From 0d63d2a7adbe4381d945228a9da4471c9409a8f1 Mon Sep 17 00:00:00 2001
From: Colin Snover
Date: Fri, 16 Jun 2017 20:04:46 -0500
Subject: VIDEO: Wrap out-of-range VMD audio samples instead of clipping

The 16-bit DPCM decompressors in SSCI and Urban Runner use a 16-bit
register to store sample data, without any special handling of
overflow. As such, out-of-range samples simply wrap around, rather
than getting clipped.

It is not totally clear if the wrapping behaviour was intentionally
exploited to handle extreme transients, but in any case, videos
like GK2 5280.VMD that generate samples outside the signed 16-bit
range cause a loud pop when using clipping, but play back correctly
when wrapping.
---
 video/coktel_decoder.cpp | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

(limited to 'video')

diff --git a/video/coktel_decoder.cpp b/video/coktel_decoder.cpp
index 905ffd6ff2..daebbd3ea9 100644
--- a/video/coktel_decoder.cpp
+++ b/video/coktel_decoder.cpp
@@ -2617,7 +2617,14 @@ int DPCMStream::readBuffer(int16 *buffer, const int numSamples) {
 			else
 				_buffer[i] += tableDPCM[data];
 
-			*buffer++ = _buffer[i] = CLIP<int32>(_buffer[i], -32768, 32767);
+			// Emulating x86 16-bit signed register overflow
+			if (_buffer[i] > 32767) {
+				_buffer[i] -= 65536;
+			} else if (_buffer[i] < -32768) {
+				_buffer[i] += 65536;
+			}
+
+			*buffer++ = _buffer[i];
 		}
 
 		samples += _channels;
-- 
cgit v1.2.3