Feature request: IEC standard dB measure...

Dear John:

How are you doing? I hope you are still kicking with REW, one of the best acoustic measurement applications out there, and it keeps on getting better. I keep on using it and I keep on getting great results.

Two months ago I found out that your method of determining dBFS RMS for sine waves and pink noise is not according to the IEC standard 61606 and also AES standard AES17-1998. I found out that you are 3 dB off from the standards when I tried to compare line level and distortion measurements made in Spectrafoo with those made by REW, on the same computer, with the same interface, so the only variable was the software.

Let me explain: For over 75 years, precision analog equipment measuring in decibels has used a sine wave for its calibration. If the equipment is peak sensitive, then the peak level of an 0.775 volts RMS sine wave was used as the 0 dB reference, so 0 dBu measured with a peak meter read the same as 0 dBu measured with an RMS or averaging meter. This produced a convenient and common reference point for analog decibel meters switching between peak and RMS. It is the way that signal to noise measurements are specified (e.g. "the RMS noise floor is -82 dBFS compared to a maximum level that can be recorded in the PCM system of 0 dBFS).

So, audio professionals were already into this standard and had (have) no problem understanding and using it. It's been intuitive for me for years and years. But computer software designers who never worked in professional audio often went for the right triangle rule: "The RMS level of a sine wave is 0.707 times its peak, so it is 3 dB below the peak level so I'll set the RMS to be -3 dBFS when the peak is 0 dBFS". But the key to understanding the IEC and other professional approach is not to think that way. To understand that a decibel REFERENCE is just that, a reference. So IEC defines the decibel reference for both peak and RMS to be the same, 0 dB. It's also true for simple averaging. In other words, a sine wave whose peak level is, say, -10 dBFS will have an RMS level of -10 dBFS. BY DEFINITION. Once you work with this approach for a while you'll see how much more convenient it becomes to calculate signal to noise ratio when using RMS measure for both the sine wave reference and the noise reference. Otherwise you will be 3 dB off of every other professional meter.

The same principle applies to marrying VU meters and peak meters for analog and eventually digital audio. For example, a sine wave at -20 dBFS (as read on a standard peak reading digital meter) was applied to a VU meter and the meter (which is an averaging meter) was set to read 0 VU. From that point it was defined that the peak to average ratio of the system could be determined by comparing the level of the peak meter with program material versus the VU meter. The average level was said to be -20 dBFS.

The vast majority of professional measurement equipment adheres to this standard so you will be in the good company of Dorrough, Audio Precision, Stanford Research, Spectrafoo, and countless others. Professionals such as myself already expect the decibel reference levels to be calibrated in this manner.

The specifications for signal to noise and distortion for IC chips, ADC and DAC are also made using the IEC standard. If you were to measure a chip with the current version of REW, you would be 3 dB off, which is quite an error.

In summary, the standard says, according to Wikipedia:
http://en.wikipedia.org/wiki/DBFS

"Peak: For the case in which the RMS value of a full-scale sine wave is designated 0 dBFS, a full-scale square wave would have an RMS of +3 dBFS.[8][9] This is the definition specified in AES Standard AES17-1998[10] and IEC 61606 and used in Dorrough meters,[7] Analog Devices[11] and Wolfson[12] digital microphone specs, etc."

I once wanted to download the standard, but it is in six parts and each part costs a couple of hundred dollars! Ridiculous, if they want to promulgate a standard it should be affordable to the user.


I started looking at the places in your system which are affected by this error: The signal generator of course, when generating a sine wave or pink noise, differs from other standard generators by 3 dB. The maximum level that you can generate you've made to be -3 dBFS because you assume that above that the peak level would exceed full scale, but as you can see from Wiki, that is intentional and understandable. So when you generate a sine wave, its RMS level can go up to 0 dBFS. When you generate pink noise, its maximum peak level cannot of course exceed 0 dBFS peak, but its RMS will be measured by the IEC standard to be x number of dB below full scale where full scale is define as 0 dBFS RMS/0 dBFS peak with sine wave.

If you have a square wave generator, at full scale peak, its RMS level will read +3 dBFS if I'm not mistaken, and this is perfectly legitimate and conforms to the standard. Since the standard was defined with a sine wave and other waves have different crest factors. Another reason for the consistency of the IEC standard. It defines which waveform shape will have the same RMS and peak level in decibels (by definition).

When calibrating SPL with a sine wave calibrator that I have like the ACO pacific, the SPL is self correcting, you will be correct, and I assume your SPL meter is RMS reading. However, the warning message you give after SPL calibration, "caution, the highest SPL you can measure is xxxx " may be off by 3 dB if and when you recalibrate to the IEC standard. I'm not sure about that, you will have to tell me since you don't know what the waveform of the external signal is. I'm feeding a sine wave from a mike calibrator.

I hope you will give a good thought to this proposition. It would be nice to be standardized and compatible so readings can be compared. Sincerely, I believe that long-time precedent, international standards and audio engineer's approach all favor the IEC/AES standard. Only a few lone wolves working in isolation have come up with a non-standard generator.

Hope this helps.

Bob