Search code examples
vb.netaudiotext-to-speechwavspeech-synthesis

Creating a WAV file with an arbitrary bits per sample value?

Do WAV files allow any arbitrary number of bitsPerSample?

I have failed to get it to work with anything less than 8. I am not sure how to define the blockAlign for one thing.

Dim ss As New Speech.Synthesis.SpeechSynthesizer
Dim info As New Speech.AudioFormat.SpeechAudioFormatInfo(AudioFormat.EncodingFormat.Pcm, 5000, 4, 1, 2500, 1, Nothing) ' FAILS
ss.SetOutputToWaveFile("TEST4bit.wav", info)
ss.Speak("I am 4 bit.")
My.Computer.Audio.Play("TEST4bit.wav")
Solution

  • AFAIK no, 4-bit PCM format is undefined, it wouldn't make much sense to have 16 volume levels of audio; quality would be horrible.

    While technically possible, I know no decent software (e.g. Wavelab) that supports it, your very own player could though.

    Formula: blockAlign = channels * (bitsPerSample / 8)

    So for a mono 4-bit it would be : blockAlign = 1 * ((double)4 / 8) = 0.5

    Note the usage of double being necessary to not end up with 0.

    But if you look at the block align definition below, it really does not make much sense to have an alignment of 0.5 bytes, one would have to work at the bit-level (painful and useless because at this quality, non-compressed PCM would just sound horrible):

    wBlockAlign

    The block alignment (in bytes) of the waveform data. Playback software needs to process a multiple of wBlockAlign bytes of data at a time, so the value of wBlockAlign can be used for buffer alignment.

    Reference:

    http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/Docs/riffmci.pdf page 59

    Workaround:

    If you really need 4-bit, switch to ADPCM format.