Broadcast audio spec: -20dbFF = 0dbVU

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Flintstone

Well-known member
I want to make sure that all my productions are broadcast legal as far as audio levels goes. I've worked at a community cable company for a while, and I'd like to find out if this standard is really a standard.

When recording on, say a BetacamSP deck, the standard 1KHz tone, when adjusted properly, should read 0VU on the needles. This leaves about 3 to 5 db room for peaks before distorting. That's fine for the analog world.

However, I find it difficult to comprehend when going digital. From what I'm told, and from what I gather from Avid NLEs, 0dbVU is translated to -20dbFS (full scale) on the digital side. Which means that the standard 1KHz tone is actually -20db on the timeline. Avid's audio meters show this quite clearly, however, Final Cut and Premiere Pro don't.

This means that on average, my audio mixers on FCP and PPro should read about -20db, with peaks not exceeding –15, right? This seems like such a waste of bandwidth. If on a BetacamSP, there are about +6db overhead above 0VU before clipping, shouldn't the tone be -6dbFS? Mixing up things even more, PPro's default tone is set at -12db!

So what’s the real standard? What’s the proper way to work audio in PremierePro? At what levels are Hollywood DVDs mastered in?
 
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Unfortunately there is no standard, though what you describe seems to be generally acceptable. PBS for one has some strange and out dated specs that they tend to be very strict about. The historical reason for the low levels is that broadcasters can get heavily fined and or loose there license if the "over modulate" because it interferes with other stations. This is largely BS now with digital processing but... The reason it is still important to you is that A) they might reject your show if it doesn't meet their specs and B) if you are hot the stations limiters will slam your track and make it sound like, well like really bad AM pop music. The best thing is to get the specs from the network you are submitting to.
SK
 
Ahhh, a meaty question. :happy:

The reason this is confusing is there is not a direct correlation between analog and digital dB. There needs to be a rosetta stone to translate between the two. Enter the infamous bars & tone. In the BetaSP days the mixer would provide a 0db tone that different types on analog equipment could use as a reference to set up their equipment to play your material optimally. If not done correctly, the playback levels would be distorted or buried in noise. One piece of analog equipment might have as high 15db above zero before distortion was unacceptable while another peice of gear could only handle 5db above zero (in fact what we are talking about is the actual voltage handling ability of a piece of equipment). It was a messy world.

Enter the digital realm where -20dbfs always plays back at the same db level. Between digital equipment no translation is needed and thus our world was made easier. Now just how to translate the imprecise world on analog to the precise world of digital. A general rule was agreed upon that if the 0db tone was set at -20dbfs then anything record on analog equipment would fall under -0dbfs. The issue being the top end of analog is not the same hard ceiling that digital contains. Some analog mixers would record very hot, always pushing the limits of distortion while others would ride it lower.

Since the relationship is not absolute you can cheat the analog signal hotter if you so desire and know the signal in question won't peak out higher then say 5db above zero.

In practical terms, if you are talking about taking a digital recording (DV) and having it transferred to BetaSP then just keep all your levels below 0dbfs (you have no choice), put a -20db tone at head of tape, and the engineer who transfers your tape will have a way of making sure the digital signal does not run into the distortion levels of the BetaSP tape.

As to how you set your peak averages in the digital realm, that is a subjective choice. Some start with the loadest sound (say an explosion or some music) and work down from there, mixing to taste.

Hope that helps.

Cheers
 
Ouch! So basicaly, no standard. I was hoping there would be... oh well! In this case I won't go at -20db (don't want to wast the audio dynamic range).

Thanks a lot for this explanation, it put things a little bit more in perspective.
 
Wait. The point is there is a standard for tone. Technically you can put the tone anywhere and as long as the engineer setting up your tape knows what it is you will be fine. But where you put the tone is of no consequence to what your transfer will sound like as long as the engineer set it for the same level. And for this the standard is -20dbfs. Even so, make sure you write on the tape itself that the tone is at -20dbfs.

After you have the tone, you can mix your digital audio tracks to whatever you like.

Perhaps you have noticed that when playing a Hollywood DVD you need to turn up the volume compared to broadcast TV. TV pretty much compresses the hell out of all it's audio, killing the dynamic range, but making everything loud and clear. Hollywood DVD will keep the dialog averages down around -20 to -15 so they have room to blow your socks off with the big explosion. So it's not wasting the db but saving it for louder sounds.

Cheers
 
Not quite. Broadcasters have very specific "standards" they just aren't the same. While your tone should be at your 0 VU you can't just go as high as you want. One spec I saw not too long ago required average dialog level at -12 dBfs and peaks no higher than -6dBfs. Often they will accept higher levels and just squash anything over the spec but sometimes they will reject it and you have to remix. So find out what the specs are.
SK
 
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