Remove FastMag

Simulation shows that FastMag is of roughly equivalent sensitivity to
MagLUT while not providing much better performance.

decode/decode.go

@@ -113,7 +113,7 @@ type Decoder struct {
 }
 
 // Create a new decoder with the given packet configuration.
-func NewDecoder(cfg PacketConfig, decimation int, fastMag bool) (d Decoder) {
+func NewDecoder(cfg PacketConfig, decimation int) (d Decoder) {
 	d.Cfg = cfg
 
 	d.Cfg.SymbolLength2 = d.Cfg.SymbolLength << 1
@@ -139,11 +139,7 @@ func NewDecoder(cfg PacketConfig, decimation int, fastMag bool) (d Decoder) {
 	d.csum = make([]float64, (d.DecCfg.PacketLength - d.DecCfg.SymbolLength2 + 1))
 
 	// Calculate magnitude lookup table specified by -fastmag flag.
-	if fastMag {
-		d.demod = NewAlphaMaxBetaMinLUT()
-	} else {
-		d.demod = NewSqrtMagLUT()
-	}
+	d.demod = NewSqrtMagLUT()
 
 	// Pre-calculate a byte-slice version of the preamble for searching.
 	d.preamble = make([]byte, len(d.Cfg.Preamble))
@@ -234,40 +230,6 @@ func (lut MagLUT) Execute(input []byte, output []float64) {
 	}
 }
 
-// Alpha*Max + Beta*Min Magnitude Approximation Lookup Table.
-type AlphaMaxBetaMinLUT []float64
-
-// Pre-computes absolute values with most common DC offset for rtl-sdr dongles.
-func NewAlphaMaxBetaMinLUT() (lut AlphaMaxBetaMinLUT) {
-	lut = make([]float64, 0x100)
-	for idx := range lut {
-		lut[idx] = math.Abs(127.4 - float64(idx))
-	}
-	return
-}
-
-// Calculates complex magnitude on given IQ stream writing result to output.
-func (lut AlphaMaxBetaMinLUT) Execute(input []byte, output []float64) {
-	const (
-		α = 0.948059448969
-		ß = 0.392699081699
-	)
-
-	decIdx := 0
-	dec := (len(input) / len(output))
-
-	for idx := 0; decIdx < len(output); idx += dec {
-		i := lut[input[idx]]
-		q := lut[input[idx+1]]
-		if i > q {
-			output[decIdx] = α*i + ß*q
-		} else {
-			output[decIdx] = α*q + ß*i
-		}
-		decIdx++
-	}
-}
-
 // Matched filter for Manchester coded signals. Output signal's sign at each
 // sample determines the bit-value since Manchester symbols have odd symmetry.
 func (d Decoder) Filter(input, output []float64) {

flags.go

@@ -37,7 +37,6 @@ var sampleFilename = flag.String("samplefile", os.DevNull, "raw signal dump file
 var sampleFile *os.File
 
 var msgType = flag.String("msgtype", "scm", "message type to receive: scm, idm or r900")
-var fastMag = flag.Bool("fastmag", false, "use faster alpha max + beta min magnitude approximation")
 
 var symbolLength = flag.Int("symbollength", 72, "symbol length in samples, see -help for valid lengths")
 

idm/idm.go

@@ -51,8 +51,8 @@ func (p Parser) Cfg() decode.PacketConfig {
 	return p.Decoder.Cfg
 }
 
-func NewParser(symbolLength, decimation int, fastMag bool) (p Parser) {
-	p.Decoder = decode.NewDecoder(NewPacketConfig(symbolLength), decimation, fastMag)
+func NewParser(symbolLength, decimation int) (p Parser) {
+	p.Decoder = decode.NewDecoder(NewPacketConfig(symbolLength), decimation)
 	p.CRC = crc.NewCRC("CCITT", 0xFFFF, 0x1021, 0x1D0F)
 	return
 }

r900/r900.go

@@ -52,8 +52,8 @@ type Parser struct {
 	quantized []byte
 }
 
-func NewParser(symbolLength, decimation int, fastMag bool) (p Parser) {
-	p.Decoder = decode.NewDecoder(NewPacketConfig(symbolLength), decimation, fastMag)
+func NewParser(symbolLength, decimation int) (p Parser) {
+	p.Decoder = decode.NewDecoder(NewPacketConfig(symbolLength), decimation)
 
 	// GF of order 32, polynomial 37, generator 2.
 	p.field = gf.NewField(32, 37, 2)

recv.go

@@ -46,11 +46,11 @@ type Receiver struct {
 func (rcvr *Receiver) NewReceiver() {
 	switch strings.ToLower(*msgType) {
 	case "scm":
-		rcvr.p = scm.NewParser(*symbolLength, *decimation, *fastMag)
+		rcvr.p = scm.NewParser(*symbolLength, *decimation)
 	case "idm":
-		rcvr.p = idm.NewParser(*symbolLength, *decimation, *fastMag)
+		rcvr.p = idm.NewParser(*symbolLength, *decimation)
 	case "r900":
-		rcvr.p = r900.NewParser(*symbolLength, *decimation, *fastMag)
+		rcvr.p = r900.NewParser(*symbolLength, *decimation)
 	default:
 		log.Fatalf("Invalid message type: %q\n", *msgType)
 	}

scm/scm.go

@@ -42,8 +42,8 @@ type Parser struct {
 	crc.CRC
 }
 
-func NewParser(symbolLength, decimation int, fastMag bool) (p Parser) {
-	p.Decoder = decode.NewDecoder(NewPacketConfig(symbolLength), decimation, fastMag)
+func NewParser(symbolLength, decimation int) (p Parser) {
+	p.Decoder = decode.NewDecoder(NewPacketConfig(symbolLength), decimation)
 	p.CRC = crc.NewCRC("BCH", 0, 0x6F63, 0)
 	return
 }