diff --git a/src/emu/sound.h b/src/emu/sound.h
index 3aefc4bef3f..84c304dab68 100644
--- a/src/emu/sound.h
+++ b/src/emu/sound.h
@@ -371,13 +371,9 @@ public:
 	}
 
 	// safely write a sample to the buffer
-	void put(s32 index, sample_t sample)
+	void put(s32 start, sample_t sample)
 	{
-		sound_assert(u32(index) < samples());
-		index += m_start;
-		if (index >= m_buffer->size())
-			index -= m_buffer->size();
-		m_buffer->put(index, sample);
+		m_buffer->put(index_to_buffer_index(start), sample);
 	}
 
 	// write a sample to the buffer, clamping to +/- the clamp value
@@ -401,12 +397,9 @@ public:
 	}
 
 	// safely add a sample to the buffer
-	void add(s32 index, sample_t sample)
+	void add(s32 start, sample_t sample)
 	{
-		sound_assert(u32(index) < samples());
-		index += m_start;
-		if (index >= m_buffer->size())
-			index -= m_buffer->size();
+		u32 index = index_to_buffer_index(start);
 		m_buffer->put(index, m_buffer->get(index) + sample);
 	}
 
@@ -421,7 +414,7 @@ public:
 	{
 		if (start + count > samples())
 			count = samples() - start;
-		u32 index = start + m_start;
+		u32 index = index_to_buffer_index(start);
 		for (s32 sampindex = 0; sampindex < count; sampindex++)
 		{
 			m_buffer->put(index, value);
@@ -436,7 +429,7 @@ public:
 	{
 		if (start + count > samples())
 			count = samples() - start;
-		u32 index = start + m_start;
+		u32 index = index_to_buffer_index(start);
 		for (s32 sampindex = 0; sampindex < count; sampindex++)
 		{
 			m_buffer->put(index, src.get(start + sampindex));
@@ -451,7 +444,7 @@ public:
 	{
 		if (start + count > samples())
 			count = samples() - start;
-		u32 index = start + m_start;
+		u32 index = index_to_buffer_index(start);
 		for (s32 sampindex = 0; sampindex < count; sampindex++)
 		{
 			m_buffer->put(index, m_buffer->get(index) + src.get(start + sampindex));
@@ -460,6 +453,17 @@ public:
 	}
 	void add(read_stream_view const &src, s32 start) { add(src, start, samples() - start); }
 	void add(read_stream_view const &src) { add(src, 0, samples()); }
+
+private:
+	// given a stream starting offset, return the buffer index
+	u32 index_to_buffer_index(s32 start) const
+	{
+		sound_assert(u32(start) < samples());
+		u32 index = start + m_start;
+		if (index >= m_buffer->size())
+			index -= m_buffer->size();
+		return index;
+	}
 };