diff --git a/decode/decode.go b/decode/decode.go
index 28097037a..f8896279d 100644
--- a/decode/decode.go
+++ b/decode/decode.go
@@ -33,8 +33,6 @@ type PacketConfig struct {
 	PreambleLength, PacketLength   int
 	BufferLength                   int
 	Preamble                       string
-
-	FastMag bool
 }
 
 func (cfg PacketConfig) Log() {
@@ -67,7 +65,7 @@ type Decoder struct {
 }
 
 // Create a new decoder with the given packet configuration.
-func NewDecoder(cfg PacketConfig) (d Decoder) {
+func NewDecoder(cfg PacketConfig, fastMag bool) (d Decoder) {
 	d.Cfg = cfg
 
 	// Allocate necessary buffers.
@@ -78,7 +76,7 @@ func NewDecoder(cfg PacketConfig) (d Decoder) {
 	d.csum = make([]float64, d.Cfg.BlockSize+d.Cfg.SymbolLength2+1)
 
 	// Calculate magnitude lookup table specified by -fastmag flag.
-	if cfg.FastMag {
+	if fastMag {
 		d.lut = NewAlphaMaxBetaMinLUT()
 	} else {
 		d.lut = NewSqrtMagLUT()
diff --git a/recv.go b/recv.go
index bcd36daf0..e65509513 100644
--- a/recv.go
+++ b/recv.go
@@ -49,10 +49,10 @@ type Receiver struct {
 func (rcvr *Receiver) NewReceiver() {
 	switch strings.ToLower(*msgType) {
 	case "scm":
-		rcvr.d = decode.NewDecoder(NewSCMPacketConfig(*symbolLength))
+		rcvr.d = decode.NewDecoder(NewSCMPacketConfig(*symbolLength), *fastMag)
 		rcvr.p = NewSCMParser()
 	case "idm":
-		rcvr.d = decode.NewDecoder(NewIDMPacketConfig(*symbolLength))
+		rcvr.d = decode.NewDecoder(NewIDMPacketConfig(*symbolLength), *fastMag)
 		rcvr.p = NewIDMParser()
 	default:
 		log.Fatalf("Invalid message type: %q\n", *msgType)