J-Test needs a change

[JEJ]

The J-Test feature in the REW software seems to have a non-adjustable level setting of 0 dBFS. This test is supposed to have the level set at -3 dBFS. If you still want to have 0 dBFS, then allow -3 dBFS as a choice.

 

[John Mulcahy]

The signal levels are correct as they are. Perhaps the "Full scale sine rms is 0 dBFS" View preference is making you think otherwise.

 

[JEJ]

So, the 11.025 kHz sine wave is at -3 dBFS?

 

[JEJ]

Here is a screen shot of the J-Test generator panel. It says 0 dBFS RMS and -3.01 dBFS PK (Peak). Peak should be the higher level, not the lower.

 

[JEJ]

1. RMS vs. Peak Levels​

• 0.00 dBFS RMS means the sine wave’s root-mean-square amplitude is normalized to full scale (maximum digital headroom).
• For a pure sine wave, peak = RMS × √2.
  That translates to:
  
  0 dBFS RMS    →    +3.01 dBFS Peak0 \, \text{dBFS RMS} \;\; \rightarrow \;\; +3.01 \, \text{dBFS Peak}0dBFS RMS→+3.01dBFS Peak
  (since 20log⁡10(2)≈3.0120 \log_{10}(\sqrt{2}) \approx 3.0120log10(2)≈3.01).

So if you define 0 dBFS at RMS, the peak must read +3.01 dBFS, not –3.01 dBFS.

2. What REW is Showing​

The panel says:

• 0.00 dBFS RMS
• –3.01 dBFS Peak

That’s contradictory. A sine wave at 0 dBFS RMS cannot have a peak lower than RMS. What’s likely happening is:

• REW internally defines full scale as peak = 0 dBFS, not RMS.
• When you select “dBFS RMS,” REW is re-labeling, but the relationship isn’t adjusted correctly in the peak display.
• So the –3.01 dBFS Peak you see is really REW saying:
  “The peak of this sine is 3 dB below what I call full-scale peak = 0 dBFS.”

3. Correct Interpretation​

• If full scale is defined by peak = 0 dBFS, then a full-scale sine wave will measure:
  
  • –3.01 dBFS RMS (because RMS is lower than peak by 3 dB).
• If instead you define RMS = 0 dBFS, then the same sine should read:
  
  • +3.01 dBFS Peak.

REW seems to be mixing those conventions in the J-Test panel.

 

[John Mulcahy]

Look in the View preferences at the Full scale sine rms is 0 dBFS setting.

Full scale sine rms is 0 dBFS determines whether the rms level of a full scale sine wave is show as 0 dBFS or -3.01 dBFS. If this option is selected the rms level may be greater than 0 dBFS for some signal types, for example, a full scale square wave would have an rms level of +3.01 dBFS. Note that this applies to rms levels, not sample values. The peak is a sample value.

The Audio Engineering Society adopted the convention that a full scale sine should indicate an rms level of 0 dBFS. It is commonly used in test equipment. You can choose to use it or not, that's why there's an option.

 

[JEJ]

OK, I changed the setting, but I cannot see the value of having that option, which ends up delivering the wrong signal level. How can I set the Y. axis to dBr or dBrA which is the standard for the J-Test? And, can I normalize the 11,025 kHz peak to 0 dBr or 0 dBV?

 

[John Mulcahy]

I cannot see the value of having that option, which ends up delivering the wrong signal level.

That option does not change the signal level.

Axis settings: