Data Acquisition Toolbox 2.15

Introduction to Analog Input

DEMOAI_INTRO illustrates a basic data acquisition session using an analog input object. This is demonstrated by configuring analog input properties, adding a channel to the analog input object and acquiring and plotting data from the analog input object.

See also ANALOGINPUT, ADDCHANNEL, GETDATA, PEEKDATA, DAQDEVICE/SET, DAQDEVICE/GET, DAQDEVICE/START, STOP, DAQHELP, PROPINFO.

In this example you will acquire data from the sound card on your computer. To get started, you first need to verify that your environment is set up so that you can record data with your sound card. If you are unsure about this, refer to Appendix A of the Data Acquisition Toolbox™ User's Guide.

To acquire data from your sound card, you need a data source, a data sensor, and a data sink. In this demonstration:

The data source - is the sound input to the sound card. This sound can come from a variety of sources including your voice, your hands clapping, or a CD in your computer.

The sensor - is a microphone on your computer and

The sink - is a channel associated with an analog input object.

Now let's create an analog input object and add a single channel to it.

ai = analoginput('winsound');
addchannel(ai, 1);

Now, let's set up the analog input object so that we acquire 5 seconds of data at 8000 Hz as soon as the object is started.

ai.SampleRate = 8000;
ai.SamplesPerTrigger = 40000;
ai.TriggerType = 'Immediate';

With the analog input object set up, let's start it and then get the data that it collects. If your sound card is set up to collect data from the microphone, then you should whistle or clap you hands just after you continue to the next demo screen. This will introduce something other than random noise into the data you are collecting.

start(ai)
[d,t] = getdata(ai);

Now that you're done collecting data, plot it.

plot(t,d);
zoom on

Previously, you used the GETDATA function to collect 5 seconds of data. The only problem is that GETDATA is a "blocking" function. This means that it will wait until all 5 seconds of data have been collected. For many applications, this is not what you want. The solution to this is to use the PEEKDATA function. For this example, let's configure an unlimited number of triggers and use PEEKDATA to see what is happening.

ai.TriggerRepeat = inf;
start(ai);
pause(0.3);
pd1 = peekdata(ai,8000);
pause(0.3);
pd2 = peekdata(ai,8000);
pause(0.3);
pd3 = peekdata(ai,8000);

Warning: The number of samples requested is not available.
The number of samples returned will be reduced.
Warning: The number of samples requested is not available.
The number of samples returned will be reduced.
Warning: The number of samples requested is not available.
The number of samples returned will be reduced.

Notice that the PEEKDATA function didn't block and that it returns only data that is available. We asked for 8000 samples but did not necessarily get that many.

whos pd1 pd2 pd3

  Name         Size            Bytes  Class     Attributes

  pd1       2048x1             16384  double
  pd2       4608x1             36864  double
  pd3       7168x1             57344  double

This completes the introduction to analog input objects. Since the analog input object is no longer needed, you should:

First stop the analog input object from running using the STOP command. Lastly, delete it with the DELETE command to free memory and other physical resources.

stop(ai);
delete(ai);