[sdiy] VSM201 Vocorder Question

[rsdio at audiobanshee.com]

This discussion has quickly become confusing. A few terms have been used incorrectly, we've diverged onto different VCA implementations, and the schematic is in German, so I'm not surprised at the confusion. Maybe we can figure this thing out?


First of all, dynamic range is a relative term. It does not always refer to (maximum) signal to noise ratio. While most vocoders surely have issues with signal to noise ratio, I don't think that's the trickiest part of a vocoder circuit operation to figure out. For most of this thread, we've been discussing limitations in the dynamic range of the VCA section, which presumably ranges from 0 dB down to some limit of available attenuation.

With a digitally-generated PWM signal, the VCA volume steps are discrete and finite, which is why we were discussing limits like -40 dB, -60 dB, and -80 dB. Actually, even the digital PWM would be capable of infinite attenuation when the pulse width is zero, but a jump from -40 dB to negative infinity would potentially be distracting.

It's been pointed out that the VSM201 vocoder does not have a digital-counter-based PWM, but uses a comparator fed by an input signal and a triangle wave. This design does mean that the PWM is not discretely stepped, but it does not mean that the signal attenuation is continuous all the way down to infinity. The comparator chip will have limited slew rate and settling times, making a particular minimum pulse output before it jumps to flat. That minimum output pulse width will determine the maximum attenuation of the signal in that band, before the jump from maximum useful attenuation to infinite attenuation. I assume that those 4053 2-channel multiplexor elements can only be On or Off, and thus there is a minimum pulse width and thus a maximum, finite attenuation before the signal drops to negative infinity (muted).


Second of all, the discussion also focused on the fact that the audio is PWM'd and not the CV. However, it seems to me that the audio is always PWM'd by this type of VCA. The writeup by Rod Elliot is not for a vocoder, and thus describes a CV input that is converted to a PWM signal that controls a SPDT switch that alternates between the input audio and ground. Even in that basic circuit, the audio is PWM'd. When adapting this kind of VCA to a vocoder, one needs to process two input audio signals, breaking each into multiple frequency bands, and then multiplying the amplitude of each band to produce an output audio signal that sounds like a combination of the two input audio signals. Ultimately, I assume that the output VCAs still operate as described by Rod Elliot, where the audio is always PWM'd.


Finally, can anyone say whether this is a true, 20-band vocoder, or maybe just a 3-band vocoder with 20-band graphic EQ on the input signal? How wide range is the "speech or signal inputs" section? Would it handle a full keyboard tone? I assume that the "modulation sound input" is full range.

Brian


On Jan 2, 2016, at 10:45 PM, Tim Ressel <timr at circuitabbey.com> wrote:
> I think if an effort was made to make a DIY copy of the VSM201, a case could be made for swapping out the switched VCAs for 13700s or something.
> 
> Another question:  It looks as though what is being PWMed is the audio and not the CV coming from the detectors. The CV signals (I think) are labelled 'KK' but I cannot find their source.  It is not helping that the schematics are copies and are in a language I don't read.
> 
> Also I can't see the values for the filter components. Is there a BOM somewhere?
> 
> --TimR