In a couple of earlier posts we described the design and construction of a voice display and driver board which would emulate that used by Kitt in the Knight Rider TV show. This has now been completed and mounted on AVA.
When we connected the voice display unit to the EMIC 2 Speech Synthesizer which AVA uses to talk, we discovered a problem. The display was driven hard on, even when AVA was silent. A quick measure with the multimeter showed that the EMIC 2 has a constant 2.4 VDC offset. As discussed in the earlier post, the LM3915 displays full scale at 1.25 V, so that was why all the LED's were on.
This is not unusual and a lot of the Arduino MP3 shields do the same thing. Unfortunately when we were testing the prototype, the EMIC 2 hadn't arrived from the USA so I tested using the piezo analog output on the Arduino (which doesn't have a DC offset).
2.0 High Pass Filter
There are a number of ways that you can remove the DC component of a signal, so that you are just left with the AC component. You could use an isolation transformer, but we didn't have one in the spare parts bucket. We did have a heap of resistors and capacitors so we decided to try a high pass filter.
A high pass filter, as the name suggests, passes signals above a selected cut-off point, ƒc eliminating any low frequency signals from the waveform. The circuit for a first order high pass filter looks like the following.
It is generally accepted that the range of frequencies audible to humans is 20Hz to 20kHz. Consequently we will design our HP filter with an fc of 20Hz. The formula to calculate fc for a first order high pass filter is:
You can pick two variables and solve for the third, or you can go to an online filter design calculator, and let it do the hard work for you.
We selected an fc of 20Hz and a capacitor of 0.01uF (because we had one of this value), and the design site spat out a resistor value of 820k. If you plug in these values into the formula above you will get an fc of 19.4 Hz which is fine. The filter design site will even plot frequency and transient analysis graphs for you.
Adding the high pass filter between the EMIC 2 Speech Synthesizer and the voice display module did the trick. It removed the DC bias and allowed the speech signal to pass through. Here is a video of AVA speaking with the voice display module installed.