How to convert PCM audio to TrueSpeech using NAudio - naudio

Question

I am trying to convert a PCM 8 bit 8 KHz Mono file to DSP TrueSpeech 1 bit 8 kHz Mono using NAudio, and I get the following error:
A first chance exception of type 'NAudio.MmException' occurred in NAudio.dll
AcmNotPossible calling acmStreamOpen
I understand that there may be an intermediate step that I am missing -- any insight would be appreciated. Here is the code I am using:
WaveFormat outWaveFormat;
outWaveFormat = new TrueSpeechWaveFormat();
Debug.Print("Sample Rate: " + outWaveFormat.SampleRate); //displays "8000"
Debug.Print("Bit Rate: " + outWaveFormat.BitsPerSample); //displays "1"
FileInfo f = new FileInfo(inputFile);
String outputFileName = this.txtDest.Text + #"\" + f.Name;
using (WaveFileReader reader = new WaveFileReader(inputFile))
{
try
{
using (WaveStream convertedStream = new WaveFormatConversionStream (outWaveFormat, reader))
{
WaveFileWriter.CreateWaveFile(outputFileName, convertedStream);
}
}
catch (Exception ex)
{
Debug.Print(ex.Message);
}
}

Answer 1

Two reasons this might be happening:
you don't have a TrueSpeech encoder. I don't think newer versions of Windows include TrueSpeech anymore - it is effectively obsolete. You can run the NAudioDemo application to see what ACM codecs are on your machine.
your input format cannot convert to the target format in one step. Are you sure your input is PCM. Also I would expect that the TrueSpeech codec wants 16 bit input not 8 bit.
There is a third reason this can happen, although I don't think it affects TrueSpeech and that is that WaveFileWriter.CreateWaveFile assumes that AverageBytesPerSecond is an exact multiple of BlockAlign, which is not always true.