From c57f861f2bbad5a45ab666b484ecd30f269c4fea Mon Sep 17 00:00:00 2001
From: Kojin <kojin@protonmail.com>
Date: Thu, 12 Nov 2020 17:15:42 -0500
Subject: [PATCH] counters: account for vertical frequency differences in
 non-interlaced anolog modes (#3862)

---
 pcsx2/Counters.cpp | 25 +++++++++++++++++++++++--
 pcsx2/GS.cpp       |  3 ---
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/pcsx2/Counters.cpp b/pcsx2/Counters.cpp
index 5d00ef86d3..b5ad96f178 100644
--- a/pcsx2/Counters.cpp
+++ b/pcsx2/Counters.cpp
@@ -278,16 +278,37 @@ const char* ReportVideoMode()
 
 Fixed100 GetVerticalFrequency()
 {
+	// Note about NTSC/PAL "double strike" modes:
+	// NTSC and PAL can be configured in such a way to produce a non-interlaced signal.
+	// This involves modifying the signal slightly by either adding or subtracting a line (526/524 instead of 525)
+	// which has the function of causing the odd and even fields to strike the same lines.
+	// Doing this modifies the vertical refresh rate slightly. Beatmania is sensitive to this and
+	// not accounting for it will cause the audio and video to become desynced.
+	//
+	// In the case of the GS, I believe it adds a halfline to the vertical back porch but more research is needed.
+	// For now I'm just going to subtract off the config setting.
+	//
+	// According to the GS:
+	// NTSC (interlaced): 59.94
+	// NTSC (non-interlaced): 59.82
+	// PAL (interlaced): 50.00
+	// PAL (non-interlaced): 49.76
+	//
+	// More Information:
+	// https://web.archive.org/web/20201031235528/https://wiki.nesdev.com/w/index.php/NTSC_video
+	// https://web.archive.org/web/20201102100937/http://forums.nesdev.com/viewtopic.php?t=7909
+	// https://web.archive.org/web/20120629231826fw_/http://ntsc-tv.com/index.html
+	// https://web.archive.org/web/20200831051302/https://www.hdretrovision.com/240p/
 	switch (gsVideoMode)
 	{
 	case GS_VideoMode::Uninitialized: // SetGsCrt hasn't executed yet, give some temporary values.
 		return 60;
 	case GS_VideoMode::PAL:
 	case GS_VideoMode::DVD_PAL:
-		return EmuConfig.GS.FrameratePAL;
+		return gsIsInterlaced ? EmuConfig.GS.FrameratePAL : EmuConfig.GS.FrameratePAL - 0.24f;
 	case GS_VideoMode::NTSC:
 	case GS_VideoMode::DVD_NTSC:
-		return EmuConfig.GS.FramerateNTSC;
+		return gsIsInterlaced ? EmuConfig.GS.FramerateNTSC : EmuConfig.GS.FramerateNTSC - 0.11f;
 	case GS_VideoMode::SDTV_480P:
 		return 59.94;
 	case GS_VideoMode::HDTV_1080P:
diff --git a/pcsx2/GS.cpp b/pcsx2/GS.cpp
index 235f2c6891..d2d4a0211f 100644
--- a/pcsx2/GS.cpp
+++ b/pcsx2/GS.cpp
@@ -36,9 +36,6 @@ void gsOnModeChanged( Fixed100 framerate, u32 newTickrate )
 
 void gsSetVideoMode(GS_VideoMode mode )
 {
-	if( gsVideoMode == mode )
-		return;
-
 	gsVideoMode = mode;
 	UpdateVSyncRate();
 }