private static short AdpcmMsExpandNibble(ref adpcmMsChannel channel, byte nibble)
{
// Get a signed number out of the nibble. We need to retain the
// original nibble value for when we access AdaptionTable[].
sbyte signedNibble = (sbyte)nibble;
if ((signedNibble & 0x8) == 0x8)
{
signedNibble -= 0x10;
}
// Calculate new sample
int predictor = (channel.Sample1 * channel.Coeff1 +
channel.Sample2 * channel.Coeff2) / 256 +
signedNibble * channel.Delta;
// Clamp result to 16-bit, -32768 - 32767
predictor = Clamp(predictor, short.MinValue, short.MaxValue);
// Shuffle samples, get new delta
channel.Sample2 = channel.Sample1;
channel.Sample1 = (short)predictor;
channel.Delta = (short)((MSAdaptationTable[nibble] * channel.Delta) / 256);
// Saturate the delta to a lower bound of 16
if (channel.Delta < 16)
{
channel.Delta = 16;
}
return((short)predictor);
}
private static void DecodeAdpcmMs(Decoder decoder, BinaryReader reader, BinaryWriter writer)
{
// https://wiki.multimedia.cx/index.php/Microsoft_ADPCM
// see also https://github.com/DeltaEngine/DeltaEngine/blob/master/Multimedia/OpenAL/Helpers/MsAdpcmConverter.cs
DecoderState state = decoder.State;
adpcmMsChannel[] channel = new adpcmMsChannel[2];
byte blockPredictor = 0;
// determine total number of samples in this block
// the initial 2 samples from the block preamble are sent directly to the output.
// therefore, deduct 2 from the samples per block to calculate the remaining samples
int totalSamples = (state.SamplesPerBlock - 2) * decoder.AudioFormat.Channels;
if (totalSamples < 2)
{
return;
}
bool isStereo = decoder.AudioFormat.Channels == 2 ? true : false;
// read predicates and deltas
blockPredictor = reader.ReadByte();
blockPredictor = (byte)Clamp(blockPredictor, 0, 6);
channel[0].Coeff1 = (short)MSAdaptationCoeff1[blockPredictor];
channel[0].Coeff2 = (short)MSAdaptationCoeff2[blockPredictor];
if (isStereo)
{
blockPredictor = reader.ReadByte();
blockPredictor = (byte)Clamp(blockPredictor, 0, 6);
channel[1].Coeff1 = (short)MSAdaptationCoeff1[blockPredictor];
channel[1].Coeff2 = (short)MSAdaptationCoeff2[blockPredictor];
}
channel[0].Delta = reader.ReadInt16();
if (isStereo)
{
channel[1].Delta = reader.ReadInt16();
}
// read first samples and write them to result
channel[0].Sample1 = reader.ReadInt16();
if (isStereo)
{
channel[1].Sample1 = reader.ReadInt16();
}
channel[0].Sample2 = reader.ReadInt16();
if (isStereo)
{
channel[1].Sample2 = reader.ReadInt16();
}
// output the samples
if (isStereo)
{
writer.Write(channel[0].Sample2);
writer.Write(channel[1].Sample2);
writer.Write(channel[0].Sample1);
writer.Write(channel[1].Sample1);
}
else
{
writer.Write(channel[0].Sample2);
writer.Write(channel[0].Sample1);
}
// decode the rest of the samples
for (int index = 0; index < totalSamples; index += 2)
{
try
{
byte nibble = reader.ReadByte();
writer.Write(AdpcmMsExpandNibble(ref channel[0], (byte)(nibble >> 4)));
writer.Write(AdpcmMsExpandNibble(ref channel[isStereo ? 1 : 0], (byte)(nibble & 0x0f)));
}
catch (System.IO.EndOfStreamException)
{
Log.Verbose("DecodeAdpcmMs: Reached end of stream - returning.");
break;
}
}
}
