onsdag 22. februar 2017

ETI Vocoder article errors - internal excitation

There is a mismatch between the schematics and the parts list for the internal excitation module.

Schematics says C11 is 22nF, parts list says 220nF.

C11 forms a low pass filter together with R39. Together with the 100k resistor on each channel of the analysis card - the whole setup is called an inverting amplifier filter or active inverting op amp low pass filter.

According to wikipedia:

https://en.wikipedia.org/wiki/Low-pass_filter

the cutoff frequency is 1 / 2 * pi * R*C

where R is the resistor in the feedback loop of the op amp.

For 22nF this is 1 /  6.28 * 47000 * 22 * 10^-9 = 154Hz
For 220nF it is 1 / 6.28 * 47000 * 220 * 10^-9 = 15.4Hz

In comparison, C12 and R40 gives:

1 / 6.28 * 10000 * 1 * 10^-6 = 1 / 0,01 = 15.9Hz.

The output from the analysis boards is the value of the envelope follower. If the volume of a frequency band is at its max continously, the envelope follower will be a DC voltage with a similar max value.

The two circuit parts sum up channels 1-9 (for the <2kHz input) and channel 13 and 14 (for the >4kHz input). For the sums to be comparable, the max sum of each parts must be the same.

R39 and R40 are selected to achieve this:
The gain of <2kHz is -10k/100k = -0.1
The gain of >4kHz is -47k/100k = -0.47

Multiplied by the number of channels on each input we get a 'maximum' sum for each input:
<2kHz: 9 channels * -0.1 = -0.9
>4kHz: 2 channels *-0.47 = 0.94

These are similar enough to be compared.

The low-pass filter is there to make sure the Vocoder only reacts to slow changes in envelope amplitudes. It is reasonable to assume that they should be roughly the same. The capacitor has been matched with the feedback resistor (R39, R40), which in turn was selected to give correct gain as described above.

The closest matching alternatives are then 15.4Hz and 15.9Hz. Thus, the correct value for C11 is 220nF.

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