Merge branch 'master' into comment-config

codec/adpcm/adpcm.go

@@ -2,9 +2,6 @@
 NAME
   adpcm.go
 
-DESCRIPTION
-  adpcm.go contains functions for encoding/compressing pcm into adpcm and decoding/decompressing back to pcm.
-
 AUTHOR
   Trek Hopton <trek@ausocean.org>
 
@@ -30,40 +27,25 @@ LICENSE
 	Reference algorithms for ADPCM compression and decompression are in part 6.
 */
 
+// Package adpcm provides functions to transcode between PCM and ADPCM.
 package adpcm
 
 import (
-	"bytes"
 	"encoding/binary"
 	"fmt"
+	"io"
+	"math"
 )
 
-// encoder is used to encode to ADPCM from PCM data.
+const (
-// pred and index hold state that persists between calls to encodeSample and calcHead.
+	byteDepth     = 2 // We are working with 16-bit samples. TODO(Trek): make configurable.
-// dest is the output buffer that implements io.writer and io.bytewriter, ie. where the encoded ADPCM data is written to.
+	initSamps     = 2 // Number of samples used to initialise the encoder.
-type encoder struct {
+	initBytes     = initSamps * byteDepth
-	dest  *bytes.Buffer
+	headBytes     = 4 // Number of bytes in the header of ADPCM.
-	pred  int16
+	samplesPerEnc = 2 // Number of sample encoded at a time eg. 2 16-bit samples get encoded into 1 byte.
-	index int16
+	bytesPerEnc   = samplesPerEnc * byteDepth
-}
+	compFact      = 4 // In general ADPCM compresses by a factor of 4.
-
+)
-// decoder is used to decode from ADPCM to PCM data.
-// pred, index, and step hold state that persists between calls to decodeSample.
-// dest is the output buffer that implements io.writer and io.bytewriter, ie. where the decoded PCM data is written to.
-type decoder struct {
-	dest  *bytes.Buffer
-	pred  int16
-	index int16
-	step  int16
-}
-
-// PcmBS is the size of the blocks that an encoder uses.
-// 'encodeBlock' will encode PcmBS bytes at a time and the output will be AdpcmBS bytes long.
-const PcmBS = 1010
-
-// AdpcmBS is the size of the blocks that a decoder uses.
-// 'decodeBlock' will decode AdpcmBS bytes at a time and the output will be PcmBS bytes long.
-const AdpcmBS = 256
 
 // Table of index changes (see spec).
 var indexTable = []int16{
@@ -87,28 +69,35 @@ var stepTable = []int16{
 	32767,
 }
 
-// NewEncoder retuns a new ADPCM encoder.
+// Encoder is used to encode to ADPCM from PCM data.
-func NewEncoder(dst *bytes.Buffer) *encoder {
+type Encoder struct {
-	e := encoder{
+	// dst is the destination for ADPCM-encoded data.
-		dest: dst,
+	dst io.Writer
-	}
+
-	return &e
+	est int16 // Estimation of sample based on quantised ADPCM nibble.
+	idx int16 // Index to step used for estimation.
 }
 
-// NewDecoder retuns a new ADPCM decoder.
+// Decoder is used to decode from ADPCM to PCM data.
-func NewDecoder(dst *bytes.Buffer) *decoder {
+type Decoder struct {
-	d := decoder{
+	// dst is the destination for PCM-encoded data.
-		step: stepTable[0],
+	dst io.Writer
-		dest: dst,
+
-	}
+	est  int16 // Estimation of sample based on quantised ADPCM nibble.
-	return &d
+	idx  int16 // Index to step used for estimation.
+	step int16
+}
+
+// NewEncoder retuns a new ADPCM Encoder.
+func NewEncoder(dst io.Writer) *Encoder {
+	return &Encoder{dst: dst}
 }
 
 // encodeSample takes a single 16 bit PCM sample and
 // returns a byte of which the last 4 bits are an encoded ADPCM nibble.
-func (e *encoder) encodeSample(sample int16) byte {
+func (e *Encoder) encodeSample(sample int16) byte {
-	// Find difference of actual sample from encoder's prediction.
+	// Find difference between the sample and the previous estimation.
-	delta := sample - e.pred
+	delta := capAdd16(sample, -e.est)
 
