Home Hard & Software What's New! Feedback Search References


Sound card properties

Not every measurement that can be performed using a full-featured data acquisition card can be also done by a sound card. There are major differences that one should know:

Sound cards limitations

bulletDC voltage measurements require add-on hardware. Sound cards use capacitors in the signal path, so any DC level is removed. This means also that there is a lower frequency limit because of the high-pass characteristic of the capacitor in line. The lower cutoff frequency is card dependent.
bulletThe input impedance is low. Measurements are done using the line input. Real data acquisition cards have impedances of 1 to 10MOhm. In general sound cards have a much lower impedance, ranging from 600 Ohm to 47kOhm. One could use the microphone input for better sensitivity, but the noise floor is higher, and many cards output a voltage for microphone supply. If that input is used, a capacitor for blocking this voltage is necessary!
bulletData acquisition cards can measure voltages with high accuracy (+/-1% or better). Furthermore they provide different measurement ranges. In contrast the sound cards recording level is set by a slider. There's no initial calibration; it must be performed by the user.
bulletThe input voltage range used for sensor input is +/-10V in general. Sound cards with line input provide an input range of about +/- 400mV. The microphone input is much more sensitive. Only sensors with low voltage AC output and low output impedance are suitable!
bulletNo trigger facilities. Some data acquisition cards usually can wait for a so-called trigger event that starts the data capture. Common trigger events include "input voltage above or below level" or "rising edge" or "falling edge at level" detected. To some degree this limitation can be overcome by software. The calibration problem remains, so the trigger level will not be very accurate.
bulletOnly two single-ended channels are available. Many data acquisition cards provide 8 or 16 channels that can be switched to single ended "non-symmetric" or differential "symmetric" inputs if signals differ in ground level.

Sound cards advantages

bulletA clear advantage of sound cards in comparison to most data acquisition cards is that two channels are captured at the same time using two A/D converters or at least two sample and hold devices. Data acquisition cards usually use one A/D converter and a multiplexer. For that reason multiple channels are scanned one after another and not in parallel. Phase shifts can be caused, such cards are not suitable for vibration analysis or phase measurements.
bulletNotebook or desktop PC with built-in 16 bit sound card or sound system. A line input is preferable in comparison to a microphone input. Pentium class CPU and 32MB RAM minimum recommended.

Here are some download links:



http://lea.hamradio.si/~s57uuu/scdsp/CheapChop/cheapchop.htm  (a way to measure DC)

http://electronicdesign.com/article/embedded/vco-transforms-sound-card-into-precision-dc-couple  (a different way to measure DC)










Send mail to dick_grier@hotmail.com with questions or comments about this web site.
Last modified: 11/25/09
Hit Counter