Brand New MFOS [Music From Outer Space] Vocoder
VIDEO
Features
Built in White/Pink Noise Source
Built in Square/Pulse Oscillator
Mic/Line Inputs for Control and Audio
Good Dynamic Response To Voice Control
Reasonably Intelligible Speech Processing
Introduction
A vocoder has got to be one of the coolest signal processors going. It allows you to impress vocal harmonic and dynamic characteristics onto other sound sources making them sound as if they are talking. This vocoder design works by using one lo-pass filter (100 Hz.), ten successively tuned band-pass filters (154, 208, 285, 395, 542, 720, 1013, 1495, 2000, 2546 Hz.) and one hi-pass filter (3330 Hz.) to separate the incoming control signal into its harmonic constituents. For each band the control signal appearing at the output of its corresponding filter is full wave rectified and low pass filtered to obtain the control signal's envelope voltage. This signal is used to drive a VCA through which the channel's filtered audio is amplitude controlled. The whole circuit is described below but all I can say is that this thing is totally cool and I'm very happy with mine (I am somewhat biased).
This vocoder design works by using one lo-pass filter (100 Hz.), ten successively tuned band-pass filters (154, 208, 285, 395, 542, 720, 1013, 1495, 2000, 2546 Hz.) and one hi-pass filter (3330 Hz.) to separate the incoming control signal into its harmonic constituents. For each band the control signal appearing at the output of its corresponding filter is full wave rectified and low pass filtered to obtain the control signal's envelope voltage.
Meanwhile the audio (signal to be modified) path is also going through a separate but identical set of twelve filters. The output of each of these filters goes through a VCA that is controlled by the envelope follower output of the corresponding filtered control signal's envelope voltage. Thus when there is harmonic content contained in a band of the control signal a proportional amount of the same harmonic content of the audio signal is passed through it's filter and VCA to the output of the vocoder.
The Vocoder Band Block Diagram shows that each band consists of two identical active filters, an envelope follower and a VCA. The control signal's active filter output feeds an envelope follower whose output voltage controls the VCA. The VCA controls the amplitude of the channel's audio filter output. Thus whenever the control signal contains harmonic content within the pass-band of it's particular filter it turns the VCA on causing the audio signal filtered with the same filter characteristics to be output to the mix bus.
The MFOS 12 Channel Vocoder Block Diagram shows how 12 of these channels, each tuned to a separate band of frequencies are routed to the main output bus. You may have heard one of those guitar effects in which a speaker drives sound into a plastic tube that the musician places in his/her mouth. In that case the musician uses his/her mouth to apply the same physical filtering to the guitar sound coming through the tube as is normally applied to the sound emanating from the musician's vocal cords. The result is that the musician can make the guitar 'talk'.
In a way the vocoder applies similar filtering to the audio signal but uses a more round about way to get there. The audio signal is routed through the lo-pass filter, ten bandpass filters and the hi-pass filter which are controlled by the VCAs. The audio signal path's VCAs and filters are the analog of the mouth's ability to apply acoustic filtering. The amount of each filter's output is directly proportional to the harmonic content contained in the control signal (voice).
The internal oscillator and noise source provide ready-to-go audio signals to experiment with. The white noise, with level adjustment, is also applied to the hi-pass filter's VCA input so that when high frequencies are present white noise is emitted. I did not employ the 'sibilance detect' circuit found in some designs that compare the energy in the lower bands to the energy in the upper bands to see if white noise should be emitted I just let it fly when frequencies above 3330 Hz. are detected. I call my method the poor man's sibilance detector and it seems to work just fine.
You can mix the raw control or audio signals into the output via the Control Mix-In and Audio Mix-In controls. You can select white or pink noise as well as the internal rectangle wave oscillator to be applied to the vocoder input. Using noise as the audio signal creates odd whisper effects and applying the internal oscillator makes you sound like a robot. The output level control adjusts the amount of the unit's signal that is fed to your amp or mixing board.
Use a good quality mic for best results
In the just so you know category: using a quality microphone with a flat frequency response between about 70 to 10,000 Hz. (or better) will result in the best speech intelligibility. Using a low quality mic will be a very disappointing experience. I use my Octava capsule mic into my mix board and then take the output of the channel via the insert jack. Just insert the tip (send) jack's output to the vocoder's Control (voice) input. On most mixers when the stereo-tipped insert plug is in a channel's insert jack the mic is effectively cut off from the rest of the mixer (unless you plug through an effect of some kind). The insert jack is switched such that it expects me to "insert" something and since the insert cable plug to route the signal back onto the mixer's bus is not connected the channel's pre-amp is not connected to the mixer's audio bus.
