Can my PC audio be modified to go down to DC? - Page 1
Forum section: electronics / projects

author: GK
author_rank: Super Contributor
timestamp: April 10, 2013, 12:35:41 pm
content: Hello all.I'd like to do more of stuff like that detailed here:https://www.eevblog.com/forum/projects/home-brew-analog-computer-system/msg215653/#msg215653; namely, generating complex test signals / arbitrary waveforms in LTspice as *.wav files and using them for electronics testing. However my PC audio is already having significant LF attention at 50Hz, which is a major limitation. I'd like to modify the motherboard audio if possible to get a signal response down to DC.Before I start fudging my motherboard does anyone know if itisn'tonly the coupling capacitors in the op-amp based audio circuitry external to the DAC that limits the LF response of the audio outputs?

author: ConnorGames
author_rank: Regular Contributor
timestamp: April 10, 2013, 12:47:59 pm
content: I don't know, but you could try probing before the caps with a DMM/Scope to see relatively risk-free!

author: GK
author_rank: Super Contributor
timestamp: April 10, 2013, 12:51:39 pm
content: That's true enough, but there are still the warranty seal stickers in the way! If it's to be a pointless experiment I'd rather keep them intact for now

author: RJSC
author_rank: Regular Contributor
timestamp: April 10, 2013, 01:01:51 pm
content: Quote from: GK on April 10, 2013, 12:51:39 pmBuy a cheap USB external sound card and then you can experiment without worries.The signal before the coupling capacitors has a DC bias of VCC/2 and thats why the capacitors are there: to center the output waveform arround 0V.

author: Balaur
author_rank: Unknown
timestamp: April 10, 2013, 01:14:06 pm
content: Yes, buy an external USB sound card ($10-20) and experiment to your heart's desires.Removing the caps on the output channels is a good start. While I'm not able to quote any datasheet info, I think that the DAC themselves will not be against going down to 0 Hz.I'm not too clear on how you would drive all this from a software perspective. as sound card/driver/audio system expects a data chunk to play at a given sampling rate. At worst, you will need to upsample your data to a sampling rate accepted by the card.

author: fcb
author_rank: Super Contributor
timestamp: April 10, 2013, 01:26:14 pm
content: Audio DAC's almost always go down to 0Hz, ADC's on the other do not (they tend to have a non-bypassable software filter inside).You may have issues with the soundcard driver (although I doubt they would bother putting in a DC filter), and you'll have to remove the DC blocking capacitors.

author: Harvs
author_rank: Super Contributor
timestamp: April 10, 2013, 01:44:28 pm
content: Something like this came up here a while ago, and at the time I traced out and reverse engineered the front end of an ASUS Xonar D1. Unfortunately I don't have access to the computer the schematics are on at the moment.But the story was the output of the DAC and input to the ADC are differential centered around half VDC (5V). This was then capacitively coupled into a differential amp stage which was powered off IIRC +/- 7V (good old 5532 opamps.)So I desoldered all the input amp stage and built up a prototype on veroboard for an input stage as a low frequency scope (1M input impedance and DC coupling.) Wired it into the card and it worked just fine. The audio ADC in the Xonar D1 has a pin that gets pulled high or low to turn on or off the ADC DC rejection filter, so that was easily overcome.

author: Alana
author_rank: Frequent Contributor
timestamp: April 10, 2013, 02:13:40 pm
content: If your PC still has LPT then you may try for simple 8bit DAC connected to LPT and used as sound card. I think it was called covox back in the old days.

author: free_electron
author_rank: Super Contributor
timestamp: April 10, 2013, 03:17:44 pm
content: Get one of those pcm2706 usb audio chippies.Those have dc in and out i believe.And you wont fry your puter if something goes wrong.

author: Harvs
author_rank: Super Contributor
timestamp: April 10, 2013, 11:58:03 pm
content: Something else to consider (and against what most have said.) The more expensive internal sound cards (~>$40) use off the shelf ADC's and DAC's which you can grab the datasheets for, making hacking very easy. The $10 USB sound cards I've seen have no such thing (all very integrated ASIC, no datasheet.)

author: GK
author_rank: Super Contributor
timestamp: April 11, 2013, 10:52:32 am
content: Hmmm.... I'm just after a quick fudge to get me going...... a 7660 to give me a negative supply rail, -vee op-amp supply pins lifted and re-routed, coupling caps shorted, etc. I just remembered I have a ~10yo but perfectly fine WinXP computer stashed away that was a freebie. Duh. It has a nasty virus preventing me from installing antiviral software and deleting it, which was why I stashed it away. Time to pull it out of the shed and format the hard drive!

author: GK
author_rank: Super Contributor
timestamp: April 11, 2013, 10:55:27 am
content: Quote from: Balaur on April 10, 2013, 01:14:06 pmYes, I would only expect it to hold a DC potential while one of my LTspice-generated *.wav files is playing. For example at the moment low frequency square waves are out due to the tilt.

author: LukeW
author_rank: Frequent Contributor
timestamp: April 11, 2013, 04:08:50 pm
content: It might be a good idea to try building your own little custom board using one of the USB sound devices like the Burr-Brown/TI PCM2902/PCM2702 etc.That way you've got complete control of the circuit, the schematic is completely documented and you don't have to stuff around tracing out the PCB or figuring out what ICs its using and that sort of thing, and there's no risk of damaging a relatively expensive motherboard if you attempt component-level modifications such as changing the coupling caps.

