Our local cable company recently asked for an ASI feed from us to replace the analog feed we have been sending the for all the years I have been here. This was no problem as we are in the same building. But now they came to me telling me that our audio was too hot! They are monitoring all the stations with a Dolby LM 100 and they told me we were hitting a -9dB with some audio. Now remember that in the new world of DTV and digital audio it’s all referenced to dBFS or dB Full Scale, FS means all the bits are ones in the digital audio signal.
This brings us to DialNorm, the cable company is trying to get all stations to feed them audio with DialNorm set to -24dBFS, so at -9dBFS we were pretty hot! DialNorm is still pretty new and not well understood by most. Basically it’s a system to get all the dialog (voice) from different sources at the same level. In this case 24 dB down from Full Scale digital, or -24dBFS.
As a viewer adjusts their TV volume to a level where they can hear and understand the dialog on screen, then the movie ends and a promo or commercial begins,the level of the dialog
from the different sources will remain at the same level, and not be much louder or softer. DTV transmits a DialNorm number as part of the Metadata in our signal (and in AES). This tells the receiver at what level the dialog will be at and when this program was mixed the audio mixer adjusted the levels of all the sources around the dialog level, as they usually do. So the gun shot is louder than dialog and the cat’s purr is softer, but dialog is always at the same level.
Now if you tune to a different station and they are using a DialNorm different than the last
station the receiver adjusts the audio, if the first station’s DialNorm was at -27dBFS and the new one is at -24dBFS then it turns down the level by 3dB so dialog the viewer hears will still be at the same level.
Since we don’t have control of the program material that comes to us the only way to control the audio level is with a compressor/limiter. We have had one on the analog audio feed to the cable company but that was my only one and they want me to keep that on going still. So now I turned to Bill and asked for help and he came up with an Orban 424a.We put it inline with the encoder’s audio input and set it up. At first the audio level going into it was way too high, the gain reduction meters were pegged, but we took a guess and brought the output down. Then we added 10dB pads to allow the gain reduction meters, and circuit, to work in their midrange.
The only meter I had to look at was the bar graph LEDs on the front of our Wohler that was looking at the ASI decoder, as we found out this was not accurate at all. Next we went down to the cable company room and looked at their Dolby LM 100, it was now reading -50dBSF, way way too low. So with Bill upstairs at the Orban controls I watched the readout on the Dolby unit and we dialed in the DialNorm number for our station.On our encoder I found the DialNorm setting and adjusted it to -24dB, this is where the cable company was trying to get the broadcasters to be at.
As a side note the Calm Act spect’s audio to be at -24LKFS, which is a much more complex subject but about the same asa DialNorm of -24dBFS.
I found some papers on the web that help explain DialNorm and the Calm Act, feel free to read and learn more. (All are in PDF format.)
Setting AC3 Dialnorm by Glen Kropuenske of Sencore