ADPCM compression

[Petr]

16 bit version - trying ADPCM Microsoft standard...

You would certainly be interested in how 16-bit sound would sound if it were compressed by ADPCM and then decompressed. This program will show you exactly this. How it works. Indeed, the differences between signal samples are also written using 4 bits in this case. But how is that possible, you ask yourself. After all, with normal linear stepping it would be jumping in the signal by a huge 32767 /15 (2184) levels in the signal! That would ruin the sound terribly!
Well, for this purpose Microsoft has developed a table (StepTable) that has 88 steps. The program simply jumps in these 88 positions to the nearest similar step value, which is listed in the table. The maximum jump size in the table is currently limited to 15 bits, in this case one bit is no longer used to determine the sign. Values 0–7 indicate the negative direction (delta 0–7). Values 8–15 indicate the positive direction (delta 0–7). This approach causes the sound to be not completely clean, however the resulting file should be 4x smaller in size than a normal 16bite wav file with similar quality. It is possible (almost certain) that the algorithm can be improved somehow...

Rename MP3 - row 44

Code: (Select All)

'ADPCM - downsize 16bit file 4x



' Original signal array (16bite value, -32768 to 32767)

Dim originalSignalL(0 To 255) As Integer

Dim originalSignalR(0 To 255) As Integer



' ADPCM Microsoft Step Table

Dim stepTable(0 To 87) As Integer

Data 7,8,9,10,11,12,13,14,16,17,19,21,23,25,28,31

Data 34,37,41,45,50,55,60,66,73,80,88,97,107,118,130,143

Data 157,173,190,209,230,253,279,307,337,371,408,449,494,544

Data 598,658,724,796,876,963,1059,1164,1278,1403,1539,1687,1849

Data 2025,2217,2426,2653,2899,3166,3456,3769,4107,4471,4863,5285

Data 5737,6222,6741,7296,7889,8521,9195,9912,10674,11483,12341

Data 13250,14213,15231,16307,17444,18644,19909,21243,22648,24127

For i = 0 To 87

    Read stepTable(i)

Next i



' Starting values for left and right channel

Dim stepIndexL As Integer, stepIndexR As Integer

Dim currentStepL As Integer, currentStepR As Integer

Dim predictedValueL As Integer, predictedValueR As Integer



stepIndexL = 0

stepIndexR = 0

currentStepL = stepTable(stepIndexL)

currentStepR = stepTable(stepIndexR)

predictedValueL = 0

predictedValueR = 0



' Output compressed data (4 bite to sample)

Dim compressedDataL(0 To 255) As Integer

Dim compressedDataR(0 To 255) As Integer

ReDim FullCompressedL(0) As Integer

ReDim FullCompressedR(0) As Integer



' Arry for decompressed values

Dim decompressedSignalL(0 To 255) As Integer

Dim decompressedSignalR(0 To 255) As Integer



' Load sound from file

Dim m As _MEM, Snd As Long

Snd = _SndOpen("n.mp3")

m = _MemSound(Snd, 0)



Dim a As Long

Do Until a& >= m.SIZE

    ' Load 256 samples into buffer

    predictedValueL = 0

    predictedValueR = 0



    For i = 0 To 255

        If a& >= m.SIZE Then Exit Do

        originalSignalL(i) = _MemGet(m, m.OFFSET + a&, Single) * 32768

        originalSignalR(i) = _MemGet(m, m.OFFSET + a& + 4, Single) * 32768

        a& = a& + 8

    Next i



    ' ADPCM compression for lft and right channel

    For i = 1 To UBound(originalSignalL)

