Late at night, by candle light, "Phil Allison" penned this immortal opus:
> wrote:
>
>
> YD wrote:
>
>> Generate a PWM square wave at a couple 100 Hz. Then all you'll need is
>> a LPF and whatever gain and DC block/restore required.
>
>
>** The problem is that sound cards incorporate capacitors in the output
>signal path - so low ( sub audio) frequencies are attenuated and lost.
>
>Any attempt to use a PWM rectangular wave to transfer such low frequencies
>via a small capacitor will suffer the exact same fate.
The PWM carrier is in the audio range. It should be lowish for good resolution but high enough to be easily filtered. Extracting the sub-AF from the variable duty cycle is done by the external circuitry. Makes perfect sense if you think about it.
>BTW
>
>" PWM square wave " = a self contradiction.
>
Well, whatever. I was too lazy to write "variable duty cycle pulse train".
- YD.