Dbfs levels - peak or RMS?

QA401
#1

When we use dBFS as the level units, and specify the generator at say -10dBFS, is that dBFS RMS or Peak? And does that assume the total differential output or a single-ended output (one BNC)?

Thanks.

edit: Let me clarify why this matters. With the QA401 there is a fixed relationship between dBFS and dBV, but when using the QA401 with other apps, that relationship no longer exists because they don’t know it’s a QA401, it just an ASIO driver device. If another app uses dBFS Peak, and we know what’s used in the QA401 software, a relationship could be established.

Yeah, I could just get out the meter and measure it I guess. When I see a 3dB spread between the two apps, I get suspicious its a method of measurement thing.

#2

Hi @ja-tech, the dBFS and dBV outputs assumes single-ended output. So, if you set the output for -5.88 dBFS (0 dBV) then you’d measure 1Vrms (single ended) or 2V rms (diff)

The primary purpose of the dBFS button is two fold: If you are intimately aware of the ADC/DAC characteristics of the QA401, then it helps to know how far away from full scale you are operating. The second is so that when you are using mirror mode to drive a DAC, you can know the level being generated on the PC relative to full scale.

For the dBFS RMS versus peak question, the answer is RMS just like dBV. The internal maximum is +/- 1 (normalized) and then RMS is computed and scaled. You can have signals greater than 0 dBFS if their crest factor is is greater than the crest factor of a sine. If you pump in a square wave, your RMS will be +3 dBFS. At first blush this seems impossible because 0 dBFS is the max. But the common RMS relationship to peak (sqrt(2)) only applies for sines.

Finally, on the display, you PEAK L or PEAK R you see (3rd column) on the display can be confusing initially. This is showing the RMS of the signal peak. That is, the RMS value of the “strongest” frequency. The RMS field in the second column will give you the RMS of the entire signal (over the specified bandwidth). Consider the plot below, with GEN1 at 1 kHz 0 dBV, and GEN2 at 2 kHz -3 dBV. The PEAK is shown as 0 dBV (the 1 kHz) and the RMS is shown as 1.7 dB (the sum of 0 dBV and -3)

In short, everything is RMS.

Hopefully this didn’t muddy the waters!

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#3

Thanks, Matt, it actually clears up the suspicion I had. I’ve been working between the QA401 software and REW, noticed about a 3dB disparity, and figured that’s where it came from. And very useful to know that when the on-screen legend says “Peak” it means signal peak, not peak value.