Scaling channel data

The value of a channel data item can represent the actual value of the measured quantity, or the channel data item can represent relative value which is scaled to between maximum and minimum values of the measured quantity.

The KSensrvPropIdChannelDataFormat property defines if channel data items are in a scaled format.

For scaled data items, the KSensrvPropIdScaledRange property defines range for the data item value and the KSensrvPropIdMeasureRange property defines range for the measured quantity.

The following example reads the maximum value of measure range for data items (KSensrvPropIdScaledRange) and the maximum value of the measured quantity (KSensrvPropIdMeasureRange). The example takes into account that the KSensrvPropIdMeasureRange property can be defined as an array property.

    TSensrvProperty property;
    TInt  channelDataFormat( ESensrvFormatAbsolute );
    TInt  channelDataScaledRange( 1 );
    TReal channelDataMeasureRangeMaxValue( 1 );

    //Read channel data format
    iSensorChannel->GetPropertyL( KSensrvPropIdChannelDataFormat, KSensrvItemIndexNone, property );
    property.GetValue( channelDataFormat );

    if( ESensrvFormatScaled == channelDataFormat )
        {
        //Read data item scaled range
        iSensorChannel->GetPropertyL( KSensrvPropIdScaledRange, KSensrvItemIndexNone, property );
        property.GetMaxValue( channelDataScaledRange );

        //Read data item measure range
        iSensorChannel->GetPropertyL( KSensrvPropIdMeasureRange, KSensrvItemIndexNone, property );

        if( ESensrvArrayPropertyInfo == property.GetArrayIndex() )
            {
            TInt arrayIndex( 0 );
            property.GetValue( arrayIndex );//Value points to array index currently in use
            iSensorChannel->GetPropertyL( KSensrvPropIdMeasureRange,
                                          KSensrvItemIndexNone,
                                          arrayIndex,
                                          property );
            }
        else
            {
            //Single property
            }
        property.GetMaxValue( channelDataMeasureRangeMaxValue );
        }
    else
        {
        //No scaling needed.
        //Value of the data item represents actual value of the measured quantity.
        }

You can convert the scaled channel data item value to the absolute value by dividing the channel data item value with the maximum value of scaled range of the channel and multiplying it with the maximum value of the measured quantity.

For example, the accelerometer channel provides the following properties:

• KSensrvPropIdChannelDataFormat with value ESensrvFormatScaled

• KSensrvPropIdScaledRange with maximum value of 127

• KSensrvPropIdMeasureRange with maximum value of 2 g
• KSensrvPropIdChannelUnit with value ESensrvUnitGravityConstant

In the above example, the accelerometer channel data item value 64 means an absolute value of 1.01g (64 / 127 * 2g = 1.01g).

You can also scale the value of the channel data item. The scaling factor is published in the KSensrvPropIdChannelScale property.