Your PC as an Audio Analyzer: Measure THD, S/N, Power Output, etc

[Cross Talk]

Kewl!
subscribing...
gotta watch... pinoy david jones. haha

[Born2BeWired]

To calibrate the PC soundcard input, we need a leveled sine signal source as a minimum.

Signal Source

The signal source must be able to generate a leveled sine. This problem stalled me for a while because this instrument, again, is very unlikely to be found on a hobbyist’s workbench. (An example of the function generator with a leveled output is the HP 3325A we used in the early part of this thread.) While the PC instrument software suite also has a function that uses the output of a sound card as a signal generator with preset output levels (i.e. leveled output), the actual output generated is hardware dependent and should be verified (or calibrated) before we can use it to calibrate the microphone input. So here once again we are stuck in a chicken or egg situation. Well, at least until we find a way to calibrate the PC signal generator output to a reasonably accuracy using common test instruments.

Naturally, my eyes zeroed in on the cheap DT830 class DMMs. I tested several class 830 DMMs models to see how well they do measure low voltage 60hz sine signal. The result is quite appalling. All units I tested displayed fairly accurate readings when tested with 110 to 220VAC signal, but gave out distressingly wrong readings when tested with 1.00Vrms 60Hz ac signal. Not that the instruments are poorly calibrated. They are simply not capable of measuring this low AC signals accurately because of their lack of low voltage FS settings.

Let me explain this limitation a bit further. The DT830 class has only two AC range, 600V (some 500V or 750VAC) and 200VAC full scale. With sufficiently large AC inputs- take 120.0VAC for example at 200VAC range, a +/- 1 count DMM error will result with a reading anywhere between 119.9 to 120.1. This error is equivalent to a less than 0.1%  hence does not amount much. But with 1.0VAC inputs at 200VAC range, +/- 1 count error will read between 0.9 to 1.1, yielding a whooping +/-10% error budget!  (note 1)

Not with DT830 class DMMs

We need a DMM with at least 0.01V resolution when measuring 1.00V rms AC sine signal. Most mid priced DMM have an AC F.S. settings capable of displaying this resolution. You should grab one - buy, borrow, by whatever means you have, but don’t steal.  If you can buy or borrow one sporting reputable brands (e.g. Fluke, HP, Sanwa, Yokogawa, etc..), even better.

Yes to 360 VOM: Long Live Analog!

I also tested a couple of ‘lowly’ type-360s analog VOMs. Guess what -  both gave a correct reading with the low AC test signal! One of the VOM I tested is even older that most of you, it is a 30 year old Sanwa 360TR copy - Standard YN-360TR (this is the VOM I was using during my college days, and yes, it is still in good working condition), still, it gave out an exact 1.00Vrms readings when tested with a 1 KHz 1.000Vrms signal.

Hence, if you can’t get a good DMM , get your analog VOMs ready. Practice reading those scales,  we will calibrate the PC analyzer next using this instrument.

Note 1: 1.00Vrms output is chosen as the test level because that is the highest output the PC software suite generator function is able to generate.

[Woots29]

^ yup pwede sis. check lang sa limitations nung sound card like sampling rate. sa mga AV forums, meron silang common na ginagamit na USB Sound Card. 'yun ding mga ginagamit sa analysis ng amp at speaker merong loopback test para i-compensate sa output na graph 'yung mga parts na may rolloff, etc.

'yung Virtins ata, test instrument talaga ang target. meron ding ibang analysis software na pang-audio talaga like TrueRTA. ginagamit ko 'yung TrueRTA na free version for quick evaluation ng speaker response using Pink Noise source (at kinuhang Electret Mic sa headset ). Speaker Workshop 'yung isa ko pang gamit. pero ngayon, gawa muna ng preamp ng ECM8000.

behringer usb sound card gamit ko eh

may RCA input

macheck nga spcs nun

[Mayor Johnaray]

Interesting...Subscribing!

[n0th1n6]

Meron na akong buffer para sa tutorial na to.



Iyan po yung schematic para sa gustong gumawa din at ng mareview na din kung ok it .

[Woots29]

yahoo! ayus yan ah

salamas sis n0th1ng, +!

[n0th1n6]

Thanks sis woots. susundan ko mga tutorials ni tita Born gamit yang buffer na yan.

[Cross Talk]

Meron na akong buffer para sa tutorial na to.



