That's what I was wondering about. Without a filter the sine wave actually has steps in it right? versus a smooth wave? I'm imagining that's not good and that's why I assumed you HAD to have a filter. Also in the bottom two lowpass, what does the large "mountain" before the spikes indicate?
DonH gently informed me I don't
really know what I'm talking about...
A delta-sigma DAC, which is most of the audio DACs these days, puts out a high-frequency pulse stream so looks even worse before filtering.
The actual modern delta-sigma DAC output (unfiltered) might look something like the red trace here if observed on an oscillosope:
I figure the traces are not "to scale" in the illustration above...
Without a filter you won't see the filtered result (which should closely resemble the original analog waveform). Stairsteps, or high-frequency varying width pulses, it won't match the original until further processing (the low pass filter).
In either case, fed to a speaker (which has low-pass properties), you'll get the sound, with added (unfiltered) HF content (if you can hear it or its effects), I would guesstimate.
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The "mountain" is the 400Hz tone - FFT spreads the displayed width of lower frequencies when the window type and size are varied. Maybe I could have picked better, but it clearly showed the high frequency content of the steps vs the lack of them when the signal is tightly filtered just above the displayed fundamental frequency.
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Maybe the "stairsteps" are applicable to a DAC type that would feed an R2R ladder? Or the output (unfiltered) of the ladder...
Don?