        ' left channel

        diffL = originalSignalL(i) - predictedValueL

        directionL = 0

        If diffL >= 0 Then directionL = 1 Else diffL = -diffL

        deltaL = diffL \ currentStepL

        If deltaL > 7 Then deltaL = 7

        compressedValueL = directionL * 8 + deltaL

        compressedDataL(i - 1) = compressedValueL

        stepChangeL = (deltaL + 0.5) * currentStepL

        If directionL = 0 Then stepChangeL = -stepChangeL

        predictedValueL = predictedValueL + stepChangeL

        If predictedValueL > 32767 Then predictedValueL = 32767

        If predictedValueL < -32768 Then predictedValueL = -32768

        decompressedSignalL(i) = predictedValueL

        stepIndexL = stepIndexL + deltaL - 4

        If stepIndexL < 0 Then stepIndexL = 0

        If stepIndexL > 87 Then stepIndexL = 87



        currentStepL = stepTable(stepIndexL)



        ' right channel

        diffR = originalSignalR(i) - predictedValueR

        directionR = 0

        If diffR >= 0 Then directionR = 1 Else diffR = -diffR

        deltaR = diffR \ currentStepR

        If deltaR > 7 Then deltaR = 7

        compressedValueR = directionR * 8 + deltaR

        compressedDataR(i - 1) = compressedValueR

        stepChangeR = (deltaR + 0.5) * currentStepR

        If directionR = 0 Then stepChangeR = -stepChangeR

        predictedValueR = predictedValueR + stepChangeR

        If predictedValueR > 32767 Then predictedValueR = 32767

        If predictedValueR < -32768 Then predictedValueR = -32768

        decompressedSignalR(i) = predictedValueR

        stepIndexR = stepIndexR + deltaR - 4

        If stepIndexR < 0 Then stepIndexR = 0

        If stepIndexR > 87 Then stepIndexR = 87

        currentStepR = stepTable(stepIndexR)



        Print "Current compressed sample value: "; compressedDataR(i); compressedDataL(i); "    "

        If compresseddataR > 15 Or compressedDataL > 15 Then Print "Compression failure!" 'never printed!

    Next i





    ReDim signalL(0 To 512) As Integer

    ReDim signalR(0 To 512) As Integer



    ' LowPassFilter decompressedSignalL(), signalL(), 10000, _SndRate

    ' LowPassFilter decompressedSignalR(), signalR(), 10000, _SndRate



    'Play decompressed signal

    For i = 0 To UBound(decompressedSignalL)

        _SndRaw decompressedSignalL(i) / 32768, decompressedSignalR(i) / 32768

        '_SndRaw signalL(i) / 32768, signalR(i) / 32768

    Next i

    Do Until _SndRawLen < .1

    Loop

Loop

_MemFree m

_SndClose Snd

End





Sub LowPassFilter (inputSignal() As Integer, outputSignal() As Integer, cutoffFreq As Single, sampleRate As Single)

    Dim kernel(0 To 31) As Single ' Filter coefficients (FIR Window, 32 samples)

    Dim sum As Single

    Dim normFactor As Single

    Dim N As Integer

    Dim halfN As Integer



    N = UBound(kernel)

    halfN = N \ 2



    ' Creating a low-pass filter (Hamming window)

    For i = -halfN To halfN

        If i = 0 Then

            kernel(i + halfN) = 2 * cutoffFreq / sampleRate

        Else

            kernel(i + halfN) = Sin(2 * 3.14159 * cutoffFreq * i / sampleRate) / (3.14159 * i)

        End If

        kernel(i + halfN) = kernel(i + halfN) * (0.54 - 0.46 * Cos(2 * 3.14159 * (i + halfN) / N)) ' Hamming window

    Next i



    ' Normalize kernel

    For i = 0 To N

        normFactor = normFactor + kernel(i)

    Next i

    For i = 0 To N

        kernel(i) = kernel(i) / normFactor

    Next i



    ' Apply filter to input signal

    For i = 0 To UBound(inputSignal)

        sum = 0

        For j = 0 To N

            If i - j >= 0 Then

                sum = sum + inputSignal(i - j) * kernel(j)

            End If

        Next j

        outputSignal(i) = Int(sum)

    Next i

End Sub