[sdiy] VSM201 Vocorder Question

[Tom Wiltshire]

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,
>>> 
>>