 	// Create and set sign bit for nibble and find absolute value of difference.
 	var nib byte
@@ -117,217 +106,250 @@ func (e *encoder) encodeSample(sample int16) byte {
 		delta = -delta
 	}
 
-	step := stepTable[e.index]
+	step := stepTable[e.idx]
 	diff := step >> 3
 	var mask byte = 4
 
 	for i := 0; i < 3; i++ {
 		if delta > step {
 			nib |= mask
-			delta -= step
+			delta = capAdd16(delta, -step)
-			diff += step
+			diff = capAdd16(diff, step)
 		}
 		mask >>= 1
 		step >>= 1
 	}
 
-	// Adjust predicted sample based on calculated difference.
 	if nib&8 != 0 {
-		e.pred -= diff
+		diff = -diff
-	} else {
-		e.pred += diff
 	}
 
-	e.index += indexTable[nib&7]
+	// Adjust estimated sample based on calculated difference.
+	e.est = capAdd16(e.est, diff)
+
+	e.idx += indexTable[nib&7]
 
 	// Check for underflow and overflow.
-	if e.index < 0 {
+	if e.idx < 0 {
-		e.index = 0
+		e.idx = 0
-	} else if e.index > int16(len(stepTable)-1) {
+	} else if e.idx > int16(len(stepTable)-1) {
-		e.index = int16(len(stepTable) - 1)
+		e.idx = int16(len(stepTable) - 1)
 	}
 
 	return nib
 }
 
+// calcHead sets the state for the Encoder by running the first sample through
+// the Encoder, and writing the first sample to the Encoder's io.Writer (dst).
+// It returns the number of bytes written to the Encoder's destination and the first error encountered.
+func (e *Encoder) calcHead(sample []byte, pad bool) (int, error) {
+	// Check that we are given 1 sample.
+	if len(sample) != byteDepth {
+		return 0, fmt.Errorf("length of given byte array is: %v, expected: %v", len(sample), byteDepth)
+	}
+
+	n, err := e.dst.Write(sample)
+	if err != nil {
+		return n, err
+	}
+
+	_n, err := e.dst.Write([]byte{byte(int16(e.idx))})
+	if err != nil {
+		return n, err
+	}
+	n += _n
+
+	if pad {
+		_n, err = e.dst.Write([]byte{0x01})
+	} else {
+		_n, err = e.dst.Write([]byte{0x00})
+	}
+	n += _n
+	if err != nil {
+		return n, err
+	}
+	return n, nil
+}
+
+// init initializes the Encoder's estimation to the first uncompressed sample and the index to
+// point to a suitable quantizer step size.
+// The suitable step size is the closest step size in the stepTable to half the absolute difference of the first two samples.
+func (e *Encoder) init(samples []byte) {
+	int1 := int16(binary.LittleEndian.Uint16(samples[:byteDepth]))
+	int2 := int16(binary.LittleEndian.Uint16(samples[byteDepth:initBytes]))
+	e.est = int1
+
+	halfDiff := math.Abs(math.Abs(float64(int1)) - math.Abs(float64(int2))/2)
+	closest := math.Abs(float64(stepTable[0]) - halfDiff)
+	var cInd int16
+	for i, step := range stepTable {
+		if math.Abs(float64(step)-halfDiff) < closest {
+			closest = math.Abs(float64(step) - halfDiff)
+			cInd = int16(i)
+		}
+	}
+	e.idx = cInd
+}
+
+// Write takes a slice of bytes of arbitrary length representing pcm and encodes it into adpcm.
+// It writes its output to the Encoder's dst.
+// The number of bytes written out is returned along with any error that occured.
+func (e *Encoder) Write(b []byte) (int, error) {
+	// Check that pcm has enough data to initialize Decoder.
+	pcmLen := len(b)
+	if pcmLen < initBytes {
+		return 0, fmt.Errorf("length of given byte array must be >= %v", initBytes)
+	}
+
+	// Determine if there will be a byte that won't contain two full nibbles and will need padding.
+	pad := false
+	if (pcmLen-byteDepth)%bytesPerEnc != 0 {
+		pad = true
+	}
+
+	e.init(b[:initBytes])
+	n, err := e.calcHead(b[:byteDepth], pad)
+	if err != nil {
+		return n, err
+	}
+	// Skip the first sample and start at the end of the first two samples, then every two samples encode them into a byte of adpcm.
+	for i := byteDepth; i+bytesPerEnc-1 < pcmLen; i += bytesPerEnc {
+		nib1 := e.encodeSample(int16(binary.LittleEndian.Uint16(b[i : i+byteDepth])))
+		nib2 := e.encodeSample(int16(binary.LittleEndian.Uint16(b[i+byteDepth : i+bytesPerEnc])))
+		_n, err := e.dst.Write([]byte{byte((nib2 << 4) | nib1)})
+		n += _n
+		if err != nil {
+			return n, err
+		}
+	}
+	// If we've reached the end of the pcm data and there's a sample left over,
+	// compress it to a nibble and leave the first half of the byte padded with 0s.
+	if pad {
+		nib := e.encodeSample(int16(binary.LittleEndian.Uint16(b[pcmLen-byteDepth : pcmLen])))
+		_n, err := e.dst.Write([]byte{nib})
+		n += _n
+		if err != nil {
+			return n, err
+		}
+	}
+	return n, nil
+}
+
+// NewDecoder retuns a new ADPCM Decoder.
+func NewDecoder(dst io.Writer) *Decoder {
+	return &Decoder{dst: dst}
+}
+
 // decodeSample takes a byte, the last 4 bits of which contain a single
 // 4 bit ADPCM nibble, and returns a 16 bit decoded PCM sample.
-func (d *decoder) decodeSample(nibble byte) int16 {
+func (d *Decoder) decodeSample(nibble byte) int16 {
 	// Calculate difference.
 	var diff int16
 	if nibble&4 != 0 {
-		diff += d.step
+		diff = capAdd16(diff, d.step)
 	}
 	if nibble&2 != 0 {
-		diff += d.step >> 1
+		diff = capAdd16(diff, d.step>>1)
 	}
 	if nibble&1 != 0 {
-		diff += d.step >> 2
+		diff = capAdd16(diff, d.step>>2)
 	}
-	diff += d.step >> 3
+	diff = capAdd16(diff, d.step>>3)
 
