Reference Level for Broadcast Audio

Steve House · October 31st, 2007, 11:39 AM

Quote:

Originally Posted by Brooks Harrington

'The Truth' is visually right here.

What's the source of that chart? Everything looks right to me except the SMPTE bar - it appears to be offset downward. According to all my info, -20dBS is supposed to correspond to +4dBu. (Holman, "Sound for Digital Video", pp144-145 among many others)

Brooks Harrington · October 31st, 2007, 12:03 PM

Steve, I got it here from SoundonSound.

http://www.soundonsound.com/sos/oct0...cles/qanda.htm

Now I need to cross ref this.

Jimmy Tuffrey · October 31st, 2007, 06:21 PM

In England we have always had the PPM as king and set 1K tone at 0db or -18 digital.

We then peak at +8db or -10 digital.

Gives you 10 db headroom in the digital domain.

The PPM meters are very fast but stil take about 10ms to register so the apparent peak of +8/-10 might in fact be +10/-8. Still gives one 8db of headroom in the digital domain.

I think digibeta records at 20 bit and no one seems to mind having 8 db of headroom. You could if you were recording to cheaper, (semi pro) equipment peak a bit higher if you feel secure with your levels ( say you have limiters).
Personally I would be happy with less of a safety margin but the standard came about to avoid mistakes and was set with a high safety factor.

The above chart is very helpful.

In the old days of 1/4 inch tape we set our tone, (100hz, 1khz, & 10 khz) at PPM 4 ( 0db) and at -4vu on the revox1/4", 1"vt, beta SP etc. And peaked at an apparent level of 8 db above tone. (+8 db or PPM 6)

Jimmy Tuffrey · October 31st, 2007, 06:46 PM

Just read the Hugh Robjohns Sound on Sound article and although complicated it is the definative article.

He says that the PPM could be letting through as much as 4 to 6 db during that initial 10 ms before it can respond.

(In the old days the PPM was considered a very fast meter and VU was slow).

That would mean peaking at +8db / -10 digital could really be hitting +14db / -4 digital.

That would mean only 4 db of safety/headroom in the digital standard which makes sense.

So there we have it.

My question is....

Are the digital meters telling the truth then?

The SOS article states that they read the sample values and not the combined value at the DA which indicates that the need to leave the stated headroom in digital systems is neccessary to avoid clipping. I think we can asume that the lowest levels of clipping are not actually noticed by us but are none the less there and that what seems like excessive safety margins are in fact required for complete fidelity. ( ie 0% clipping).

Funny how this debate rages on these forums and never seems to go away. And we all thought digital would be straight forward compared to anologue!

Chuck Fadely · November 5th, 2007, 09:10 PM

I've read this thread three times and am still confused.

We're doing web video and trying to match volume levels to pre-roll ads, which I'm assuming are broadcast level at some point in their lives.

We're editing in Final Cut and can't get near the audio volume of the ads without clipping. We're not going to tape but straight to the web. I keep pushing the levels up higher and it ain't enough.

Can anyone point me to some sort of reference for this kind of work? We've experimented with Levelator and 'normalize' in Soundtrack but it sure slows things down for news work.

thanks much

Ty Ford · November 5th, 2007, 09:46 PM

Hello Chuck,

Here's the dirty little secret of audio production. You're right. You can't use gain to make other content sound as loud as spots. The reason is that most spots are compressed and then limited.

Next time you have your editing program open use the waveform view. Notice that the spots are more like thick rectangles, whereas the other audio is not as dense. That's what compression does.

Do all of us a favor and knock down the level of the spots a bit and compress and limit your other audio a bit until they match.

Regards,

Ty Ford

Chuck Fadely · November 7th, 2007, 03:55 PM

Thanks, Ty. We're feeding local video into a national product, so we can't take the ad levels down.

Is there some way to compress and limit our audio within Final Cut -- a plug-in or hardware add-on? I see compressors in the catalogs, but they seem to be designed for analog-to-tape workflow?

Ty Ford · November 7th, 2007, 04:27 PM

Yes FCP has that in audio filters and if you don't find satisfasction there, Sountrack Pro does as well. Just remember NOT to process the spot audio.

