Acquire Data in Background with Live Plot Updates

This example shows how to acquire data in the background using callbacks while MATLAB^® continues to run.

A background acquisition uses callbacks to access data as the hardware collects it or to handle errors as they occur. In contrast, foreground acquisition causes MATLAB to wait for the entire acquisition to complete before you can execute your next command. In this example, you acquire data in the background from a microphone with ID Audio1, and you define a callback to update a plot in real time while acquisition continues.

Device Discovery and Setup

Get a list of devices available on your machine and identify the microphone you want to use. The partial listing here indicates the vendor ID and device ID used in this example, a DirectSound headset microphone with device ID Audio1.

daqlist

      VendorID       DeviceID                         Description
    _____________    ________    ____________________________________________________

    "directsound"    "Audio1"    "DirectSound Headset Microphone (Plantronics BT600)"

Create a directsound DataAcquisition object with a microphone input channel on Audio1.

mic = daq("directsound");
ch = addinput(mic,"Audio1",1,"audio");

Background Acquisition and Live Plot Setup

Configure a callback function using the ScansAvailableFcn property to update a live plot with the data from background acquisition. This example uses the plotMyData callback function defined in the next section.

The callback is triggered when the number of accumulated scans exceeds the value of the ScansAvailableFcnCount property. By default, this value is set to Rate/10, so the callback is triggered 10 times per second.

mic.ScansAvailableFcn = @plotMyData;

Start the acquisition to run for 5 seconds in the background.

start(mic,"Duration",seconds(5))

To create a live-updating line plot, assign an animated line plot to the UserData property of the data acquisition object, mic. During background acquisition, the callback function adds points to this plot dynamically.

figure;
xlabel("Time (s)");
ylabel("Amplitude");
mic.UserData = animatedline;

Callback Function

This callback function updates the live plot with the data acquired in the background. The function takes two inputs, the callback source and event information. MATLAB passes the DataAcquisition object that triggers the callback as the first input, src. The second input contains event information, which is not used in this example. The function reads data from src and updates the live plot by adding the data to the animated line stored in the UserData property.

function plotMyData(src,~)
     [data,timestamps,~] = read(src,src.ScansAvailableFcnCount,OutputFormat="Matrix");
      addpoints(src.UserData,timestamps,data);
 end

During the background acquisition, speak into the microphone and see the plot update in real time.