 	// Account for sign bit.
 	if nibble&8 != 0 {
 		diff = -diff
 	}
 
-	// Adjust predicted sample based on calculated difference.
+	// Adjust estimated sample based on calculated difference.
-	d.pred += diff
+	d.est = capAdd16(d.est, diff)
 
 	// Adjust index into step size lookup table using nibble.
-	d.index += indexTable[nibble]
+	d.idx += indexTable[nibble]
 
 	// Check for overflow and underflow.
-	if d.index < 0 {
+	if d.idx < 0 {
-		d.index = 0
+		d.idx = 0
-	} else if d.index > int16(len(stepTable)-1) {
+	} else if d.idx > int16(len(stepTable)-1) {
-		d.index = int16(len(stepTable) - 1)
+		d.idx = int16(len(stepTable) - 1)
 	}
 
 	// Find new quantizer step size.
-	d.step = stepTable[d.index]
+	d.step = stepTable[d.idx]
 
-	return d.pred
+	return d.est
 }
 
-// calcHead sets the state for the encoder by running the first sample through
+// Write takes a slice of bytes of arbitrary length representing adpcm and decodes it into pcm.
-// the encoder, and writing the first sample to the encoder's io.Writer (dest).
+// It writes its output to the Decoder's dst.
-// It returns the number of bytes written to the encoder's io.Writer (dest) along with any errors.
+// The number of bytes written out is returned along with any error that occured.
-func (e *encoder) calcHead(sample []byte) (int, error) {
+func (d *Decoder) Write(b []byte) (int, error) {
-	// Check that we are given 1 16-bit sample (2 bytes).
+	// Initialize Decoder with first 4 bytes of b.
-	const sampSize = 2
+	d.est = int16(binary.LittleEndian.Uint16(b[:byteDepth]))
-	if len(sample) != sampSize {
+	d.idx = int16(b[byteDepth])
-		return 0, fmt.Errorf("length of given byte array is: %v, expected: %v", len(sample), sampSize)
+	d.step = stepTable[d.idx]
-	}
+	n, err := d.dst.Write(b[:byteDepth])
-
-	intSample := int16(binary.LittleEndian.Uint16(sample))
-	e.encodeSample(intSample)
-
-	n, err := e.dest.Write(sample)
-	if err != nil {
-		return n, err
-	}
-
-	err = e.dest.WriteByte(byte(uint16(e.index)))
-	if err != nil {
-		return n, err
-	}
-	n++
-
-	err = e.dest.WriteByte(byte(0x00))
-	if err != nil {
-		return n, err
-	}
-	n++
-	return n, nil
-}
-
-// encodeBlock takes a slice of 1010 bytes (505 16-bit PCM samples).
-// It writes encoded (compressed) bytes (each byte containing two ADPCM nibbles) to the encoder's io.Writer (dest).
-// The number of bytes written is returned along with any errors.
-// Note: nibbles are output in little endian order, eg. n1n0 n3n2 n5n4...
-// Note: first 4 bytes are for initializing the decoder before decoding a block.
-// 		- First two bytes contain the first 16-bit sample uncompressed.
-// 		- Third byte is the decoder's starting index for the block, the fourth is padding and ignored.
-func (e *encoder) encodeBlock(block []byte) (int, error) {
-	if len(block) != PcmBS {
-		return 0, fmt.Errorf("unsupported block size. Given: %v, expected: %v, ie. 505 16-bit PCM samples", len(block), PcmBS)