Iyan po yung schematic para sa gustong gumawa din at ng mareview na din kung ok it .

very... very familar. hehe

[Born2BeWired]

Meron na akong buffer para sa tutorial na to.



Iyan po yung schematic para sa gustong gumawa din at ng mareview na din kung ok it .

Thanks Tita Noth1n6, this buffer might prove useful especially for those who may want to work with preamplifier circuits. In fact, we may use this later to experiment with some real world effects of opamps. As a preview of things to come, I will show you why you may want to use an 'audio grade' OPAMP instead of the TL082 used in this circuit.

[rdpzycho]

quite interesting, sinubukan kong i-check ang levels nung output ng sound card ng laptop ko at mukha siyang may maliliit na notches (around 6dB) sa odd multiples ng 90Hz, pero pag-abot ng 1kHz above, ok naman na siya.

[Born2BeWired]

Sa tagal ng installment, inabot na ng expiration ng evaluation period ang  Multi-Instrumens 3.1 na ginagamit ko. Napagana ko ulit gamit ang 1. system restore (pick a date bago nyo nilagay ang MI software) 2. burahin manually ang folder ng MI. 3. Install uli. Remember, if you find this software useful, buy it

INPUT ATTENUATORS

The microphone input is designed to accept signals from electret type of microphone, so it works with fairly large input signal ( >50mVrms). In fact, actual measurements (COMPAQ CQ40) showed input values can go as high as 84mVrms before input overloading occurs.

But when used to evaluate audio power amplifier performance, you may find this input handling range preposterously inadequate.  To make the PC analyzer useful even with moderately high power amplifier circuits, an input attenuator must be inserted between the microphone input of your PC and the device (i.e. power amp) under test.

You can easily build an input attenuator probes yourself using the schematic diagram as shown in the figure below.



R3 and R2 voltage divider forms the attenuator circuit. R3 essentially sets the input impedance of the measuring circuit to 100K, a value most audio measurement equipment takes.  The value for R2 must be chosen according to the desired full scale range. D1 and D2 (as suggested by audiomachines) protects the microphone input from damage in case the input swings dangerously high. C1 keeps the DC mic bias away from D1-D2. R1 provides a DC bleed for the microphone input (this helps keep the residual distortion low).

I would recommend building three separate attenuator probes, one for each range. Then, each probe must be calibrated using the PC analyzer software (Virtex Multi-instruments 3.1) calibration utility.

Calibration Preparation

Microphone gain should be set to 0db. The microphone gain can be adjusted using the sound card driver functions that can either be accessed through the control panel group (Sound) or through a software utility provided by the sound chipset manufacturer.

Prepare a phone plug by removing its cover baring its solder lug terminal. The terminal connected with the plug tip should be readily accessible. You will use this as a calibration test point when we get to the calibration stage.

Preparing the signal source

1. Activate the Multi-instruments 3.1 generator function by clicking the menu Instrument>Signal Generator. The signal generator control window should appear with the default values as shown below.



Check to ensure the displayed settings are correct: Output frequency should be 1000Hz, and the Output Amplitude should be 1 V(rms).

2. Click the PLAY button (upper right hand corner of the signal generator control window). If your PC is equipped with a loudspeaker, you should hear a continuous 1000 Hz tone playing. Adjust the master volume/mixer control if you hear nothing (or if it is playing too loud).
3. Plug the phone plug prepared as described earlier into the speaker (or headphone) jack.
4. Using your VOM (or a DMM with sufficient range) set at 10VAC range, measure the generator output level (probe across the ground terminal and tip terminal of the phone plug inserted in step 3.) Adjust the master volume control until you get a reading as close as possible to 1.00Vrms.

Calibration procedure follows... Kaunting tiyaga pa at pwede na natin magamit ito

[Born2BeWired]

quite interesting, sinubukan kong i-check ang levels nung output ng sound card ng laptop ko at mukha siyang may maliliit na notches (around 6dB) sa odd multiples ng 90Hz, pero pag-abot ng 1kHz above, ok naman na siya.

Subukan ko tita rdp kung ganun din dito sa laptop ko.

[Born2BeWired]

Tita rdp

I swept the signal generator output from 100Hz to 10KHz, wala naman akong napansin na dip. Output flatness measured 0.3db within this range. Kailangan mo na ng bagong PC tita

[rdpzycho]

^ 'yun din nga naisip ko Tita Born. 'di naman siya problem doon sa Desktop ko.

[Ugin]

Nice thread by the Master himself!!! Subscribing..