Regards,

Ty Ford

October 31st, 2007, 06:21 PM	#18
Jimmy Tuffrey Major Player Join Date: Jul 2007 Location: Bristol U.K. Posts: 244	UK Style In England we have always had the PPM as king and set 1K tone at 0db or -18 digital. We then peak at +8db or -10 digital. Gives you 10 db headroom in the digital domain. The PPM meters are very fast but stil take about 10ms to register so the apparent peak of +8/-10 might in fact be +10/-8. Still gives one 8db of headroom in the digital domain. I think digibeta records at 20 bit and no one seems to mind having 8 db of headroom. You could if you were recording to cheaper, (semi pro) equipment peak a bit higher if you feel secure with your levels ( say you have limiters). Personally I would be happy with less of a safety margin but the standard came about to avoid mistakes and was set with a high safety factor. The above chart is very helpful. In the old days of 1/4 inch tape we set our tone, (100hz, 1khz, & 10 khz) at PPM 4 ( 0db) and at -4vu on the revox1/4", 1"vt, beta SP etc. And peaked at an apparent level of 8 db above tone. (+8 db or PPM 6)

November 5th, 2007, 09:46 PM	#21
Ty Ford Inner Circle Join Date: Nov 2004 Location: Baltimore, MD USA Posts: 2,337	Hello Chuck, Here's the dirty little secret of audio production. You're right. You can't use gain to make other content sound as loud as spots. The reason is that most spots are compressed and then limited. Next time you have your editing program open use the waveform view. Notice that the spots are more like thick rectangles, whereas the other audio is not as dense. That's what compression does. Do all of us a favor and knock down the level of the spots a bit and compress and limit your other audio a bit until they match. Regards, Ty Ford __________________ Ty Ford's Audio Bootcamp Field Guide Ty Ford's Audio Video Blog

November 7th, 2007, 04:27 PM	#23
Ty Ford Inner Circle Join Date: Nov 2004 Location: Baltimore, MD USA Posts: 2,337	Yes FCP has that in audio filters and if you don't find satisfasction there, Sountrack Pro does as well. Just remember NOT to process the spot audio. Regards, Ty Ford __________________ Ty Ford's Audio Bootcamp Field Guide Ty Ford's Audio Video Blog

October 31st, 2007, 12:03 PM	#17
Brooks Harrington Regular Crew Join Date: May 2007 Location: Pasadena, CA Posts: 152	Steve, I got it here from SoundonSound. http://www.soundonsound.com/sos/oct0...cles/qanda.htm Now I need to cross ref this.

October 31st, 2007, 06:46 PM	#19
Jimmy Tuffrey Major Player Join Date: Jul 2007 Location: Bristol U.K. Posts: 244	Just read the Hugh Robjohns Sound on Sound article and although complicated it is the definative article. He says that the PPM could be letting through as much as 4 to 6 db during that initial 10 ms before it can respond. (In the old days the PPM was considered a very fast meter and VU was slow). That would mean peaking at +8db / -10 digital could really be hitting +14db / -4 digital. That would mean only 4 db of safety/headroom in the digital standard which makes sense. So there we have it. My question is.... Are the digital meters telling the truth then? The SOS article states that they read the sample values and not the combined value at the DA which indicates that the need to leave the stated headroom in digital systems is neccessary to avoid clipping. I think we can asume that the lowest levels of clipping are not actually noticed by us but are none the less there and that what seems like excessive safety margins are in fact required for complete fidelity. ( ie 0% clipping). Funny how this debate rages on these forums and never seems to go away. And we all thought digital would be straight forward compared to anologue!

November 5th, 2007, 09:10 PM	#20
Chuck Fadely Major Player Join Date: Jul 2006 Location: United States Posts: 306	I've read this thread three times and am still confused. We're doing web video and trying to match volume levels to pre-roll ads, which I'm assuming are broadcast level at some point in their lives. We're editing in Final Cut and can't get near the audio volume of the ads without clipping. We're not going to tape but straight to the web. I keep pushing the levels up higher and it ain't enough. Can anyone point me to some sort of reference for this kind of work? We've experimented with Levelator and 'normalize' in Soundtrack but it sure slows things down for news work. thanks much

November 7th, 2007, 03:55 PM	#22
Chuck Fadely Major Player Join Date: Jul 2006 Location: United States Posts: 306	Thanks, Ty. We're feeding local video into a national product, so we can't take the ad levels down. Is there some way to compress and limit our audio within Final Cut -- a plug-in or hardware add-on? I see compressors in the catalogs, but they seem to be designed for analog-to-tape workflow?