-	}
-
-	n, err := e.calcHead(block[0:2])
-	if err != nil {
-		return n, err
-	}
-
-	for i := 3; i < PcmBS; i += 4 {
-		nib1 := e.encodeSample(int16(binary.LittleEndian.Uint16(block[i-1 : i+1])))
-		nib2 := e.encodeSample(int16(binary.LittleEndian.Uint16(block[i+1 : i+3])))
-		err = e.dest.WriteByte(byte((nib2 << 4) | nib1))
-		if err != nil {
-			return n, err
-		}
-		n++
-	}
-
-	return n, nil
-}
-
-// decodeBlock takes a slice of 256 bytes, each byte after the first 4 should contain two ADPCM encoded nibbles.
-// It writes the resulting decoded (decompressed) 16-bit PCM samples to the decoder's io.Writer (dest).
-// The number of bytes written is returned along with any errors.
-func (d *decoder) decodeBlock(block []byte) (int, error) {
-	if len(block) != AdpcmBS {
-		return 0, fmt.Errorf("unsupported block size. Given: %v, expected: %v", len(block), AdpcmBS)
-	}
-
-	// Initialize decoder with first 4 bytes of the block.
-	d.pred = int16(binary.LittleEndian.Uint16(block[0:2]))
-	d.index = int16(block[2])
-	d.step = stepTable[d.index]
-	n, err := d.dest.Write(block[0:2])
 	if err != nil {
 		return n, err
 	}
 
 	// For each byte, seperate it into two nibbles (each nibble is a compressed sample),
 	// then decode each nibble and output the resulting 16-bit samples.
-	for i := 4; i < AdpcmBS; i++ {
+	// If padding flag is true (Adpcm[3]), only decode up until the last byte, then decode that separately.
-		twoNibs := block[i]
+	for i := headBytes; i < len(b)-int(b[3]); i++ {
+		twoNibs := b[i]
 		nib2 := byte(twoNibs >> 4)
 		nib1 := byte((nib2 << 4) ^ twoNibs)
 
-		firstBytes := make([]byte, 2)
+		firstBytes := make([]byte, byteDepth)
 		binary.LittleEndian.PutUint16(firstBytes, uint16(d.decodeSample(nib1)))
-		_n, err := d.dest.Write(firstBytes)
+		_n, err := d.dst.Write(firstBytes)
 		n += _n
 		if err != nil {
 			return n, err
 		}
 
-		secondBytes := make([]byte, 2)
+		secondBytes := make([]byte, byteDepth)
 		binary.LittleEndian.PutUint16(secondBytes, uint16(d.decodeSample(nib2)))
-		_n, err = d.dest.Write(secondBytes)
+		_n, err = d.dst.Write(secondBytes)
 		n += _n
 		if err != nil {
 			return n, err
 		}
 	}
-
+	if b[3] == 0x01 {
-	return n, nil
+		padNib := b[len(b)-1]
-}
+		samp := make([]byte, byteDepth)
-
+		binary.LittleEndian.PutUint16(samp, uint16(d.decodeSample(padNib)))
-// Write takes a slice of bytes of arbitrary length representing pcm and encodes in into adpcm.
+		_n, err := d.dst.Write(samp)
-// It writes its output to the encoder's dest.
-// The number of bytes written out is returned along with any error that occured.
-func (e *encoder) Write(inPcm []byte) (int, error) {
-	numBlocks := len(inPcm) / PcmBS
-	n := 0
-	for i := 0; i < numBlocks; i++ {
-		block := inPcm[PcmBS*i : PcmBS*(i+1)]
-		_n, err := e.encodeBlock(block)
 		n += _n
 		if err != nil {
 			return n, err
 		}
 	}
-
 	return n, nil
 }
 
