Testing Sources with No Input

[Dean_OAV]

Hi - Firstly I would like to thank QA for such an awesome device
For the money this thing is incredible

I have a repair bench and be using it to fault find and verify repairs

Mainly to pro audio and DJ Equipment

I only received it yesterday but already put it to good use!

My question, I do a lot of repairs to DJ CD Players and controllers
A lot of these have a USB connection and can function as a DAC
I have tried the PC Mirroring and don’t seem to get good results.. bands widen, and what looks like comb filtering - but it happens randomly - sometimes get a good measurement sometimes not. I assume this is down to how windows handles audio. I have tried playing with many settings and sometimes it seems to help - but its never perfect

So, I thought I would try the Export Chirp Wav for Triggered Testing
The devices also have a USB Port to play back audio files
I had to convert the files to 24bit in audacity as the device dosent support 32bit
This actually works really well for frequency response using the exported Expochirp

What I really want though is a stepped sine 1 or 2 points per octave at 96khz and at levels from -20 to 0dBV in 2dB increments so I can visualize THD+N at various levels and frequencies

Exactly like the automated test AMP THD vs Frequency

Is this possible? Currently I have each octave center Frequency as its own track 30 seconds long and at various levels but this is really time consuming and I can’t see a simple way to include it all on 1 graph

Many Thanks for any help you can offer
Best Regards from the UK

[matt]

Hi @Dean_OAV,

A problem that comes from using devices as DACs in mirroring mode is that Windows will do sample rate conversions at the drop of a hat. So, you need to be very diligent in getting the correct sample rate in window, trying different drivers, etc.

In the Edit->Settings, you see there are 3 different drivers you can pick.

If Windows has opened your DAC at 44.1 kHz, and you try to send it a tone at 48 kHz, then the only thing windows can do is re-sample the 48k content down to 44.1k. And that re-sampling will leave you with degraded harmonics and noise.

So, before you do anything with the QA403, make sure the device (your CD player) is seen by Windows as the primary device and as a 24-bit or floating point device with 48K sample rate. And keep an eye on that, because Windows can change that without telling you. And then, once that is sorted you can try the different drivers on the QA403 and hopefully you’ll see a great quality tone out of your DUT. And then you can use that for swept tests.

Please try and feel free to post screen shots of spectrum if you aren’t sure. This forum makes posting a snap: Just copy the image to your clipboard and CTRL-v to paste it into your message.

PS. Make sure your windows volume is set to max when testing.

[Dean_OAV]

Hi Matt, thanks for quick response

Firstly I cannot seem to find a way to do a single stepped frequency response..

The Frequency response tab only has the option for ExpoChirp, I assume stepped sine should be an option here? along with THD? and or THD+N? I probably have something set wrong

I can do it from automated tests, but without THD and I have to run tests at 2 levels

This test is actually a Mixer also with USB inputs, and a different driver (The Pioneer driver)

I have managed to get a frequency response of it as a dac

However I cannot run the AMP THD vs Freq with these settings and get this error message

The issue with the CD Player is the driver I think, it uses Generic Usb Audio driver from microsoft version 10.0.26100.3912 I assume thats the problem, they havent spent any time on the generic driver as it has an ASIO driver for proffesional use

Any chance asio could be added as an output option on PC MIrror?

I repair a lot of these (Pioneer CDJ2000 nxs2)

I am able to get good amp frequency vs thd plots when using qa403 output into the mixer as below

Many Thanks

[matt]

Hi @Dean_OAV, I just ran your settings on my unit in loopback and it completed, which suggests your input might be seeing overload maybe? Could you run with Autoset Input Range disabled and put the full scale input to +18 dBV before you run?

Also, make very sure you are getting a good THDN reading with a single tone before you start an automated test. If a single reading isn’t making sense (due to overload, bad mirror driver selection, etc) then an automated test won’t make sense.

Once you pick the Frequency Response button, you are telling the software you want to deal with chirps. To do a frequency response with sines, you will generally go to Automated Tests → Amp Frequency Response and that will step the sines for you.

Some specifics you asked:

Firstly I cannot seem to find a way to do a single stepped frequency response..

See Automated Tests->AMP Frequency Response (which I think you know based on next question)

I can do it from automated tests, but without THD and I have to run tests at 2 levels

You can “stack” results. For example, below I do a single sweep at -10 dBV for distortion and then another for gain at -10 dBV, and they will plot on the same graph if I ask them to. And then I can manually re-name traces and axis. You can use this technique of stacking to put a lot of measurements on the same graph. Just make sure they make sense.

I assume thats the problem, they havent spent any time on the generic driver as it has an ASIO driver for proffesional use

The issue probably isn’t in the manufacturer’s driver. It’s in Windows. You just need to find the right windows driver that won’t re-sample. And I think WASAPI Exclusive will often do the trick

Any chance asio could be added as an output option on PC MIrror?

Probably not in the near term as it would be a big change.

In your plot below, I’m not sure of your setup, but remember when you specify an amplitude and are using mirroring, your specified level is taken as dBFS. So, if you specify a QA403 output level >0, then you are specifying a level in excess of what the digital device can output. You will see a “Digital Clip” on the main dipslay in the upper left if this happens (same place Atten is indicated).

It kind of looks like the degraded THDN you are seeing is happening when the output level goes above 0 dBV/0 dBFS. I think the QA403 will soft limit and display the Digital Clip message when this is happening. The soft clip means it won’t clip hard, but it will have its amplitude re-scaled on the fly to try and fit it ino the limited output range.

[Dean_OAV]

Hey Matt, with auto range disabled I am able to take a THD stepped sine and seperate stepped sine frequency response reliably with some devices but not others (and specifically not the devices I repair a load of), as we both think the issue lies with the windows drivers or sub system. (All the devices that work have thier own driver, this device uses the generic usb audio driver)

The last graph you didnt quite grasp what I meant that was a clean reading of the DJ Mixer I was testing fed from QA403 and back into QA403 not fed from USB. the distortion is typical of this mixer at high levels (Crazy for a mixer over £1500) I have found the setting to increase system bandwidth now so my graphs extend to 20khz for THD+THD+n

I had a thought that might be easier to implement to resolve the CD player issue (Pioneer CDJ2000 nxs2).. as I still can’t take reliable readings from the CD Player using mirror, I have been playing 2496 sine waves at octave centers and taking readings and sticking them in a spreadsheet.. this is time consuming!

I just had a though I could make a file in audacity with 10-20 seconds of sine wave at 3rd octave bands, (31 files, with gaps) load it onto a memory key and play it in player then how would I make use of the REST interface to reset averages, wait 5 seconds and then send back the THD, THD+N + RMS level every 25 seconds or something, so it stays in sync with the audio track playing? I’m not fussed if it takes an hour I can leave it running off the workbench. Im a service tech not a programmer I did have a read of the REST Api a bit beyond me!

Or a simpler way just an automation that can record live THD, thd+n and gain every N seconds for X amount of times?

Might also be useful for tracking devices that distort when hot/cold? to view THD over time?

Thanks
Dean

[restorer-john]

@Dean_OAV

Or a simpler way just an automation that can record live THD, thd+n and gain every N seconds for X amount of times?

Might also be useful for tracking devices that distort when hot/cold? to view THD over time?

This would be very useful down the track in a future software update/version. Kind of using the QA403 as a THD data-logger for intermittent faults in vintage gear repairs.