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1995-05-15
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The GUS Programmer's Digest Monday, 15 May 1995 Volume 19 : Number 008
Today's Topics:
Re: DC offset
RE: DC offset
Standard Info:
- Meta-info about the GUS can be found at the end of the Digest.
- Before you ask a question, please READ THE FAQ.
----------------------------------------------------------------------
From: Jani Mattsson <janmat@utu.fi>
Date: Sun, 14 May 1995 16:56:18 +0300 (EET DST)
Subject: Re: DC offset
Ricky.Houghton@cs.cmu.edu wrote:
> Our HPs 735s, with the 4231 chip, have the same problem, constant DC
> offset that is some function of the gain setting. [...]
> I'm not sure about the drift. On the Analog Devices 1848, it was
> constant, I've not done enough with the 4231 to know. If you are seeing
> drift to a null offset, when do you get the null offset? Whenever you
> change gain?
I've not been following this DC offset discussion, but
could this have to do with that when I once tried to do
some recording with my GUS Max CODEC, the middle-point
of the waveform wasn't in the middle of the sample space
(i.e. was not signed 0 but something else) but it had
shifted. When I retried, the sampling was ok, but this
occurred approximately every third time I started sampling
a new piece of sample.
- -jpm
------------------------------
From: Chris James <JAMESC@mail.medoto.unimelb.edu.au>
Date: Mon, 15 May 1995 16:12:00 -0700 (PDT)
Subject: RE: DC offset
>Our HPs 735s, with the 4231 chip, have the same problem, constant DC
>offset that is some function of the gain setting. Analog Devices
>aknowledged that their counterparts have this problem. I've not
>contacted Crystal for confirmation that it is a CODEC problem, but it
>sure seems that way.
Basically with 0 dB gain (i.e. 0 poked in the record gain reg) there seems
to
be no offset problem with the windows drivers. I yet haven't tried saving
the mixer
settings with non-zero gain in the vain hope that if the mixer settings are
made
when windows starts up before the codec is "initialised/reset/set-up" that
the
problem may disappear, read on for explaination of why this might work:
> I'm not sure about the drift. On the Analog Devices 1848, it was
>constant, I've not done enough with the 4231 to know. If you are seeing
>drift to a null offset, when do you get the null offset? Whenever you
>change gain? I may be able to live with this. That is, if I can set
>the gain and at some point in the near future have no offset.
Now I more clearly remember the problem I had with the codec when
programming
with the SDK. I note in my code still that I positively switch the codec
into the 16-bit
PCM mode that I use. I do this by switching to the ADPCM mode
and then back to PCM to exert a calibration (it mentions this vaguely in the
SDK). The
Crystal Semi. data sheet also mentions that auto-calibration is done when
certain mode changes occur. I mentioned in the last post that I remember a
"decay" effect when I used post-mux gain with my driver, decaying from some
offset down to zero. The data sheet does say that auto-calibration take 168
ms so this could account for it. So if I delay for 168 ms after I force a
switch I should see no evidence of an offset.
This is a pain when recording speech because a lot can happen in 168 ms (and
I have to take this into account when going to sample!). This may fix the
symptom, but
the post-mux gain is still introducing DC which it shouldn't in an ideal
world. This also
doesn't help end users of the codec who are stuck with the Windows or other
vendor supplied software which doesn't force a recalibration after gain
adjust or before record. Tonight therefore I'll try to figure out whether I
can switch-on the codec's high-pass filters and whether they will remain set
after Windows etc has loaded sound drivers. Gravis/Forte could do a quick
fix by asserting a calibration
whenever the record-gain is adjusted, if my explanation here is well
founded...
Chris James
Ricky
------------------------------
End of The GUS Programmer's Digest V19 #8
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