When looking at this again, it's clear that all of them work out to be simple multiples of the sample clock period, which is highly likely, assuming there's a single clock divider for all of these circuits, followed by even simpler/smaller counters.

This patch only changes how these constants are calculated; a more accurate/faithful implementation would update these counters only when the sample clock period occurs. I may implement this in a later patch.

rustual-boy-core/src/vsu/mod.rs

@@ -9,23 +9,23 @@ use self::mem_map::*;
 //  Note that the documentation rounds values in a lot of places, so that's probably what happened here.
 pub const SAMPLE_RATE: u32 = 41667;
 
-// 20mhz / 41.7khz = ~480 clocks
+// 20mhz / 41.7khz
 const SAMPLE_CLOCK_PERIOD: u32 = 480;
 
-// 20mhz / 260.4hz = ~76805 clocks
-const DURATION_CLOCK_PERIOD: u32 = 76805;
+// 20mhz / 260.4hz
+const DURATION_CLOCK_PERIOD: u32 = SAMPLE_CLOCK_PERIOD * 160;
 
-// 20mhz / 65.1hz = ~307218 clocks
-const ENVELOPE_CLOCK_PERIOD: u32 = 307218;
+// 20mhz / 65.1hz
+const ENVELOPE_CLOCK_PERIOD: u32 = SAMPLE_CLOCK_PERIOD * 640;
 
-// 20mhz / 5mhz = 4 clocks
+// 20mhz / 5mhz
 const FREQUENCY_CLOCK_PERIOD: u32 = 4;
 
-// 20mhz / 1041.6hz = ~19200 clocks
-const SWEEP_MOD_SMALL_PERIOD: u32 = 19200;
+// 20mhz / 1041.6hz
+const SWEEP_MOD_SMALL_PERIOD: u32 = SAMPLE_CLOCK_PERIOD * 40;
 
-// 20mhz / 130.2hz = ~153600 clocks
-const SWEEP_MOD_LARGE_PERIOD: u32 = 153600;
+// 20mhz / 130.2hz
+const SWEEP_MOD_LARGE_PERIOD: u32 = SAMPLE_CLOCK_PERIOD * 320;
 
 // 20mhz / 500khz = 40 clocks
 const NOISE_CLOCK_PERIOD: u32 = 40;