Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!uakari.primate.wisc.edu!dali.cs.montana.edu!milton!uw-beaver!Teknowledge.COM!unix!hplabs!hpda!hpcuha!hpcuhd!edwardm
From: edwardm@hpcuhd.HP.COM (Edward McClanahan)
Newsgroups: comp.sys.next
Subject: Re: DAC
Message-ID: <108170005@hpcuhd.HP.COM>
Date: 17 Dec 90 21:26:59 GMT
References: <16894@csli.Stanford.EDU>
Organization: Hewlett Packard, Cupertino
Lines: 74

poser@csli.Stanford.EDU asks:

> Can anyone tell me what sampling rates are supported by the
> digital-to-analog converters on the NeXT machine?

jsd@arcadien.rice.edu answers:

> If I read the man pages right, you have a choice of 11kHz or 22kHz
> with either 8bit logrithmic or 16 bit quality.

I believe the two rates supported are 22KHz and 44KHz stereo 16-bit/channel.

The 8-bit logrithmic rate is the sampling rate of the A->D converter
attached to the microphone input.  Several manufacturers (such as Ariel)
offer 16-bit/ch stereo A->D converters which connect to the DSP Port.

> The 22kHz w/ 16 bits is the same as CD sound.  

Uh...  The 44KHz rate is that used in Audio CDs.  This rate was chosen
       so that frequencies up to 20KHz (actually 22KHz) could be represented
       digitally and be reconstructed accurately in an D->A converter later.
       Nyquist is credited with the theory that twice the sampling rate is
       required to represent an analog signal of a particular frequency
       (20KHz is higher than most people can here).  As the 44KHz standard
       was being proposed, some manufacturers wanted a 50KHz signal (giving
       more breathing room at the high frequency end), but the technology of
       the day suggested using the slower rate.  If I'm not mistaken, a 48KHz
       rate was also proposed because it matched a "standard" used already
       in the recording process.  Skeptics doubt the accuracy of converting
       48KHz to 44KHz sampled data.  Also, I believe current digital recording
       equipment samples at twice and/or four times the 44KHz rate.  The
       conversion from 88KHz or 176KHz to 44KHz is obviously mathematically
       simpler than the 48KHz to 44KHz conversion and has quieted most of
       the skeptics (such as Chip Davis of Mannheim Steamroller fame).

> I'm sure it will support about any other speed except the packaged
> software won't support it.

Well, you could probably pump data to the DAC connected to the DSP chip
at an arbitrary rate, but recall that an extremely steep low-pass filter
is required before and/or after the D->A step.  This filter is difficult
to design because it must try to avoid phase (and other) distortion near
the Nyquist frequency (half the sampling frequency).  The 22KHz (Nyquist)
frequency may be well beyond your hearing, but filters arn't perfect.  The
low-pass filters in CD players (and the NeXT) tend to invert or shift the
phase of frequencies near their "cutoff" point.  Also, those frequencies
may be boosted or attenuated as well.  Suffice it to say that in order to
minimize these effects, the low-pass filters are designed for set "cutoff"
points.  The one in the NeXT apparently supports two points corresponding
to the 22KHz and 44KHz sampling rates.

Incidently, this filter is far more important during the sampling phase
(A->D).  That is why Ariel's (and others') stuff costs so much.  The D->A
function is much cheaper.  I'd even go so far as to say the reason many
of my CDs sound worse than their corresponding albums is due to the A->D
conversion process used at the recording studios.  On several Leo Kottke
CDs, an audible ring at around 10KHz cannot be avoided by playing them
on any CD player I have tried (some costing over $3000).  Alas, even albums
arn't immune to this problem if the studio used digital mastering (more
and more common these days).

>                             Supposedly (and I heard this from a 
> NeXT rep), the DAC is fast enough to emulate 9600 buad modems and
> even a fax machine.

The hottest question in comp.sys.next last year...

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

  Edward McClanahan
  Hewlett Packard Company     -or-     edwardm@cup.hp.com
  Mail Stop 42UN
  11000 Wolfe Road                     Phone: (480)447-5651
  Cupertino, CA  95014                 Fax:   (408)447-5039