mediamtx/internal/aac/adts.go

package aac

import (
	"fmt"
)

// ADTSPacket is an ADTS packet
type ADTSPacket struct {
	SampleRate   int
	ChannelCount int
	Frame        []byte
}

// DecodeADTS decodes an ADTS stream into ADTS packets.
func DecodeADTS(byts []byte) ([]*ADTSPacket, error) {
	// refs: https://wiki.multimedia.cx/index.php/ADTS

	var ret []*ADTSPacket

	for len(byts) > 0 {
		syncWord := (uint16(byts[0]) << 4) | (uint16(byts[1]) >> 4)
		if syncWord != 0xfff {
			return nil, fmt.Errorf("invalid syncword")
		}

		protectionAbsent := byts[1] & 0x01
		if protectionAbsent != 1 {
			return nil, fmt.Errorf("ADTS with CRC is not supported")
		}

		pkt := &ADTSPacket{}

		profile := (byts[2] >> 6)
		if profile != 0 {
			return nil, fmt.Errorf("only AAC-LC is supported")
		}

		sampleRateIndex := (byts[2] >> 2) & 0x0F
		switch sampleRateIndex {
		case 0:
			pkt.SampleRate = 96000
		case 1:
			pkt.SampleRate = 88200
		case 2:
			pkt.SampleRate = 64000
		case 3:
			pkt.SampleRate = 48000
		case 4:
			pkt.SampleRate = 44100
		case 5:
			pkt.SampleRate = 32000
		case 6:
			pkt.SampleRate = 24000
		case 7:
			pkt.SampleRate = 22050
		case 8:
			pkt.SampleRate = 16000
		case 9:
			pkt.SampleRate = 12000
		case 10:
			pkt.SampleRate = 11025
		case 11:
			pkt.SampleRate = 8000
		case 12:
			pkt.SampleRate = 7350
		default:
			return nil, fmt.Errorf("invalid sample rate index: %d", sampleRateIndex)
		}

		channelConfig := ((byts[2] & 0x01) << 2) | ((byts[3] >> 6) & 0x03)
		switch channelConfig {
		case 1:
			pkt.ChannelCount = 1
		case 2:
			pkt.ChannelCount = 2
		case 3:
			pkt.ChannelCount = 3
		case 4:
			pkt.ChannelCount = 4
		case 5:
			pkt.ChannelCount = 5
		case 6:
			pkt.ChannelCount = 6
		case 7:
			pkt.ChannelCount = 8
		default:
			return nil, fmt.Errorf("invalid channel configuration: %d", channelConfig)
		}

		frameLen := int(((uint16(byts[3])&0x03)<<11)|
			(uint16(byts[4])<<3)|
			((uint16(byts[5])>>5)&0x07)) - 7

		fullness := ((uint16(byts[5]) & 0x1F) << 6) | ((uint16(byts[6]) >> 2) & 0x3F)
		if fullness != 1800 {
			return nil, fmt.Errorf("fullness not supported")
		}

		frameCount := byts[6] & 0x03
		if frameCount != 0 {
			return nil, fmt.Errorf("multiple frame count not supported")
		}

		if len(byts[7:]) < frameLen {
			return nil, fmt.Errorf("invalid frame length")
		}

		pkt.Frame = byts[7 : 7+frameLen]
		byts = byts[7+frameLen:]

		ret = append(ret, pkt)
	}

	return ret, nil
}

// EncodeADTS encodes ADTS packets into an ADTS stream.
func EncodeADTS(pkts []*ADTSPacket) ([]byte, error) {
	var ret []byte

	for _, pkt := range pkts {
		frameLen := len(pkt.Frame) + 7
		fullness := 1800

		var channelConf uint8
		switch pkt.ChannelCount {
		case 1:
			channelConf = 1
		case 2:
			channelConf = 2
		case 3:
			channelConf = 3
		case 4:
			channelConf = 4
		case 5:
			channelConf = 5
		case 6:
			channelConf = 6
		case 8:
			channelConf = 7
		default:
			return nil, fmt.Errorf("invalid channel count: %v", pkt.ChannelCount)
		}

		var sampleRateIndex uint8
		switch pkt.SampleRate {
		case 96000:
			sampleRateIndex = 0
		case 88200:
			sampleRateIndex = 1
		case 64000:
			sampleRateIndex = 2
		case 48000:
			sampleRateIndex = 3
		case 44100:
			sampleRateIndex = 4
		case 32000:
			sampleRateIndex = 5
		case 24000:
			sampleRateIndex = 6
		case 22050:
			sampleRateIndex = 7
		case 16000:
			sampleRateIndex = 8
		case 12000:
			sampleRateIndex = 9
		case 11025:
			sampleRateIndex = 10
		case 8000:
			sampleRateIndex = 11
		case 7350:
			sampleRateIndex = 12
		default:
			return nil, fmt.Errorf("invalid sample rate: %v", pkt.SampleRate)
		}

		header := make([]byte, 7)
		header[0] = 0xFF
		header[1] = 0xF1
		header[2] = (sampleRateIndex << 2) | ((channelConf >> 2) & 0x01)
		header[3] = (channelConf&0x03)<<6 | uint8((frameLen>>11)&0x03)
		header[4] = uint8((frameLen >> 3) & 0xFF)
		header[5] = uint8((frameLen&0x07)<<5 | ((fullness >> 6) & 0x1F))
		header[6] = uint8((fullness & 0x3F) << 2)
		ret = append(ret, header...)

		ret = append(ret, pkt.Frame...)
	}

	return ret, nil
}