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Old 13 March 2017, 21:40   #20
demolition
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Join Date: Sep 2012
Location: Copenhagen / DK
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Quote:
Originally Posted by Foebane View Post
Are you sure it's not down to the samples used? I always thought Paula was a simple 4-channel 8-bit DAC with independent volume controls, so there should be no problems, unless the frequency response is uneven?
The low pass filter can be considered a part of the DAC circuit, also known as a reconstruction filter:
https://en.wikipedia.org/wiki/Reconstruction_filter

Mirror images of the frequencies in the audio signal are created around the half of the sampling frequency of the DAC and some kind of low pass filter must be used if you want to remove these, or at least attenuate them.

This theory also applies to the samples being played back which are played at a lower sampling rate than the DAC/Paula itself, thus some kind of sample rate conversion is involved (and nearest neighbor is a very crude way of doing it). The overtones from the sample rate conversion is probably the main reason why you would want a low pass filter, however ideally the filter should be applied separately to each channel with a cut off frequency tracking each sample's sampling frequency, so having just one filter and being able to turn it on and off was probably as good as what could realistically be done.
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