[sdiy] Arpeggiator1 / Arduino UNO

[Richie Burnett]

One of the biggest problems with using PWM instead of a proper DAC is that 
the reference voltage for the logic '1' (high) state is often a noisy and 
poorly-regulated logic power supply rail, rather than a precision analogue 
voltage source with a noise free output.  (That is unless you actually use 
the digital PWM signal to switch an output line between a precision analogue 
reference and analogue ground.) There is obviously a trade-off between 
response time and adequate suppression of the switching carrier, but you can 
improve this situation significantly by using a high-order elliptic low-pass 
filter that has a notch in it's frequency response right at the PWM carrier 
frequency (and possibly again at some of the lower harmonics.)

Those tips make PWM okay for control purposes, but it still really sucks for 
audio output purposes.  A lot of people don't realise how much distortion is 
introduced into audio by the asymmetric modulation (leading-edge or 
trailing-edge only).  You are actually phase-modulating the PWM carrier as 
well as imparting the wanted amplitude modulation.  This produces an FM-like 
spectrum with lots of horrible side-bands.  Not what you want for hi-fi 
audio usage.  If you can achieve symmetric modulation (leading & trailing 
edge equally) then the distortion products go away, and there is only 
harmonic of the switching frequency left to filter out.  That's why 
"Digital" Class-D audio power amps usually use a triangular waveform for the 
PWM generation instead of a sawtooth/ramp.

PWM is well suited for achieving regulation in switching power supplies, and 
for motor speed control, but caution is needed in places where it's trying 
to replace a DAC.

Sorry for the rant, but the list was relatively quiet and hopefully some 
helpful info for someone!?

-Richie,


-----Original Message----- 
From: Jacob Watters
Sent: Friday, April 10, 2015 2:15 PM
To: Rick Jansen
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Arpeggiator1 / Arduino UNO

You can increase the PWM frequency. That will help to remove ripple.

https://arduino-info.wikispaces.com/Arduino-PWM-Frequency

http://arduino.cc/en/Tutorial/SecretsOfArduinoPWM


On Fri, Apr 10, 2015 at 7:07 AM, Rick Jansen <rick.jansen at xs4all.nl> wrote:
>
>> On 10 Apr 2015, at 11:49, Nantonos <nantonos at epona.net> wrote:
> ...
>>>> I would be surprised if the ENOB is even as high as 12; probably more
>>>> like 10. Which is fine if it suits the purpose, but describing this as
>>>> "16 bit" is misleading for anyone else thinking of building it.
>>
>>> That is why I labeled it "~16 bit DAC" :-)
>>
>> My point is that ~10 is more likely.
>>
>>> Even an 8-bit (ZN426) DAC would have been good enough, it will give
>>> you 256 discrete voltages.
>>
>> Yes, if an error of +/- 25 cents is ok then a theoretically perfect
>> 8bit DAC will suffice
>
> ok ok
>
>> Then you learn about INL, DNL, zero offset, gain error, thermal drift,
>> etc. At which point the wisdom of saving $5 comes into question and
>> you get a very good 12bit DAC or an okay 16bit one.
>
> If you settle for 64 discrete output CV values and an 8-bit DAC there are 
> 4 bits per step and a smaller error? I'm after "good enough" here, not 
> perfection.
>
> Even 32 discrete values would work, I think, for an arpeggiator.
>
> rick
>
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