author: GK
author_rank: Super Contributor
timestamp: May 03, 2013, 10:53:21 am
content: Turns out my PC motherboard has a Realtek ALC662 audio codec:http://www.hardwaresecrets.com/datasheets/ALC662_DataSheet_1.3.pdfHere is a 50Hz square wave probed directly at the pin 36 DAC output (2V per division):A quick skim through the datasheet found a digital filter spec. for a low pass of 0.45 * Fc but apparently no mention of a high pass.However the response clearly doesn't go down to DCWhat a heap of poo.I'm not sure the high pass is implemented in the codec. Perhaps it is in the driver software? Any ideas?? I know there is a control panel somewhere for the audio interface with equalizer sliders.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 03, 2013, 11:53:59 am
content: Page 68 of the datasheet says Magnitude response min 0 max >20,000.Are you sure your scope is on DC?

author: GK
author_rank: Super Contributor
timestamp: May 03, 2013, 11:55:56 am
content: Quote from: PA0PBZ on May 03, 2013, 11:53:59 amYep. Double checked then triple checked.

author: GK
author_rank: Super Contributor
timestamp: May 03, 2013, 12:49:11 pm
content: It appears that a fixed subsonic software filter is implemented in the Realtek driver. There is a whole host of software equalization controls and options in the "Realtek HD audio manager". I can play with the equalizer settings and produce a whole heap of funky distortions to my 50Hz square wave, but even with all equalizer settings disabled the subsonic filtering remains in effect. How annoying.I think I'll drive down to MSY tomorrow morning and pick up an "ASUS Xonar DG" sound card and fudge that. It's less than 30 buck so is worth the punt.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 03, 2013, 01:22:17 pm
content: Whatever software filter is in place, it's still a bit fishy that it looks exactly like an AC coupled output. If they are suppressing subsonic waves I'd expect to see something resembling what you put in, only with a lower amplitude. What happens when you do the same with a sinewave?Oh well, maybe the best is to let it rest and spend the $ 30.

author: GK
author_rank: Super Contributor
timestamp: May 03, 2013, 02:31:28 pm
content: A subsonic filter is just a high pass filter. It has the same effect on square wave tilt as does AC coupling. You can't attenuate a square wave with a filter to any significant degree without waveform distortion.

author: TMM
author_rank: Frequent Contributor
timestamp: May 03, 2013, 02:34:11 pm
content: Xonar DX:Note: Volume was not adjusted between these measurements.

author: GK
author_rank: Super Contributor
timestamp: May 03, 2013, 02:47:10 pm
content: Quote from: TMM on May 03, 2013, 02:34:11 pmThanks!The tilt on the 50Hz is a great deal less sever than what I am seeing, indicating a much lower frequency high-pass. That will just be due to (defeatable) AC coupling of the DAC output. I'm not exactly sure what is going with your 1Hz waveform though.

author: Unknown
author_rank: Unknown
timestamp: May 03, 2013, 08:59:24 pm
content: Have you considered building an oscillator, this would be your carrier frequency input, say 1Khz; then DC amplitude modulate the 1Khz to provide your low frequency input; not sure if you can software engineer this to null-out the 1Kz carrier, then leave the amplitude signal trace for display !...I have done this in a simple form for a PC strip-chart recorder, using a programme from Radio Sky...http://radiosky.com/skypipeishere.htmlAnd this looks like the free basic version, using one channel...http://radiosky.com/radio_skypipe_1X_support.htm

author: ivan747
author_rank: Super Contributor
timestamp: May 03, 2013, 10:01:11 pm
content: Why does every sound card manufacturer feels the need to include this proprietary crap in every product they make?This is what pisses me off about Windows. Sorry for the offtopic.

author: TMM
author_rank: Frequent Contributor
timestamp: May 04, 2013, 03:57:06 am
content: keeps the devs busy for the 300 days a year they aren't coding drivers i'd imagine

author: SeanB
author_rank: Super Contributor
timestamp: May 04, 2013, 05:34:31 am
content: Adds to the tick boxes that they can use in Marketing to give "features" more than the competitors. That they are as useful as a teapot made from Tibetan Blackbird is another point.

author: ivan747
author_rank: Super Contributor
timestamp: May 04, 2013, 11:52:28 am
content: Isn't the best solution just to get the simplest sound card you can get, install generic drivers and remove the dc-blocking caps on it?

author: GK
author_rank: Super Contributor
timestamp: May 05, 2013, 06:15:38 am
content: All done. I will design and etch a proper PCB with more refined circuitry in SMD in the future, but this fudge to the ASUS Xonar sound card will get me going in the meanwhile. The circuitry is just an amplifier/filter and DC level shifter, along with a charge-pump PSU to provide +/- 9V for the TL074. The circuitry on card runs entirely on a single supply rail, with a pair of DRV601's to drive the line outputs. The two trimpots are for centering the output signals about ground. I tapped the stereo signal pair directly from the codec DAC outputs. The sound card itself is unmodified and continues to operate as standard.Now I can run things like this:https://www.eevblog.com/forum/projects/home-brew-analog-computer-system/msg227592/#msg227592

author: ivan747
author_rank: Super Contributor
timestamp: May 05, 2013, 01:37:44 pm
content: I love the mod. It's gonna look so clean from the outside...

author: c4757p
author_rank: Super Contributor
timestamp: May 05, 2013, 01:54:00 pm
content: Quote from: ivan747 on May 05, 2013, 01:37:44 pmIt looks clean from the inside!Nice job.