... | ... | @@ -396,8 +396,111 @@ Non of the open-source projects found during the research are able to perform si |
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== Soundcard-based
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A especially for hobbyists interesting solution is to use a standard PC sound card as main component for signal measuring.
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Generally every PC available on the market is equipped with this kind of device and ready for basic measurement.
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The microphone or line-in input of the sound card is then used to input the measured signal. +
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Sound cards are usually a feature of PC or notebook mainboards, but are also available as external USB devices. +
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A typically sound chip, as it is located on mainboards, can be seen in figure 4.1. Figure 4.2 shows an USB solution.
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/SOTA/SOTA_SC_ALC892.jpg[caption="Figure 4: ",title="Realtek ALC892 sound chip installed on the ASUS P8Z68-V PRO PC mainboard (?)",height=320,align="center"]
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/SOTA/SOTA_SC_USB.jpg[caption="Figure 4: ",title="Hama USB Sound Card (29)",height=320,align="center"]
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{empty} +
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=== Characteristics of sound cards
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Sound cards not only vary in their number of input and output channels, for example surround-sound versus stereo sound, but also regarding more specific characteristics like the maximum allowed input voltage.
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Those specifications are often not mentioned in any publicly available document.
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So to get those informations it is common to measure the value of interest by yourself or to rely on estimations. +
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The amount of input channels also depends on the manufacturer and model of the sound card. Generally their are two different methods to input signals into a PC audio device.
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==== Microphone input (MIC-IN)
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The first one is the Microphone Input (MIC-IN) often labeled by a pink connector. This connector is available by the majority of PCs and notebooks.
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A considerable disadvantage of this port is the bias voltage which is needed for some microphones to work.
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This DC voltage shifts any input signal by its value. So for developing a sound card oscilloscope this means that an offset is constantly added to the input signal.
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This will require either a conditioning circuit before connecting the signal to the sound card, or a software filter. +
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More information about the characteristics of a microphone input can be found on www.hobbyhour.com
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==== LINE-IN input
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The second one is called LINE-IN and is generally labeled by a light blue connector. This connector is less often used by todays sound cards.
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It differs from the microphone input by the fact that this input does not use any bias voltage.
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LINE-IN is used to connect self powered devices, for example CD/DVD-players or television sets. +
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The following picture, figure 4.3, shows a standard sound card used in computers with its connectors labeled.
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/SOTA/SOTA_SC_BOARD.jpg[caption="Figure 4: ",title="Connectors of a standard sound card (30)",height=320,align="center"]
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==== Specification
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Because of the great variety of available sound cards, the specifications of such devices can not be generalized.
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However, most sound cards are able to sample signals at a max frequency of about 44.1kHz and a resolution of 16bits.
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This limits the bandwidth of the measurable signal to around 20khz. The voltage of the input signal should not exceed 3V peak to peak.
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With the usage of the output of a sound card, it is possible to implement a function generator as well. +
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To extend the maximum voltage range a conditioning circuit, which usually consists of a few analog components, can be used.
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==== Sound card measurement example
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During this state of the art research a first sample measurement has been taken. The device under
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test, a ALC662[31] audio codec by Realtek, is used as a typical internal sound card for standard
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computers. At the University of Applied Sciences Technikum Wien[32] this specific model is used
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in nearly every student accessible PC. +
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The ALC662 audio codec is directly integrated on the mainboard of the Lenovo ThinkCentre M92p
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Tower-PC[33]. It features one LINE-IN, one LINE-OUT as well as an MIC-IN port at the rear of
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the computer. The front panel adds an additional MIC-IN and a headphone out port. All audio
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ports are implemented as 3.5mm plug connectors. However, this test handles only the front audio
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out(respectively headphone out) connector. +
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In the course of this analysis, the maximum peak- and peak to peak output voltage of the ALC662
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should be measured. To generate waveforms with different frequencies a software called "Wav
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Test Tone Generator" was used. The program can be downloaded for free at the project page
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wavb.sourceforge.net. Figure 4.4 shows the graphical user interface of the software.
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/SOTA/SOTA_SC_GEN.jpg[caption="Figure 4: ",title="Main window of the test tone generator program (34)",height=320,align="center"]
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=== Coming soon!
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This software enables the user to easily generate waveforms and save them as files. Additionally it allows the user to adjust the frequency, amplitude and sample rate in fine steps. +
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During this test, a sinus wave was outputed by the program. The start frequency was set to 1.0Hz and goes step wise up to 20kHz.
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The output of the sound card was then measured by a professional oscilloscope and entered in a table.
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In figure 4.5 one can clearly see, that the maximum frequency, generated without distortion, lies around 16.2kHz.
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Figure 4.6 shows the minimum and maximum peak voltage during measurement.
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/SOTA/SOTA_SC_Peakpeak.png[caption="Figure 4: ",title="Peak to Peak voltage output, of the ALC662 sound card, versus frequency",height=320,align="center"]
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image::https://es.technikum-wien.at/openlab/openlab_wiki/wikis/img/SOTA/SOTA_SC_Peak.png[caption="Figure 4: ",title="Minimum and maximum Peak voltage of the ALC662 sound card",height=320,align="center"]
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In conclusion, the results of the analysis shows that a maximum of 3.4V peak to peak can be outputted at a maximum frequency of 16.2kHz.
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Of course those values differ from sound card to sound card but they can be used as an estimation. |
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