• EEVblog #713 – VCP200 Voice Recognition – 1980’s Style

    Dave breadboards a 1988 vintage Tandy / Radio Shack VCP200 speaker independent voice recognition chip from Voice Control Products Inc.
    Will it work after 27 years?
    Will it recognise his Australian accent?

    Forum HERE

    Radio Electronics magazine April 1991
    MC68HC04 Datasheet
    M6804 User Manual

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      • Ivan Berton

        Just to go on with my mindfapping….

        At 10:49 the capacitor in parallel to the 470k resistor affect the frequency response of the OPamp, like Dave already said.

        But the LM324 has a gain-bandwidth product of 1.2MHz and with the high gain due the chosen feedback resistors, the first dominant pole of the OPamp begin to roll-off at around a few kHz. The 39pF cap in parallel to the 470k works in a much higher frequency.

        So it could be that the idea was to stabilize the OPamp, they can get unstable at high gains.
        This technique is called pole splitting. In the open loop transfer function of the OPamp the 39pF cap splits the first dominant pole from the second pole resulting in a increasing of the phase margin. The idea is to push away the second pole, which turns the phase from -90° to -180°, from the region where the gain is still above 1. A phase margin (phase distance from the -180° phase turn at gain 1) is needed to get a stable OPamp when the feedback loop is closed. The phase margin is also related to the damping of the system, the more phase margin the more damping, which results in a better ring-down.

        • Yes, I completely forgot it was an LM324 which has a poor GBWP

      • comox

        But can it wreck a nice beach?

      • Hans

        When you use the ‘ symbol in the URL for the link to the forum something is cutting everything after this symbol, so then the link to the forum is not valid.

        “http://www.eevblog.com/forum/blog/eevblog-713-vcp200-voice-recognition-1980’s Style” become “http://www.eevblog.com/forum/blog/eevblog-713-vcp200-voice-recognition-1980”, not good.
        Ok this examples also become corrupt so who cares?

      • Paul

        Will it recognise his Australian accent? Your pushing it a bit now 🙂

      • blipton

        How did you find the D.Coopers’ ‘Experiments in Voice Recognition’ article? Didn’t look like it came from the net.. did you scan the original magazines, do an ocr on them for easy look-up?

        Also, I get why you have the op-amp with positive feedback (comparator), but why do you need 2 negative feedback portions for the filter/gain? Couldn’t you raise the gain on the first stage so it does both, instead having a separate gain stage?

        • I didn’t scan it, I found it referenced on the web.

      • Brian Mantel

        That’s pretty cool. There is an easier way to add voice recognition to your scope or other instrumentation. Here is an Android App I wrote where you can set it up to match any words you speak to any commands the instrument supports on its programmable interface. Feedback is welcome.

        Trial Version: https://play.google.com/store/apps/details?id=com.instrumental.instrumenttalkfree&hl=en

        Paid Version: https://play.google.com/store/apps/details?id=com.instrumental.instrumenttalkapp&hl=en

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