Add global Downsampling feature (#1575)

* Add global downsampling feature

* Default to Opus &  consider player transcoder

* Add a test case for DefaultDownsamplingFormat

Co-authored-by: Deluan <deluan@navidrome.org>

core/media_streamer.go

@@ -142,6 +142,10 @@ func selectTranscodingOptions(ctx context.Context, ds model.DataStore, mf *model
 			if p, ok := request.PlayerFrom(ctx); ok {
 				cBitRate = p.MaxBitRate
 			}
+		} else if reqBitRate > 0 && conf.Server.DefaultDownsamplingFormat != "" {
+			// If no format is specified and no transcoding associated to the player, but a bitrate is specfied, and there is no transcoding set for the player, we use the default downsampling format
+			log.Debug("Default Downsampling", "Using default downsampling format", conf.Server.DefaultDownsamplingFormat)
+			cFormat = conf.Server.DefaultDownsamplingFormat
 		}
 	}
 	if reqBitRate > 0 {

core/media_streamer_test.go

@@ -8,6 +8,7 @@ import (
 	"sync"
 
 	"github.com/navidrome/navidrome/conf"
+	"github.com/navidrome/navidrome/conf/configtest"
 	"github.com/navidrome/navidrome/log"
 	"github.com/navidrome/navidrome/model"
 	"github.com/navidrome/navidrome/model/request"
@@ -24,6 +25,7 @@ var _ = Describe("MediaStreamer", func() {
 	ctx := log.NewContext(context.TODO())
 
 	BeforeEach(func() {
+		DeferCleanup(configtest.SetupConfig())
 		conf.Server.DataFolder, _ = os.MkdirTemp("", "file_caches")
 		conf.Server.TranscodingCacheSize = "100MB"
 		ds = &tests.MockDataStore{MockedTranscoding: &tests.MockTranscodingRepo{}}
@@ -115,6 +117,15 @@ var _ = Describe("MediaStreamer", func() {
 				Expect(format).To(Equal("raw"))
 				Expect(bitRate).To(Equal(320))
 			})
+			It("returns the DefaultDownsamplingFormat if a maxBitrate but not the format", func() {
+				conf.Server.DefaultDownsamplingFormat = "opus"
+				mf.Suffix = "FLAC"
+				mf.BitRate = 960
+				format, bitRate := selectTranscodingOptions(ctx, ds, mf, "", 128)
+				Expect(format).To(Equal("opus"))
+				Expect(bitRate).To(Equal(128))
+
+			})
 		})
 
 		Context("player has format configured", func() {