-// Write takes a slice of bytes of arbitrary length representing adpcm and decodes in into pcm.
+// capAdd16 adds two int16s together and caps at max/min int16 instead of overflowing
-// It writes its output to the decoder's dest.
+func capAdd16(a, b int16) int16 {
-// The number of bytes written out is returned along with any error that occured.
+	c := int32(a) + int32(b)
-func (d *decoder) Write(inAdpcm []byte) (int, error) {
+	switch {
-	numBlocks := len(inAdpcm) / AdpcmBS
+	case c < math.MinInt16:
-	n := 0
+		return math.MinInt16
-	for i := 0; i < numBlocks; i++ {
+	case c > math.MaxInt16:
-		block := inAdpcm[AdpcmBS*i : AdpcmBS*(i+1)]
+		return math.MaxInt16
-		_n, err := d.decodeBlock(block)
+	default:
-		n += _n
+		return int16(c)
-		if err != nil {
-			return n, err
-		}
 	}
-
+}
-	return n, nil
+
+// EncBytes will return the number of adpcm bytes that will be generated when encoding the given amount of pcm bytes (n).
+func EncBytes(n int) int {
+	// For 'n' pcm bytes, 1 sample is left uncompressed, the rest is compressed by a factor of 4
+	// and a start index and padding-flag byte are added.
+	// Also if there are an even number of samples, there will be half a byte of padding added to the last byte.
+	if n%bytesPerEnc == 0 {
+		return (n-byteDepth)/compFact + headBytes + 1
+	}
+	return (n-byteDepth)/compFact + headBytes
 }

codec/adpcm/adpcm_test.go

@@ -37,14 +37,13 @@ import (
 // then compare the result with expected ADPCM.
 func TestEncodeBlock(t *testing.T) {
 	// Read input pcm.
-	pcm, err := ioutil.ReadFile("../../../test/test-data/av/input/raw-voice.pcm")
+	pcm, err := ioutil.ReadFile("../../../test/test-data/av/input/original_8kHz_adpcm_test.pcm")
 	if err != nil {
 		t.Errorf("Unable to read input PCM file: %v", err)
 	}
 
 	// Encode adpcm.
-	numBlocks := len(pcm) / PcmBS
+	comp := bytes.NewBuffer(make([]byte, 0, EncBytes(len(pcm))))
-	comp := bytes.NewBuffer(make([]byte, 0, AdpcmBS*numBlocks))
 	enc := NewEncoder(comp)
 	_, err = enc.Write(pcm)
 	if err != nil {
@@ -52,7 +51,7 @@ func TestEncodeBlock(t *testing.T) {
 	}
 
 	// Read expected adpcm file.
-	exp, err := ioutil.ReadFile("../../../test/test-data/av/output/encoded-voice.adpcm")
+	exp, err := ioutil.ReadFile("../../../test/test-data/av/output/encoded_8kHz_adpcm_test.adpcm")
 	if err != nil {
 		t.Errorf("Unable to read expected ADPCM file: %v", err)
 	}
@@ -66,14 +65,13 @@ func TestEncodeBlock(t *testing.T) {
 // resulting PCM with the expected decoded PCM.
 func TestDecodeBlock(t *testing.T) {
 	// Read adpcm.
-	comp, err := ioutil.ReadFile("../../../test/test-data/av/input/encoded-voice.adpcm")
+	comp, err := ioutil.ReadFile("../../../test/test-data/av/input/encoded_8kHz_adpcm_test.adpcm")
 	if err != nil {
 		t.Errorf("Unable to read input ADPCM file: %v", err)
 	}
 
 	// Decode adpcm.
-	numBlocks := len(comp) / AdpcmBS
+	decoded := bytes.NewBuffer(make([]byte, 0, len(comp)*4))
-	decoded := bytes.NewBuffer(make([]byte, 0, PcmBS*numBlocks))
 	dec := NewDecoder(decoded)
 	_, err = dec.Write(comp)
 	if err != nil {
@@ -81,7 +79,7 @@ func TestDecodeBlock(t *testing.T) {
 	}
 
