diff --git a/downloader.go b/downloader.go
index 7471a82..9cca3c4 100644
--- a/downloader.go
+++ b/downloader.go
@@ -43,9 +43,15 @@ type Downloader struct {
 
 	// A method to call when a segment was downloaded
 	OnSegmentDownload func(segment *m3u8.MediaSegment, current, total int, file *os.File) error
+	// If LockOnSegmentDownload is true, only one OnSegmentDownload function can be called at
+	// once. Normally (because of the use of goroutines while downloading) multiple could get
+	// called simultaneously
+	LockOnSegmentDownload bool
 
-	// If FFmpeg is true, ffmpeg will used to merge and convert files
-	FFmpeg bool
+	// If FFmpegOpts is not nil, ffmpeg will be used to merge and convert files.
+	// The given opts will be used as ffmpeg parameters while merging.
+	// Some opts are already used, see mergeSegmentsFFmpeg in format.go for more details
+	FFmpegOpts []string
 }
 
 // NewDownloader creates a downloader with default settings which should
@@ -72,7 +78,7 @@ func NewDownloader(context context.Context, filename string, goroutines int, onS
 // 		The actual crunchyroll video is split up in multiple segments (or video files) which have to be downloaded and merged after to generate a single video file.
 //		And this function just downloads each of this segment into the given directory.
 // 		See https://en.wikipedia.org/wiki/MPEG_transport_stream for more information
-func download(context context.Context, format *Format, tempDir string, goroutines int, onSegmentDownload func(segment *m3u8.MediaSegment, current, total int, file *os.File) error) error {
+func download(context context.Context, format *Format, tempDir string, goroutines int, lockOnSegmentDownload bool, onSegmentDownload func(segment *m3u8.MediaSegment, current, total int, file *os.File) error) error {
 	req, err := http.NewRequest(http.MethodGet, format.Video.URI, nil)
 	if err != nil {
 		return err
@@ -99,6 +105,7 @@ func download(context context.Context, format *Format, tempDir string, goroutine
 	}
 
 	var wg sync.WaitGroup
+	var lock sync.Mutex
 	chunkSize := int(math.Ceil(float64(len(segments)) / float64(goroutines)))
 
 	// when a onSegmentDownload call returns an error, this channel will be set to true and stop all goroutines
@@ -145,11 +152,21 @@ func download(context context.Context, format *Format, tempDir string, goroutine
 						return
 					}
 					if onSegmentDownload != nil {
+						if lockOnSegmentDownload {
+							lock.Lock()
+						}
+
 						if err = onSegmentDownload(segment, int(atomic.AddInt32(&total, 1)), len(segments), file); err != nil {
 							quit <- true
+							if lockOnSegmentDownload {
+								lock.Unlock()
+							}
 							file.Close()
 							return
 						}
+						if lockOnSegmentDownload {
+							lock.Unlock()
+						}
 					}
 					file.Close()
 				}
diff --git a/format.go b/format.go
index f3d548f..b1b21a4 100644
--- a/format.go
+++ b/format.go
@@ -56,12 +56,12 @@ func (f *Format) Download(downloader Downloader) error {
 	if downloader.DeleteTempAfter {
 		defer os.RemoveAll(downloader.TempDir)
 	}
-	if err := download(downloader.Context, f, downloader.TempDir, downloader.Goroutines, downloader.OnSegmentDownload); err != nil {
+	if err := download(downloader.Context, f, downloader.TempDir, downloader.Goroutines, downloader.LockOnSegmentDownload, downloader.OnSegmentDownload); err != nil {
 		return err
 	}
 
-	if downloader.FFmpeg {
-		return mergeSegmentsFFmpeg(downloader.Context, downloader.TempDir, downloader.Filename)
+	if downloader.FFmpegOpts != nil {
+		return mergeSegmentsFFmpeg(downloader.Context, downloader.TempDir, downloader.Filename, downloader.FFmpegOpts)
 	} else {
 		return mergeSegments(downloader.Context, downloader.TempDir, downloader.Filename)
 	}
@@ -116,7 +116,7 @@ func mergeSegments(context context.Context, tempDir string, outputFile string) e
 // mergeSegmentsFFmpeg reads every file in tempDir and merges their content to the outputFile
 // with ffmpeg (https://ffmpeg.org/).
 // The given output file gets created or overwritten if already existing
-func mergeSegmentsFFmpeg(context context.Context, tempDir string, outputFile string) error {
+func mergeSegmentsFFmpeg(context context.Context, tempDir string, outputFile string, opts []string) error {
 	dir, err := os.ReadDir(tempDir)
 	if err != nil {
 		return err
@@ -129,12 +129,21 @@ func mergeSegmentsFFmpeg(context context.Context, tempDir string, outputFile str
 	for i := 0; i < len(dir); i++ {
 		fmt.Fprintf(f, "file '%s.ts'\n", filepath.Join(tempDir, strconv.Itoa(i)))
 	}
-	cmd := exec.Command("ffmpeg",
+
+	// predefined options ... custom options ... predefined output filename
+	command := []string{
 		"-f", "concat",
 		"-safe", "0",
 		"-i", f.Name(),
 		"-c", "copy",
-		outputFile)
+	}
+	if opts != nil {
+		command = append(command, opts...)
+	}
+	command = append(command, outputFile)
+
+	cmd := exec.Command("ffmpeg",
+		command...)
 	if err := cmd.Start(); err != nil {
 		return err
 	}
@@ -148,6 +157,7 @@ func mergeSegmentsFFmpeg(context context.Context, tempDir string, outputFile str
 	case err := <-cmdChan:
 		return err
 	case <-context.Done():
+		cmd.Process.Kill()
 		return context.Err()
 	}
 }