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Digital audio clipping

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Matt JonesDigital audio clipping
by on Feb 17, 2009 at 5:04:53 pm

The day that we have been waiting for has arrived!
No, it's not the end of analog broadcast. It's the capability of my local ABC and CBS affiliates to accept digital files for broadcast playback. Unfortunately, I'm having some problems with audio clipping.

I submitted 6 spots for air and received complaints from the stations that the audio levels were too hot. I went back and checked the levels on the files I gave them and they are all under 0db. I checked them in Soundtrack Pro as well as the M100 timeline.

One station told me that all audio levels must be below -12db. What?!? How can that be? Why would it be so low? Should I just mix as normal, and then compress the crap out of the final audio?

Thanks,

Matt





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Floh PetersRe: Digital audio clipping
by on Feb 17, 2009 at 5:17:48 pm

[Matt Jones] "One station told me that all audio levels must be below -12db. What?!? How can that be? Why would it be so low? Should I just mix as normal, and then compress the crap out of the final audio? "

Oh man, that´s a topic. And as far as I can tell it even varies between the countries.
Here in Germany we have a conclusion that digital audio for broadcast (e.g. on DigiBeta) is not allowed to exceed -9dBFS (-9dB digital level) at all. This corresponds to 0dB on our analog RTW peakmeters (although there are sometimes too slow to catch digital peaks, so we add a hardware broadcast audio limiter at -9dBFS). But e.g. CDs are mastered to 0dBFS, so if we import a track from CD into Media 100 we have to level it down at least 9dB (if we want to have it at full volume).
This all works as long as you stay digital (e.g. you use SDI embedded audio or AES/EBU). If you use DA-converters somewhere (e.g. the ones on the Kona board, or as a standalone hardware device), you have to make sure that they match your levels. Unfortunately level for the conversion from digital to analog is not specified for all situations identically (e.g. as I said in Germany it is said to be 0dB analog to -9dBFS digital for TV broadcast), so e.g. the Media 100 LHe systems do have a pref setting for the digital to analog audio level conversion.
Anyway, if your broadcaster says that you are not allowed to exceed -12dBFS you need to adjust accordingly. And best would be to have a hardware peakmeter attached to your Kona outputs to monitor these levels, since the VU Meters in Media 100 are not really exact, as far as I can tell. Then you need to find out how your digital and analog levels do correspond, and then do your mix accordingly.
Or do lower your levels in Soundtrack (not by applying a compressor as you suggested, but by adding some compression and by reducing the master output level).
Your digital audio files will be below 0dBFS anyway, since digital audio cannot exceed 0dBFS. Everything that would be above 0dB will be clipped digitally; that´s why the levels have to be set below this to avoid digital distortion and harsh clipping, and to have some headroom for "accidents".



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Matt JonesRe: Digital audio clipping
by on Feb 17, 2009 at 7:14:22 pm

Thanks for your response Floh. Sorry, but I'm still not sure I understand.

The bottom line is that if I want to deliver a digital spot to this station, my files must meet their requirements for broadcast. I'm just trying to understand why they ask for their digital audio levels to be set so low.

They specify -12db limit on spots delivered in H264 or DVCProHD quicktime formats.

So my producer supplied a digital VO file in .mp4 format. It does not exceed 0dBFS. There's no D/A conversion happening here. I import the file into M100HD, cut it to exactly 30 sec, add some video and output a ref file out to After Effects. I do my graphics comp in AE and then save my file as a DVCProHD file with 48K audio for the stations. The file still does not exceed 0dB.

So 0dB on the supplied digital file should equal 0dB on my final digital file, right? -12dBFS sounds like a lot of headroom for 'accidents' If I'm providing a digital file in the server's native playback format, where could the "accidents" happen?









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Floh PetersRe: Digital audio clipping
by on Feb 17, 2009 at 7:19:22 pm

[Matt Jones] "So 0dB on the supplied digital file should equal 0dB on my final digital file, right? -12dBFS sounds like a lot of headroom for 'accidents' If I'm providing a digital file in the server's native playback format, where could the "accidents" happen? "

If -12dBFS is what they accept as their broadcast level limit, that´s what you have to deliver. We have -9dBFS here in Germany, but I know that it is different in the US. Not sure if it is -12, though, but it sounds like it may be the fact. The reason for this is that some years ago most companies have agreed on a digital level (be that from tape, like from DigiBeta or from a digital file), and that it is extremely difficult to change an already existing convention on something like this. Many workflows would break if somebody would decide to now use 0dBFS as his max work level, since most broadcast equipment expects to get a much lower level.



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