Here's another software related to this topic that will also be useful for such testing. Maybe Tita Born or someone else will also evaluate it:

Screenshot:



Link 1

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B

Link 2

1D96b^S//91+V99Ta_7\ZFW

Use our standard key.....

[Woots29]

tnx po +1

[diy_master]

In a related thread, I was chided by one member for suggesting a power amplifier measurement procedure that requires the use of a distortion analyzer. Rightly so, because this piece of equipment is not cheap; it is unreasonable to assume that hobbyists will ever have access to this equipment to do the tests.

You may not know it, but already on your desk is a piece of equipment capable of doing these measurements- your PC! Yes, PC has all the necessary hardwares to do these measurements with reasonable accuracy (albeit to a limited extent). With the right software, you can convert and use your PC to measure audio amplifier figure of merits, such as THD, S/N, Output Power and so on.

PC Audio Analyzer

Using the PC sound circuitry as digitizer to display sampled waveform (i.e. as digital oscilloscope) is not a new idea. It has been with us for years. But only few took interest because it suffers a few but important shortcomings that severely limit its usefulness:

   Bandwidth – it works only with audio frequencies
   Input Coupling- it work with AC inputs only

And then a few talented individuals came up with an idea. Realizing that current PC processors now have enough processing surplus to do a lot of other things other than displaying those pretty desktop graphics, they worked and successfully add a real time FFT (Fast Fourier Transform) function to the PC soundcard oscilloscope suite. In other words, your PC now can perform the function of an audio spectrum analyzer!

This wonderful development may completely changed the way you look upon these PC oscilloscope applications. The powerful FFT function enables you to directly measure (not just imagine) THD, noise, output power, and score of other parameters of audio circuits. Your newfound capability to measure these parameters will enable you further to take your audio related work to the next level and do some real engineering work. For instance, you can now study, measure, and fully appreciate the effect of “ground loop” or “star grounding” technique that is often mentioned by the analog gurus in this forum, which I suspect most of you simply regard as just one of our deranged fantasy.

Who should read and participate in this thread

This thread is not just for audio enthusiast, it is for everyone who would like to venture into the analog realm and gain deeper understanding into this discipline. Audio circuit is just one of the many branches of the analog tree. The knowledge and skill you earn working with audio circuits will apply as well to other analog application circuits.

Download PC Oscilloscope Software Tools

Googling “PC FFT Analyzer” will yield a number of sites offering soundcard based PC oscilloscope suite. Most of them are shareware, they will allow you free use of the software for a limited period, after which you have to buy the software. The PC oscilloscope software I downloaded for evaluation is

Virtins Technology Multi-Instruments 3.1:

http://www.virtins.com/

The evaluation version expires in 21 days after installation. The lite version costs USD49.95 - sounds reasonable. But I know that cash strapped third world creatures like us may still find this unaffordable. If you find a freeware equivalent, please let me know, perhaps we can base our future discussion on your find.

What will I do in this thread

Using the Virtins software, I will do some quick verification tests, compare measurement results with real instruments, and report the result here. The purpose is to determine whether the PC analyzer measurement results are close enough to the real analyzer equipment readings to be trustworthy.

What I will do next and hope we will accomplish

I will build an experimental power amp and perform some experiments that you can duplicate using this PC software tool. By this time, I hope we will be joined by our analog gurus and get their precious views and inputs. At the conclusion of this project, I hope this effort will result in all of us broadening our knowledge that will make us even better engineers. 

*****
Be patient though. Since I can only to this at home, it may take some few days between updates. I will try my best to make at least three updates a week. The truth is, early this evening, I already started performing a number of tests even as I started writing this intro, but at this point, I am now too tired to write a report. Expect some results in the next couple of days.

Mga si r sa po nakakabili ng  Pocket Multi-Instrument 1.0, dito locally? need ko po talaga # 3577990 Anthony. 

[anthonydp]

mga ka-elab! i-share ko lang sa inyo itong PC Oscilloscope with Spectrum Analyzer:

http://www.zeitnitz.de/Christian/scope_en

subscribing na rin ako dito...

[enriz622]

patambay nga ok dito ah

[Born2BeWired]

My apology, di ko natapos ang thread because my (evaluation) copy of the software expired and stopped working before I even completed the input calibration procedure..

My new tablet convertible HP tm2-2050us is padating na, hopefully I can reinstall a fresh copy and get another shot para matapos natin.