Vocoders, vocoders everywhere.........

[Sean Costello]

Dr S Grainger wrote:
  
>         May wait and get the demo disk of the Quasimidi Sirius to
>         hear it's vocoder (I know this is not exactly DIY !) any of
>         you tried this ? Is it any good ? The magazines seem to have
>         differing opinions on the whole system and on the performance
>         of the vocoder.

I tried it briefly.  The built-in microphone delivered a potent shock to
my lips.  As a guitarist, I have received enough nasty shocks in my life
to take that as a warning not to mess with the device anymore.

The MAM-11 Vocoder has a VERY nice sound to it.  I think it only has 11
bands, but it proves that the design and tuning of those bands goes a
long way.  Very intelligible speech.

Sean Costello

P.S.  My current favorite vocoder-like technique is my own
implementation of PSOLA (Pitch-synchronous overlap and add), a form of
pitch synchronous granular synthesis.  Basically, the key is to chop up
the input signal into units (grains) that are exactly one or two periods
in length (this, of course, requires some form of pitch detection); the
resulting grains are then triggered at the audio rate of the desired
pitch.  Sounds very nice - vocoderish, but easier to understand.  Much
better than Linear Predictive Coding, in my opinion.  I am using my
crude version of PSOLA in my final project for my current Computer Music
class; I am using the fog opcode in Csound, with a preset window length
(.028 seconds) that allows the formants to come through clearly, but
without too much of the pitch coming through.  The advantage of this
technique is that you have independent control over the duration, pitch,
and formant tuning of the sound.  You can shift pitch as much as you
like, without the formants getting "munchkinized."  Just thought I would
throw this out as an alternative to vocoding.  I am sure there are
devices out there that currently implement this technique (I know that
WaveMechanics sells Pro Tools plug ins that implement a form of this).