Real Time Audio Spectrum Analyzer with Signal Generator
On some computer platforms, an Input or an Output device could fail when selected. If this happens, something with the audio driver or with the audio hardware is not standard. It can be a bad or obsolete Windows audio driver; it can be a lack of needed audio card hardware capability, such as full duplex (when Audio OUT works at the same time with Audio INPUT - for example, to listen to some music on the speakers and, at the same time, to record singing through microphone); or something else. The easiest way to overcome this problem is to go to Windows Control Panel - Sound section and manually choose the DEFAULT Audio Input and Output device. When this is done, a user has to choose Default Audio Input and Output devices in the EasyRTA Audio Devices Selection menu.
Some Audio Cards have a very low signal output on the Line OUT connector. It can be below 200 mV rms, which is lower than -10 dBu (245 mV rms). In such a case, EasyRTA Signal Generator cannot output signal amplitude bigger than -20 dBu. This will affect Signal Generator amplitude range in the EasyRTA application. To establish a proper amplitude on the Line OUT connector, a user can turn on the Signal Generator at -10 dBu and connect the True RMS AC Voltmeter with mV range to the Line OUT connector of the Audio Card. In the Windows sound mixer, a user can change the Audio Output Device level until 245 mV rms reading appears on the True RMS AC Voltmeter. After accomplishing that, a user can continue to the procedure described in the paragraph below.
Some Audio Cards don't have a Line IN connector, but only a Mic IN connector. This generates at least two problems in signal measurements. First, Mic Input has a DC offset for driving electret microphones. This offset can be removed programatically, but then, the results are far from precise. The second problem is that Audio Card Input has a very low, or very high sensitivity. Both of the mentioned problems are difficult to resolve in EasyRTA only. The easiest way is to change the Audio Input Device level in the Windows sound mixer. To set the appropriate level for measurements, a user can turn on the Signal Generator in EasyRTA at the -10 dBu level, then turn on Spectrum Analyzer and change the Audio Input Device level in the Windows sound mixer to the level where side harmonics disappear. After that, a user can start the calibration process in EasyRTA, as described in the "Audio I/O - Line IN / Line OUT Calibration" section.