Getting Audio Data From MP3 File using NAudio

Question

I want to be able to get audio data from an MP3 file with NAudio, average out the data in the left and right channels to create one dataset and then resample the averaged 44.1KHz audio data to 8Khz but I am having trouble understanding how data is represented in an NAudio Wavestream.

If I had 1 sec worth of MP3 audio, then how many bytes would I have in the WaveStream? By looking at a few code samples it seems one sample is 4 bytes and audio is sampled at 44100Hz and we have 2 different channels, so would that mean we would have (44100 * 4 * 2) bytes in the wavestream, is that right?

Which of the following 3 streams - AStream,PCM and inputStream - should I use to get audio data from? And how to I access left and right channel data separately?

var AStream = new MP3FileReader(myFilePath);
var PCM = new WaveConversionStream.Createpcm(AStream);
var inputStream = new WaveChannel32(new BlockAlignStream(PCM));

I have been thinking of converting the WaveStream using the WaveFormatConversionStream but the code below throws a NAudio.MmException with a message saying "AcmNotPossible calling Acmstreamopen".

 var targetFormat = new WaveFormat(8000,1);
 var resampled = new WaveFormatConversionStream(targetFormat, inputStream);

The above code doesn't even work if targetFormat is equal to inputStream's format, so I don't know what I am doing wrong here.

//Still throws NAudio.MmException
var resampled = new WaveFormatConversionStream(inputStream.WaveFormat, inputStream);

Other Info: VS2012, WPF, NAudio 1.6.

Mark Heath Mark Heath · Accepted Answer · 2013-11-25T20:55:02

You seem to have copied a code sample that belongs to a much earlier version of NAudio. The Mp3FileReader class will emit 16 bit samples, and uses the ACM MP3 frame decompressor by default. If you'd prefer your samples directly in floating point, then you can make use of the AudioFileReader.

Resampling 44.1kHz straight down to 8kHz is not a particularly good idea, as you'd end up with a lot of aliasing, so a low pass filter would ideally be applied first. Left and right channels are stored interleaved, so you get a left sample, followed by a right sample, and so on.

Getting Audio Data From MP3 File using NAudio

1 Answers