[sdiy] PWM or DAC better for audio out?

Scott Gravenhorst music.maker at gte.net
Thu Jun 2 19:17:29 CEST 2011


This is an addendum to my reply...

In fact, you can run your PWM carrier at a higher frequency than you update it.  This would likely
affect positively on the slew rate because the output filter Fc doesn't need to be set as low.

I still like DACs better, if there are sound output issues, it's difficult blame the output circuit
and there's no fiddly analog filter to mess with.

synth-diy at dropmix.xs4all.nl wrote:
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>johnspeth at yahoo.com wrote:
>>Can anyone give a good practical tradeoff analysis for choosing PWM vs DAC for 
>>digital audio output?
>>
>>When considering how to output audio from an FPGA synth, PWM is so simple and 
>>cheap compared to a DAC.  I personally can't think of any reason to use a DAC 
>>over PWM but my hunch is that I'm missing something obvious.
>
>The following is my opinion:
>
>As you probably know, PWM is accomplished by outputting a 
>rectangular pulse wave and varying the duty cycle (or pulse 
>width). This is used to charge a capacitor which then ought to 
>produce a given voltage with respect to the pulse width. If a 
>high frequency is used for this pulse train, it is easier to get 
>a decent signal out of it. PWM at lower frequencies can require a 
>filter on the output. Response time (slew rate of the output may 
>also be limited by lower frequency pulse trains. I would think 
>that using a high frequency pulse train implies that your sample 
>rate is also high which limits what you can do with DSP in terms 
>of voice count and/or complexity. I'm not sure - but linearity 
>may also be a problem at the low and high ends of the pulse width 
>range. 
>
>I use DAC hardware as low as 12 bits wide and I get some rather nice sounds even at low
>sample rates (44.1 KHz). A DAC requires no output filter 
>regardless of it's sample rate. I also use 24 bit DACs and of 
>course, high quality is easy to get. 
>
>If you're targetting high quality sound and using low sample rates (such as 44.1 KHz) you
>also want high voice count in polysynts with good complex voice 
>structure, I would recommend DAC hardware. 
>
>Remember that an FPGA has the advantage of being able to do whatever you want whenever you
>want just by sending a new bit file.  If you use a DAC, you can do anything.
>
>-- ScottG
>________________________________________________________________________
>-- Scott Gravenhorst
>-- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/FPGA_synth/
>-- FatMan: home1.gte.net/res0658s/fatman/
>-- NonFatMan: home1.gte.net/res0658s/electronics/
>-- When the going gets tough, the tough use the command line.
>
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-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/FPGA_synth/
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.




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