Originally Posted by Roger Dressler
I looked at the TrueRTA product info, and I didn't see any reference to TrueRMS detection. But it's a software product, so doing it carries no cost burden, so I'd assume it's true RMS.
Easy enough to check. Generate a square wave, 1Vp, (as measured in oscilloscope mode), and see what the AC voltmeter reads. It should be 1V. If it reads closer to 1.1V, then it's an average responding detector.
I tried that test and the TrueRTA generated square wave is not smooth enough to do that test. Perhaps that is a soundcard issue.
I checked the help menu, and it indicated input voltage readings are RMS values. You have to calibrate the input and output each time you fiddle with volume levels if you want accurate voltage and SPL readings.
Crest factors are also calculated based on RMS numbers.
Funny thing is the Digital RS SPL meter has a MAX reading button on the display. That MAX reading pretty much matches up with the PEAK SPL reading on TrueRTA.
I am not sure how the chip in the RS Digital SPL meter calculates the SPL output numbers. I am not sure if the analog and digital SPL meters work the same way. You can do a lot of things in a chip on the cheap.
Input Level Meter
The input voltmeter reads the signal selected at either the spectrum analyzer input or the scope input depending on which mode is active. If you are using the scope then the input you select for the scope will also feed the meters. Likewise for the analyzer, if you are using the real time analyzer then the analyzer’s input will feed the meters.
The voltmeter can display the following levels:
Input Level in millivoltsInput Level in dBu rms (0.774597 Vrms reference, or 1mW @ 600 Ohms)
Input Level in dBu peak
When the dB display is selected a solid vertical bar is drawn at the left edge of the plot screen to indicate the overall dB level of the signal. Floating above the top of the bar is a line that indicates the peak signal level.
Upon first installing TrueRTA you should calibrate the voltmeter for your sound card. See the Audio I/O Menu for full details on how to calibrate your system so that it reads input signal levels accurately. You will need an AC voltmeter separate from TrueRTA so that you can accurately measure the signal level. You will enter that value at the calibration dialog and then your TrueRTA will be calibrated for your sound card and provide you with precise measurement of input signal levels. Here is a link to the Edit menu for the calibration commands: Line Input and Output Calibration."