[sdiy] VSM201 Vocorder Question

[Mattias Rickardsson]

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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