Offset waveform from on-board audio...

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cstv
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Joined: Jul 26 2002

Hi, recently built a new edit PC around a Gigabyte GA-7N400 Pro2. The systems works perfectly apart from the audio.

Output sounds fine, but anything captured via the line input (haven't tried the mic input yet) seems to be offset below the "zero" (-infinity) line. see the picture below...

i can't work out why the left channel is so much worse than the right.

i can't hear any difference but it causes real problems when if i try to amplify anything because i can amplify it beyond zero dB so there's no signal!

any help would be greatly apreciated!

mark.

[This message has been edited by cstv (edited 21 January 2004).]

harlequin
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Joined: Aug 16 2000

what are you feeding the signal from , to the line in.
i've seen some recordings from tv on a nicam recorder where one channel is a couple of db different all the time, like someone has partially panned the sound image

Gary MacKenzie

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Nigel Longman
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Joined: Apr 28 1999

Mark

Your waveforms are exhibiting a dc offset, ie a fixed voltage bias offset from the zero voltage line. These offsets are undesirable (I can't think of a reason for actually wanting one) but can be removed using an audio editing program (eg Sound Forge) which has a 'remove dc offset' process.

I have to say I've never seen such a large offset as your waveforms are exhibiting. This may indicate a problem with your audio card/chips.

Incidentally have you tried recording from a different source? It could be that for some reason there is a standing dc voltage between the equipment. Is one or the other pieces of equipment efficiently earthed? Maybe there are some dc leakage currents floating around in your system causing the problem.

HTH, NL

[This message has been edited by Nigel Longman (edited 21 January 2004).]

cstv
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Joined: Jul 26 2002

Ahhhh... now i know what that means! i've been wondering for a while what the "dc adjust" button was for... now i know!

sorry, i should've said, the waveform in the picture was amplified to make the problem easier to see, it's not actually that bad.

the waveform in the picture was from a mic into a mixer and then into an amp before heading for the computer (excessive i know but the set up into the amp was already there and it was the easiest way at the time) but i've tried it without anything connected to the line in and it's still there, so is an earthing fault on the mobo likely?

i'll try plugging the PC into a different power ring, and give the mic input a try tomorrow.

cheers,
mark.

[This message has been edited by cstv (edited 21 January 2004).]

Nigel Longman
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Joined: Apr 28 1999

Mark

Are you making these recordings using Sound Forge? The picture you have posted looks like it could be.

If so I assume you are using the Special/Transport/Record menu item? On the panel for that feature you will see there’s a tick-box called ‘dc adjust’. Selecting this will compensate for dc offset during the recording. There’s also a calibrate button to null dc offset, the use of which is described in the help file.

My earlier comments about earthing related to earthing your equipment rather than suggesting your mobo had an earthing problem.

You comment that you cannot increase the gain above 0dB, but that is correct of course since here 0dB relates to absolute maximum volume, ie digital clipping level. Silence is the axis labelled ‘–inf’ and is of course where the audio signal should sit when there is no signal, no noise and no dc offset.

You mentioned that you are going through a mixer and an amp before the computer. I would have thought the output level from the mixer would have been sufficient for the sound card line-in without needing further amplification. Are you using the low-level output from the amp or connecting to the speaker terminals? Using the speaker terminals is less than ideal for many reasons, not least of which is that its not uncommon to find a small dc voltage present at the terminals.

Hope that lot may be of some assistance. Apologies if I’m teaching you how to suck eggs.

Good luck, NL

cstv
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Joined: Jul 26 2002

nothing wrong with sucking eggs Nigel, and i've always wanted to know, would that be hard-boiled or raw...?

anyway, output from amp is a tape output it's just that the signal was quite low when i did that recording and i was in a bit of a hurry. it does the same when the level's up higher.

the recording was using sound forge, and i had seen the "dc adjust" options, just didn't know what they were for! a bit of egg licking here, and very useful it is too!

the fact that it does it even with no input (just hitting record with nothing connected) suggests to me that maybe there's a potential difference across the line-in socket. but wouldn't that normally just produce hum?

i'll check the mic input tomorrow.

cheers,
mark.

Nigel Longman
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Joined: Apr 28 1999

quote:Originally posted by cstv:
Snipped by NL .... but wouldn't that normally just produce hum?
cheers,
mark.

Hi again Mark

Just a quick note that a dc offset won't produce hum - that's usually caused by mains induced interference. Hum is an ac (not dc) signal having a frequency of 50Hz and sometimes higher harmonics of 100, 150Hz etc

The dc offset you are suffering with won't produce any audible effects until it causes the audio to clip early. AFAIK only a few specialist audio amps have a bandwidth that goes down to dc.

The fact that you get this offset without any input does seem to suggest that you may have a fault on the board.

Sometimes offsets like this can be caused by errors in supply voltage rails from the power supply. If possible could you try a different power supply in the computer?

HTH NL

[This message has been edited by Nigel Longman (edited 22 January 2004).]

cstv
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Joined: Jul 26 2002

Ok, just tested the other recording sources - line, mic, CD(analogue) and stereo mix.

all had a DC offset, but sound forge's DC Adjust function fixes the problem.

for the line input:
left = -1129
right = 135

i can't really change the PSU at the moment because it's in pretty much constant use. i may get a chance in a month or so, but since SF fixes the problem i think i'll leave it be for now.

thanks for your help with this Nigel,
mark.

cstv
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Joined: Jul 26 2002

interestingly i discovered that a friend's gigabyte mobo has exactly the same problem with it's on-board audio... maybe it's a "feature" of the design...

mark.