go2rtc/pkg/pcm/pcma.go

// Package pcm
// https://www.codeproject.com/Articles/14237/Using-the-G711-standard
package pcm

const alawMax = 0x7FFF

func PCMAtoPCM(alaw byte) int16 {
	alaw ^= 0xD5

	data := int16(((alaw & 0x0F) << 4) + 8)
	exponent := (alaw & 0x70) >> 4

	if exponent != 0 {
		data |= 0x100
	}

	if exponent > 1 {
		data <<= exponent - 1
	}

	// sign
	if alaw&0x80 == 0 {
		return data
	} else {
		return -data
	}
}

func PCMtoPCMA(pcm int16) byte {
	var alaw byte

	if pcm < 0 {
		pcm = -pcm
		alaw = 0x80
	}

	if pcm > alawMax {
		pcm = alawMax
	}

	exponent := byte(7)
	for expMask := int16(0x4000); (pcm&expMask) == 0 && exponent > 0; expMask >>= 1 {
		exponent--
	}

	if exponent == 0 {
		alaw |= byte(pcm>>4) & 0x0F
	} else {
		alaw |= (exponent << 4) | (byte(pcm>>(exponent+3)) & 0x0F)
	}

	return alaw ^ 0xD5
}