Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!tnc!m0154
From: m0154@tnc.UUCP (GUY GARNETT)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: A1000 filter
Message-ID: <770@tnc.UUCP>
Date: 26 Mar 91 21:30:50 GMT
References: <48599@nigel.ee.udel.edu>
Reply-To: m0154@tnc.UUCP (GUY GARNETT)
Organization: The Next Challenge, Fairfax, Va.
Lines: 61

In article <48599@nigel.ee.udel.edu> Dickson@system-m.phx.bull.com (Paul Dickson) writes:
>> It it the audio-filter I am talking about.  If I am not totally
>> mistaken the low-pass filter on a A1000 is permanently on and it
>> can't be turned off as is possible on all the other Amigas
>> unless you do some hardware fixing.  In other words you are
>> stuck with a poor sound output.
>
>There is no audio filter on the A1000.  The filter was added to the
>other Amigas to reduce aliasing noise on the audio output, but can be
>turned off by software.
>
>          -Paul Dickson


For the last time, Let's Get This Straight!

There is a Low-Pass audio filter installed on the audio output of all
Amiga models.  This filter is there to improve the audio output
quality of the sound generators.  It removes high-frequency artifacts
(which would otherwise be audible to many people) caused by aliasing
in the Amiga's audio output system.  The filter starts attenuating at
above 7Khz, and completely blocks everything above 14Khz.  These
frequencies were chosen because the Amiga's audio system has a maximum
sampling rate of about 14,000 samples/second.  By application of the
Nyquist Theorem, artifacts will appear at approximately half of this
(about 7Khz), and any sound above the sampling rate is composed of
nothing but artifacts (therefore the cutoff at about 14Khz).

On the Amiga 1000, this filter is built into the audio output
circuitry, and cannot be controlled by software.  Armed with the A1000
schematic, a talented hacker can pick up the signal before it goes to
the filter, and route it to an extra set of jacks.

The unfiltered signal is desirable for certain applications where the
filter is un-needed (like driving galvos in a laser light show) and
would simply muddy the signal.  Talented programmers have also made
the Amiga do things that the original hardware designers didn't
consider, and so the unfiltered signal is also desirable for audio
applications (where the aliasing effects will be removed by external
equipment, or where the artifacts are part of the desired effect).

Therefore, on all subsequent Amigas, a set of bypass switches was
added to the circuit.  They are controlled by the power light
intensity setting: when the light is dim, the filters are switched
out, and when it is bright, they are switched in.  This can be
controlled by setting or clearing a certain bit in the custom
hardware, and many Amiga audio programs provide an option to do this.

A very clever hacker figured out a way to add this capability to the
Amigia 1000 (by hacking a similar circuit onto  the A1000
motherboard).

Amazing Computing has run articles on both hacks.  Back issues are
available from PiM Publications, PO Box 869, Fall River, MA 02722;
telephone 1-800-345-3360.  You want Vol 3 No 4 (April 1988) which
contains an article "Upgrade Your Amiga 1000 to A500/2000 Audio Power"
by H. Bassen.

I hope all of this helps clear up the confusion.

Wildstar