 	// Read expected pcm file.
-	exp, err := ioutil.ReadFile("../../../test/test-data/av/output/decoded-voice.pcm")
+	exp, err := ioutil.ReadFile("../../../test/test-data/av/output/decoded_8kHz_adpcm_test.pcm")
 	if err != nil {
 		t.Errorf("Unable to read expected PCM file: %v", err)
 	}

codec/pcm/pcm.go

@@ -24,6 +24,8 @@ LICENSE
   You should have received a copy of the GNU General Public License in gpl.txt.
   If not, see [GNU licenses](http://www.gnu.org/licenses).
 */
+
+// Package pcm provides functions for processing and converting pcm audio.
 package pcm
 
 import (

exp/adpcm/decode-pcm/decode-pcm.go

@@ -2,9 +2,6 @@
 NAME
   decode-pcm.go
 
-DESCRIPTION
-  decode-pcm.go is a program for decoding/decompressing an adpcm file to a pcm file.
-
 AUTHOR
   Trek Hopton <trek@ausocean.org>
 
@@ -25,6 +22,7 @@ LICENSE
   If not, see [GNU licenses](http://www.gnu.org/licenses).
 */
 
+// decode-pcm is a command-line program for decoding/decompressing an adpcm file to a pcm file.
 package main
 
 import (
@@ -54,8 +52,7 @@ func main() {
 	fmt.Println("Read", len(comp), "bytes from file", inPath)
 
 	// Decode adpcm.
-	numBlocks := len(comp) / adpcm.AdpcmBS
+	decoded := bytes.NewBuffer(make([]byte, 0, len(comp)*4))
-	decoded := bytes.NewBuffer(make([]byte, 0, adpcm.PcmBS*numBlocks))
 	dec := adpcm.NewDecoder(decoded)
 	_, err = dec.Write(comp)
 	if err != nil {

exp/adpcm/encode-pcm/encode-pcm.go

@@ -2,9 +2,6 @@
 NAME
   encode-pcm.go
 
-DESCRIPTION
-  encode-pcm.go is a program for encoding/compressing a pcm file to an adpcm file.
-
 AUTHOR
   Trek Hopton <trek@ausocean.org>
 
@@ -25,6 +22,7 @@ LICENSE
   If not, see [GNU licenses](http://www.gnu.org/licenses).
 */
 
+// encode-pcm is a command-line program for encoding/compressing a pcm file to an adpcm file.
 package main
 
 import (
@@ -54,8 +52,7 @@ func main() {
 	fmt.Println("Read", len(pcm), "bytes from file", inPath)
 
 	// Encode adpcm.
-	numBlocks := len(pcm) / adpcm.PcmBS
+	comp := bytes.NewBuffer(make([]byte, 0, adpcm.EncBytes(len(pcm))))
-	comp := bytes.NewBuffer(make([]byte, 0, adpcm.AdpcmBS*numBlocks))
 	enc := adpcm.NewEncoder(comp)
 	_, err = enc.Write(pcm)
 	if err != nil {

exp/pcm/resample/resample.go

@@ -2,9 +2,6 @@
 NAME
   resample.go
 
-DESCRIPTION
-  resample.go is a program for resampling a pcm file.
-
 AUTHOR
   Trek Hopton <trek@ausocean.org>
 
@@ -24,6 +21,8 @@ LICENSE
   You should have received a copy of the GNU General Public License in gpl.txt.
   If not, see [GNU licenses](http://www.gnu.org/licenses).
 */
+
+// resample is a command-line program for resampling a pcm file.
 package main
 
 import (

exp/pcm/stereo-to-mono/stereo-to-mono.go

@@ -2,9 +2,6 @@
 NAME
   stereo-to-mono.go
 
-DESCRIPTION
-  stereo-to-mono.go is a program for converting a mono pcm file to a stereo pcm file.
-
 AUTHOR
   Trek Hopton <trek@ausocean.org>
 
@@ -24,6 +21,8 @@ LICENSE
   You should have received a copy of the GNU General Public License in gpl.txt.
   If not, see [GNU licenses](http://www.gnu.org/licenses).
 */
+
+// stereo-to-mono is a command-line program for converting a mono pcm file to a stereo pcm file.
 package main
 
 import (