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Old 06 December 2013, 15:36   #103
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Originally Posted by mc6809e View Post
I understand that. But real Amigas don't have perfect and infinitely adjustable reconstruction filters with variable cutoff frequencies.

A two point square wave output at 1000 points per second requires a low pass filter with a cutoff frequency of 500 Hz to construct a sine wave. There is no such filter on the Amiga. The cutoff frequency is much higher. To get perfect reconstruction of the original signal, the cutoff frequency must be half the the output frequency and since on the Amiga the output frequency is variable, we'd also have to have a programmable filter.
This is only partially correct - filter is present but his efficiency may be not enough to completely fulfill reconstruction requirement, also it is very important to understand that aliasing is introduced in ADC not in DAC thus even not efficient enough reconstruction filter will perform correctly if signal is without aliasing.

Originally Posted by mc6809e View Post
Oh sure. For the highest output rates, the low pass filter starts to be effective as a reconstruction filter. For lower rates, not much at all.

And maybe that's why this discussion has been so contentious. At higher output rates, the Amiga audio hardware performs more like a sample player. At lower output rates it looks more like a synthesizer. The Amiga gives us the choice.

Thanks for the link!
Once again - aliasing is introduced when signal is sampled with to low sample rate i.e. in ADC - DAC will reproduce signal without aliasing even if reconstruction filter is not efficient enough - DAC without oversampling (Amiga case) will introduce additional loss for high frequency signal - this can be compensated in analog reconstruction filter or compensated in digital stage (or by additional digital filter before DAC or by software filter that modify samples itself - then you need additional room (bit or 2) or you are forced to accept lower level of signal (peak and rms)).
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