Doing More With Analog Input (Data Acquisition Toolbox)

Data Acquisition Toolbox

Setting and Verifying Hardware Properties

For the ChannelSkew, InputRange, and SampleRate properties, data acquisition devices have a finite (though sometimes large) number of valid values that can be set. If you specify a property value that does not match one of the valid device values, then the data acquisition engine will choose a valid value automatically. The rules the engine uses to select a valid property value are described for each property in Chapter 7, "Property Reference".

You can use the propinfo function to obtain information about the valid values for some of these properties. For example, suppose you create the analog input object AI for a sound card. You can use propinfo to display information about the SampleRate property.

AI = analoginput('winsound') propinfo(AI, 'SampleRate') ans = Type: 'double' Constraint: 'Bounded' ConstraintValue: [8000 44100] DefaultValue: 8000 ReadOnly: 0 ReadOnlyRunning: 1 DeviceSpecific: 0 The Constraint field describes the constraints on the property value, while the ConstraintValue field gives the property value constraint. In the example above, Bounded means there are a large number of valid values for SampleRate and ConstraintValue gives the upper and lower limits (the bounds) for SampleRate. For more information about propinfo, refer to Chapter 6, "Function Reference."

Note:
For some sound cards, you can set the sampling rate to any value between the minimum and maximum values defined by the hardware. This is due to onboard decimation.

To find out if the engine sets a property to a value that differs from the one you specified, you should retrieve the value after it is set. For example, after setting the sampling rate for a National Instruments device, you should retrieve the actual rate set since you may want to use this value to set additional property values.

AI = analoginput('nidaq', 1)
addchannel(AI, 0:7)
set(AI, 'SampleRate', 5000)
ActualRate = get(AI,'SampleRate')
duration = 10 % Ten second acquisition
set(AI, 'SamplesPerTrigger', duration*ActualRate)

As an alternative to the syntax shown above, you can use the setverify function. setverify is equivalent to the commands

set(obj, 'Property', Value)
Actual = get(obj, 'Property')

Using setverify, the previous example is written as

AI = analoginput('nidaq', 1)
addchannel(AI, 0:7)
ActualRate = setverify(AI,'SampleRate', 5000)
duration = 10 % Ten second acquisition
set(AI, 'SamplesPerTrigger', duration*ActualRate)

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