Convert PCM int16 to float32 iOS

December 29, 2024

What's inside this article ⌄

KMP IOS convert PCM int 16bit to PCM float 32bit
PCM int16 to float32 IOS
Kotlin Multiplatform PCM conversion
IOS audio format conversion KMP
Convert PCM audio format IOS

This code is for converting audio in ByteArray in PCM int 16bit mono format to audio in ABAudioPCMBuffer in PCM float 32bit stereo format:

 1fun convertMonoPCMInt16ToStereoPCMFloat32(
 2        inputData: ByteArray, inputSampleRate: Double, outputSampleRate: Double
 3    ): AVAudioPCMBuffer? {
 4        val inputFrameLength = inputData.size / 2  // 2 bytes per Int16 sample
 5        val resampleRatio = outputSampleRate / inputSampleRate
 6        val outputFrameLength = (inputFrameLength * resampleRatio).toInt()
 7
 8        val audioFormat = AVAudioFormat(standardFormatWithSampleRate = outputSampleRate, channels = 2u)
 9        val outputBuffer = AVAudioPCMBuffer(audioFormat, frameCapacity = outputFrameLength.toUInt()) ?: return null
10
11        val leftChannel = outputBuffer.floatChannelData!![0]
12        val rightChannel = outputBuffer.floatChannelData!![1]
13
14        for (outputFrame in 0 until outputFrameLength) {
15            val inputFrameFloat = outputFrame / resampleRatio
16            val inputFrame = inputFrameFloat.toInt()
17            val fraction = inputFrameFloat - inputFrame
18
19            val sample1 = readInt16Sample(inputData, inputFrame, inputFrameLength)
20            val sample2 = readInt16Sample(inputData, inputFrame + 1, inputFrameLength)
21            val interpolatedSample = (1 - fraction) * sample1 + fraction * sample2
22
23            // Convert to Float and apply gain control
24            val floatSample = (interpolatedSample / Short.MAX_VALUE.toFloat()) * 0.7f  // Apply -3dB attenuation
25
26            // Duplicate to stereo
27            leftChannel!![outputFrame] = floatSample.toFloat()
28            rightChannel!![outputFrame] = floatSample.toFloat()
29        }
30
31        outputBuffer.frameLength = outputFrameLength.toUInt()
32        return outputBuffer
33    }
34
35    private fun readInt16Sample(inputData: ByteArray, index: Int, length: Int): Int {
36        if (index >= length) return 0
37        val low = inputData[index * 2].toInt() and 0xFF
38        val high = inputData[index * 2 + 1].toInt()
39        return (high shl 8) or low
40    }
41}