GUIDE: How To Properly Encode Dolby Digital Audio (AC3)

[SomeJoe]

Originally posted by Sunix
In Sound forge, I get these values:

RMS: -18.4 db WITHOUT use equal loudness contour option checked
RMS: -21.1 db WITH use equal loudness contour option checked

And if I run Cool Edit on the same file, I get these values:

In FS Square mode:

Minimum RMS Power: -57.74 dB -54.83 dB
Maximum RMS Power: -5.01 dB -6.13 dB
Average RMS Power: -31.11 dB -31.37 dB
Total RMS Power: -22.34 dB -22.91 dB


In FS Sine mode:

Minimum RMS Power: -54.73 dB -51.82 dB
Maximum RMS Power: -1.99 dB -3.12 dB
Average RMS Power: -28.1 dB -28.36 dB
Total RMS Power: -19.33 dB -19.9 dB


I read in a previous message that Average RMS Power from cool edit should be used when you just have cool edit as program, but these value are far away from those in Sound Forge. Even in Sound forge value change depending of selected options.

Interesting. I don't have Cool Edit, so I haven't been able to read the documentation to see what the difference is between "Average RMS Power" and "Total RMS Power". However, if you look back in this thread, previous posters who have asked me this question about CoolEdit, their Average RMS and Total RMS values were very close to each other. Yours are very different.

Also I haven't seen any documentation on what the difference is between "FS Square" mode and "FS Sine" mode, but if I had to guess I would pick FS Sine, because that seems to say that the RMS value is being computed from a Fourier series based on sine wave computations, which is how it should be done.

As far as Sound Forge goes, I'm reasonably confident that you should use the RMS value you get without the equal loudness contour.

Since the RMS value is a definite, fixed value for the file, the differences we see in values between all methods must come from algorithm differences in the way each is computed. I would say that from your file, it appears as if Cool Edit's FS Sine, Total RMS Power is the closest algorithm to Sound Forge's RMS without equal loudness contour.

Because of that, I would say your appropriate DialNorm setting is -19 dBFS.

If you or anyone else has appropriate documentation from CoolEdit on an explaination of their reported RMS parameters, I'd be able to make a more educated recommendation as to which value to use. Also keep in mind that the true setting of dialnorm is the LAeq level -- using RMS is our low-budget, attainable, close approximation.

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Edit:

I went and looked at the help file for Sound Forge and found this regarding the equal loudness contour:

Checking the Use equal loudness contour option causes the scan to take into effect the Fletcher-Munson Equal Loudness Contours. Essentially, very low and high frequency material is less audible than mid-range audio. This option forces the scan to weigh this factor into the RMS calculation.

In view of that, I would recommend that you use the equal loudness contour setting when performing a scan for normalization. This is contrary to my above recommendation.

Also in view of that, I would then change my recommendation for your DialNorm level. Since Total RMS Power, FS Sine from CoolEdit is -19 dB, while RMS, equal loudness contour enabled level from Sound Forge is -21 dB, you could use either with confidence or use their average of -20 dB. A +/- 1 dB error will not make a very (if any) audible difference in the resulting AC3, and indeed the error imposed by using RMS instead of true LAeq could be more than this anyway.