How do a simple loopback?

rkruz

Registered
Thread Starter
Joined
Apr 19, 2019
Messages
16
I have a Windows 10 Dell Laptop. I'm trying to REW to do a simple loopback to characterize the onboard audio. I have attached a splitter cable to the laptop's headphone/mic connector and connected audio out to mic input. Using the Sound Card Check Levels I cannot get only an output, never an input after trying the many combinations of configurations offered in the Sound Card Check. I can verify the sound card is working by connecting a headset and mic and can see both in and out are working when viewing the REW Sound Card Check.

How can I loopback the laptop sound card using an external cabled loopback.?

My goal here is to insert a Unit Under Test or a wireless audio transmitter in the loop once I can get it to work.

Tips appreciated.
 
Last edited:

sam_adams

Member
Joined
May 23, 2017
Messages
130
Most current laptops have a Universal Audio Connector on them—see this Dell specific page. You will need a splitter cable that has a TRRS connector to plug into the laptop and an M-M stereo cable. There really is no 'Line' input on this connector as it is really setup to use an external mic, but you can get away with a loopback connection.

Quit REW if running.

Open the Sound control panel and click on the Recording tab. Highlight External Mic and then click the Properties button. Click the Listen tab and uncheck 'Listen to this device'.

Click the Levels tab and set the Microphone Boost to zero (0.0 dB). Set the level to around 85 to start.

Click the Advanced tab. The single channel mic input is usually going to be limited to 16 bit/48KHz, select that. Uncheck Signal Enhancements. Click Apply and close the recording dialog.

In the Sound control panel, click the Playback tab. Highlight the audio output—might be labelled 'RealTek HD Audio 2nd output' or something similar. Click the Properties button.

Click the Levels tab and set the output to 100.

Click the Advanced tab and set the Default Format to: 2 channel, 16 bit, 48000 Hz. Uncheck the Signal Enhancements checkbox.

Click the Spatial Sound tab and set the format to Off. Click the Apply button and close the dialog.

Back in the Sound control panel, click the Recording tab, highlight the External Mic and click the Properties button. Click the Levels tab. Leave the dialog open.

Start REW and open the Preferences. Select the Java driver. Set the Sample rate to 48KHz. Select the Output Device to match the Sound Control panel Output device. Select the Output as HEADPHONE. Select the channels—usually L+R.

Select the Input Device as External Mic. Select the Input as MICROPHONE. Select the input channel you want to use. Select the Timing Reference Output and Loopback Input channel.

Set the Sweep Level to -12 dBFS.

Set Levels to 'Use main speaker test signal to check/set levels'. Click the Check Levels button. Click Next. The test signal will begin to play.

Switch back to the Sound control panel and adjust the mic level so that it is at or below the Output signal level in REW—you won't get it exactly on as there isn't that kind of resolution in that slider to get you there. The REW Preferences dialog will stay on top of everything else so you will be able to see the meters to set the level. When the levels are set click finish in the REW preferences and the signal will stop.

Close the Preferences and close the External Mic settings and Sound control panel.

You can now run your sweeps.

You should not expect much in the way of performance from the internal sound card. Since the input is designed as a Mic input and not a line-level input, it will be very noisy and the levels of distortion are going to be very high. Although it would cost a significant amount, you probably should get yourself a good quality USB audio interface for this kind of testing.
 

rkruz

Registered
Thread Starter
Joined
Apr 19, 2019
Messages
16
Sam: Thanks so much for the thoughtful and incredibly detailed instruction set. I carefully followed the procedure, but still cannot get any audio loopback thru the mic. The output is at -12dbfs but no comparable levels on the input. I can see an audio noise floor at -75 dbfs from the mic and it changes when I change the Microphone levels in the properties dialog.
I must not be looping audio back into the mic. I have a TRRS plugged into the laptop that provides 3 RCA pigtails for stereo out and mic input, Red, White and Yellow. and I believe yellow is the mic. I've tried different combinations of loop back connections but no luck. Tomorrow I'm getting another TRRS splitter to try as perhaps I have a bad splitter.

I'll repeat the entire process and note any differences in the settings when I have different options than in the instructions and follow up again.

Thanks again for such a great procedure!
.
 

sam_adams

Member
Joined
May 23, 2017
Messages
130
Since you're using a splitter with three outputs you would want to use a mono cable—tip and ring like a 12V trigger cable—to connect the output to the input.
 

rkruz

Registered
Thread Starter
Joined
Apr 19, 2019
Messages
16
Understood. The splitter I have has RCAs. I have another different type arriving this afternoon and Ill try it and if it doesn't provide loopback there there is still some oddness about the laptop to figure out.
 

sam_adams

Member
Joined
May 23, 2017
Messages
130
When you plug in the adapter; does a little microphone icon appear at the bottom of the screen for a few seconds? That icon indicates that the OS has detected the external mic becoming active.
 

rkruz

Registered
Thread Starter
Joined
Apr 19, 2019
Messages
16
Not a mic icon. But I get this pop up window for example when I plug in a pair of earbuds/mic and the same window when I plug in the splitter and then I have to select one of the options.
43599
 

DanDan

Moderator
Staff member
Joined
Mar 10, 2018
Messages
512
I am not sure if I am understanding this, but it looks to me that you need to buy an audio interface.
 

rkruz

Registered
Thread Starter
Joined
Apr 19, 2019
Messages
16
OK. I discovered the issue. As I mentioned I was using a splitter that had 3 RCA plugs off of the TRRS. As it turns out Video TRRS uses a different connector pinout than audio TRRS. The ground band and the microphone band are swapped. So I was using a TRRS splitter that grounding the mic input. I have since gotten an audio TRRS splitter and now I am able to loop back and make measurements with REW.
Thanks so much. I'm sure I would have given up if you had not jumped in with your expert help.
thanks again!
 
Top Bottom