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 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 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 ***************************************** To post to tomorrow's digest: To (un)subscribe or get help: To contact a human (last resort): FTP Sites Archive Directories --------- ------------------- Main N.American Site: ftp.orst.edu pub/packages/gravis wuarchive.wustl.edu systems/ibmpc/ultrasound Main Asian Site: nctuccca.edu.tw PC/ultrasound Main European Site: src.doc.ic.ac.uk packages/ultrasound Main Australian Site: ftp.mpx.com.au /ultrasound/general /ultrasound/submit South African Site: ftp.sun.ac.za /pub/packages/ultrasound Submissions: archive.epas.utoronto.ca pub/pc/ultrasound/submit Newly Validated Files: archive.epas.utoronto.ca pub/pc/ultrasound Mirrors: garbo.uwasa.fi mirror/ultrasound ftp.st.nepean.uws.edu.au pc/ultrasound ftp.luth.se pub/msdos/ultrasound Gopher Sites Menu directory ------------ -------------- Main Site: src.doc.ic.ac.uk packages/ultrasound WWW Pages --------- Main Site: http://www.xmission.com/~grue/gus.html Main European Site: http://src.doc.ic.ac.uk/packages/ultrasound/ Main Australian Site: http://ftp.mpx.com.au/archive/ultrasound/general/ http://ftp.mpx.com.au/archive/ultrasound/submit/ http://ftp.mpx.com.au/gravis.html Mirrors: http://www.st.nepean.uws.edu.au/pub/pc/ultrasound/ GUS digest: http://gpu.srv.ualberta.ca/~itam/digest.html MailServer For Archive Access: Email to Email to New Submit Files Mailing List: Email to with content "subscribe epas-list " Hints: - Get the FAQ from the FTP sites or the request server. - Mail to for info about other GUS related mailing lists (general use, musician's, etc.).