Re: [ARSCLIST] Cassette obsolescence - digitizing standards

[Jerome Hartke <jhartke@xxxxxxxxxxxx>]

> -----Original Message-----
> From: Association for Recorded Sound Discussion List
> [mailto:ARSCLIST@xxxxxxx] On Behalf Of Dave Bradley
> Sent: Wednesday, February 22, 2006 10:52 PM
> To: ARSCLIST@xxxxxxxxxxxxxxxx
> 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