> -----Original Message----- > From: Association for Recorded Sound Discussion List > [mailto:[log in to unmask]] On Behalf Of Dave Bradley > Sent: Wednesday, February 22, 2006 10:52 PM > To: [log in to unmask] > Subject: Re: [ARSCLIST] Cassette obsolescence - digitizing standards > > >Both analog-to-digital (A/D), and its inverse digital-to-analog (D/A), > >conversion require filtering. When digital audio is "played", the > >output of the D/A converter is filtered to remove the "steps", resulting > in a > >continuously varying "smoothed" analog output that can closely > >replicate the analog input. Fidelity to the original depends on the > >initial sampling frequency, i.e. how close together the "steps" are, > >and also bits per sample that determines how accurately the recorded > >amplitude represents the original amplitude and also determines the > >noise floor. > > The filtering is not to "remove the steps" or even to smooth them. > The steps don't exist. When you hear digital playback, you aren't > hearing digital, you're hearing analog. Those steps are simply > details given to the D/A converter to tell it how to generate an > analog waveform that matches what the original was. That analog > waveform will not have any steps to it. Connect a decent D/A to a > oscilloscope (sp?) and you'll notice smooth analog waveforms. No steps. > > The filtering is to remove noise that the sampling frequency would > cause. For example, if you are sampling at 44.1 KHz, then any signal > over 22,050 Hz would get incorrectly digitized. The filtering on > playback is to prevent anything like that from causing interference > with the desired signal. > > Filtering can't smooth steps, it would only muddy the picture so that > you didn't notice them. That's definitely not what it's for. > > You can get a better worded explanation here: > http://www.earlevel.com/Digital%20Audio/Oversampling.html > > It covers the filters as well as what oversampling has to do with > anything. > > > > ----------------- > Diamond Productions > Preserving the past for the future. > Dave Bradley President Your oscilloscope is observing the filtered output. The steps are present because the internal output of the D/A produces discrete voltage levels, not a continuum. The filter you describe is an anti-aliasing filter. I do not require the book you referenced, having designed, built, and used A/D and D/A converters for data acquisition purposes for over 40 years. Jerry