[sdiy] VSM201 Vocorder Question

[Magnus Danielson]

Yes, extra filtering needed between them to get the effect.
The downside will be increased sensitivity to timing.
Oh, and the extra filtering was bandpass filtering.

That normal VCA sounds like a pretty good solution all of a sudden.

At the same time, the effective dynamics of a vocoder isn't very huge 
dynamic numbers. For it to work fairly well, a companding system helps,
which is what some used to get more articulation.

So, we might spend more effort on reducing noise and then add a 
companding system.

Cheers,
Magnus

On 01/02/2016 11:29 PM, Mattias Rickardsson wrote:
> Why not use 2 analog switches in series and square the attenuation
> (double the dB numbers)?
> Some extra filtering needed in between them, but -40 dB PWM-VCAs could
> be doubled to reach -80 dB or tripled to get -120. :-)
>
> /mr
>
> On 2 January 2016 at 23:10, Tom Wiltshire <tom at electricdruid.net> wrote:
>> Ok, maybe -80dB is *really really* impossible! My judgement was "certainly tough, but probably not impossible", but I'm very willing to go with "extremely difficult bordering impossible" or "totally impossible" if you feel that's a more accurate evaluation! Certainly a 50KHz carrier isn't that high, so I'd probably want to cut your timings in half (e.g. 100KHz carrier), which makes it even more difficult. Certainly we need significantly sub-nanosecond switching times to get good results.
>>
>> The point was just that -80dB isn't that good for a modern VCA, and it's pretty much out of reach for PWM VCAs. Lots of Blackmer-cell-based designs do much better than that, and even 13600's can probably pull -80dB out of the hat ( http://hem.bredband.net/bersyn/VCA/vca_shootout.htm )
>>
>> Tom
>>
>>
>> On 2 Jan 2016, at 21:12, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>>
>>> With carrier frequency of 50kHz, 0.01% duty ratio (for 80dB attenuation) represents a pulse width of just 2ns! That's getting near the sort of time mismatch you can get in turn-on and turn-off times for the switches. So the switch might not turn on at all, or might stay on twice as long!
>>>
>>> -Richie,
>>>
>>> Sent from my Xperia SP on O2
>>>
>>> ---- Tom Wiltshire wrote ----
>>>
>>>> +1 totally agree
>>>>
>>>> If you can produce a 1% pulse width, you still only get -40dB. You need to get a 0.01% pulse wave to get -80dB. Tough to do. Not impossible, but awkward enough to make it stop seeming like such a great solution.
>>>>
>>>> I've thought about this a bit because of using the PIC's PWM module so much. The best case output from that is either 8-bit or 10-bit, which means that -60dB is about as good as I'd get using it for a VCA, and that implies having a switching frequency which is much too low (31KHz) for many jobs.
>>>>
>>>> Tom
>>>>
>>>>
>>>> On 2 Jan 2016, at 19:42, rburnett at richieburnett.co.uk wrote:
>>>>
>>>>> <snip>
>>>>> Control range of PWM'd CMOS switches acting as VCAs isn't that great though.
>>>>>
>>>>> -Richie,
>>>>>
>>>>
>>
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