Firstly, as mentioned the multimeter in DC mode will only give you the DC level of the signal, which if it's swinging around 0V will be 0V (or close enough) You would need a half decent meter with a low range AC mode to get a reasonable reading.
Secondly the sound card oscilloscope software will almost certainly need to be calibrated. All it receives from the card is a value ranging between 0 and its full scale, e.g. in a 16-bit card this would be from 0 to 65,536.
It does not know how these values translate into voltage without calibration (the software may have a default setting based on usual sound card ranges, e.g. +/- 2V or whatever, which may be what you are seeing now, and may or may not be so accurate)
For example if your sound card range is +/- 1V then 65,536 would equate to +1V. If the software is set to a default of +/- 2V for full range then it will see a value of 65,536 as 2V, when in face the actual signal level would be 1V.
The fact you have your line out feeding mic in may cause the default calibration and reported levels to be a fair bit off, as line level out is a fair bit higher than what a mic input will expect.
If the software is any good it should have a calibration setting which you can use to set things up correctly. This will probably involve feeding a signal of a known level into the card and telling the software what the level is, then it can work the rest out. Since most soundcards have a DC blocking capacitor you will need an AC signal for this.
发表于 2018-7-31 09:49:36