diff --git a/src/decoder/DecoderAPI.cxx b/src/decoder/DecoderAPI.cxx
index b214b91a8..ae600260f 100644
--- a/src/decoder/DecoderAPI.cxx
+++ b/src/decoder/DecoderAPI.cxx
@@ -31,6 +31,7 @@
 #include "DetachedSong.hxx"
 #include "input/InputStream.hxx"
 #include "util/Error.hxx"
+#include "util/ConstBuffer.hxx"
 #include "Log.hxx"
 
 #include <assert.h>
@@ -472,9 +473,8 @@ decoder_data(Decoder &decoder,
 		assert(dc.in_audio_format != dc.out_audio_format);
 
 		Error error;
-		data = decoder.convert->Convert(data, length,
-						&length,
-						error);
+		auto result = decoder.convert->Convert({data, length},
+						       error);
 		if (data == nullptr) {
 			/* the PCM conversion has failed - stop
 			   playback, since we have no better way to
@@ -482,6 +482,9 @@ decoder_data(Decoder &decoder,
 			LogError(error);
 			return DecoderCommand::STOP;
 		}
+
+		data = result.data;
+		length = result.size;
 	} else {
 		assert(dc.in_audio_format == dc.out_audio_format);
 	}
diff --git a/src/filter/plugins/ConvertFilterPlugin.cxx b/src/filter/plugins/ConvertFilterPlugin.cxx
index 27e6774f8..535330cfe 100644
--- a/src/filter/plugins/ConvertFilterPlugin.cxx
+++ b/src/filter/plugins/ConvertFilterPlugin.cxx
@@ -24,6 +24,7 @@
 #include "filter/FilterRegistry.hxx"
 #include "pcm/PcmConvert.hxx"
 #include "util/Manual.hxx"
+#include "util/ConstBuffer.hxx"
 #include "AudioFormat.hxx"
 #include "poison.h"
 
@@ -130,8 +131,9 @@ ConvertFilter::FilterPCM(const void *src, size_t src_size,
 		return src;
 	}
 
-	return state->Convert(src, src_size, dest_size_r,
-			      error);
+	auto result = state->Convert({src, src_size}, error);
+	*dest_size_r = result.size;
+	return result.data;
 }
 
 const struct filter_plugin convert_filter_plugin = {
diff --git a/src/pcm/PcmConvert.cxx b/src/pcm/PcmConvert.cxx
index ad79e60fc..ba9a691fc 100644
--- a/src/pcm/PcmConvert.cxx
+++ b/src/pcm/PcmConvert.cxx
@@ -117,12 +117,9 @@ PcmConvert::Close()
 #endif
 }
 
-const void *
-PcmConvert::Convert(const void *src, size_t src_size,
-		    size_t *dest_size_r,
-		    Error &error)
+ConstBuffer<void>
+PcmConvert::Convert(ConstBuffer<void> buffer, Error &error)
 {
-	ConstBuffer<void> buffer(src, src_size);
 	AudioFormat format = src_format;
 
 	if (format.format == SampleFormat::DSD) {
@@ -164,6 +161,5 @@ PcmConvert::Convert(const void *src, size_t src_size,
 		format.channels = dest_format.channels;
 	}
 
-	*dest_size_r = buffer.size;
-	return buffer.data;
+	return buffer;
 }
diff --git a/src/pcm/PcmConvert.hxx b/src/pcm/PcmConvert.hxx
index c65835421..9d63e07c9 100644
--- a/src/pcm/PcmConvert.hxx
+++ b/src/pcm/PcmConvert.hxx
@@ -70,16 +70,12 @@ public:
 	 *
 	 * @param src_format the source audio format
 	 * @param src the source PCM buffer
-	 * @param src_size the size of #src in bytes
 	 * @param dest_format the requested destination audio format
-	 * @param dest_size_r returns the number of bytes of the destination buffer
 	 * @param error_r location to store the error occurring, or nullptr to
 	 * ignore errors
 	 * @return the destination buffer, or nullptr on error
 	 */
-	const void *Convert(const void *src, size_t src_size,
-			    size_t *dest_size_r,
-			    Error &error);
+	ConstBuffer<void> Convert(ConstBuffer<void> src, Error &error);
 };
 
 bool
diff --git a/test/run_convert.cxx b/test/run_convert.cxx
index 47d9eae86..8b9b15cf0 100644
--- a/test/run_convert.cxx
+++ b/test/run_convert.cxx
@@ -28,6 +28,7 @@
 #include "AudioFormat.hxx"
 #include "pcm/PcmConvert.hxx"
 #include "config/ConfigGlobal.hxx"
+#include "util/ConstBuffer.hxx"
 #include "util/StaticFifoBuffer.hxx"
 #include "util/Error.hxx"
 #include "Log.hxx"
@@ -48,7 +49,6 @@ config_get_string(gcc_unused enum ConfigOption option,
 int main(int argc, char **argv)
 {
 	AudioFormat in_audio_format, out_audio_format;
-	const void *output;
 
 	if (argc != 3) {
 		fprintf(stderr,
@@ -104,16 +104,15 @@ int main(int argc, char **argv)
 
 		buffer.Consume(src.size);
 
-		size_t length;
-		output = state.Convert(src.data, src.size,
-				       &length, error);
-		if (output == NULL) {
+		auto output = state.Convert({src.data, src.size}, error);
+		if (output.IsNull()) {
 			state.Close();
 			LogError(error, "Failed to convert");
 			return EXIT_FAILURE;
 		}
 
-		gcc_unused ssize_t ignored = write(1, output, length);
+		gcc_unused ssize_t ignored = write(1, output.data,
+						   output.size);
 	}
 
 	state.Close();