- December 21, 2011 at 11:49 pm
Has anyone else had a difference in levels between the analog and digital audio outs? I’m using the ATEM 1 M/E switcher with the software control in a church and can’t figure this out. Audio is mixed on a mixer with peaks at 0 on the VU meter. From the mixer it goes into the ATEM via the analog XLRs. The analog XLR outs go into a DA that feeds several consumer HDTVs. Digital audio comes out the program HDMI (with the video) and goes into a Matrox MXO2 Mini that is used with a laptop for streaming on our website. The problem is I seem to either have good analog levels and the digital audio is really low (peaks at -20dbfs on the Ultrascope) and barely audible on the stream, or I boost the audio out and get good digital levels (peaks at -8 dbsf on the Ultrascope) but all the tvs are overloaded. It’s not the MX02 Mini because when I hook a camera up to it instead I get good audio levels on our streaming. Also, does anyone know what level +/- db the ATEM is expecting for audio in? I can’t find any info on this at Black Magic.
- December 22, 2011 at 4:27 am
0 VU in a digital world is typically around -20dbfs. So, working as intended.
Jack up the input levels on your streamer rather than on the output of your board, you should have less of a problem with overmodulation and get the ‘loudness’ you’re looking for.
- December 22, 2011 at 3:45 pm
The steamer (Adobe Flash Media Live Encoder) already has the volume up as far as it will go. A camcorder (HV20) hooked directly via HDMI works great. It must be sending out a hotter audio signal? As for the ATEM levels, during a event most of the time they are between -40 and -30 dbfs. Only peaks hit -20 dbfs. This seems low to me? When I edit, I mix to have most sound hitting from -20 to d -12 dbfs with extremes hitting -6 or so.
- December 23, 2011 at 2:18 pm
Just a bit of background to help you interpret what’s going on…
Unlike most units of measurement, decibels are a relative measure, always compared to something else. It’s not like measuring X volts or saying something is N inches long; rather, it’s like saying a signal is three times more than… something. So a reading in decibels is not just the bare number; you also need to know what that “something” is — the reference.
In the world of analog equipment, one use of decibels is to describe the voltage of an audio signal; the reference used is the amount of voltage that you have when the signal delivers one milliwatt of power into a 600 ohm load. That measurement is usually shown as dBm (the “m” signifying “milliwatt”). Most professional analog audio gear nowadays uses +4dBm as the nominal operating level; back when I started in television, +8dBm was the more common reference level. Generally speaking, most analog equipment is capable of handling peak voltages around +24dBm, which means there’s about 20dB of headroom above the nominal operating level before clipping begins. The exact point of clipping varies from device to device, so a systems designer needs to be mindful to avoid serious mismatches that would lead to early distortion.
Now if you look at a piece of analog equipment with a VU meter — a console or tape recorder will do — you’ll see a scale in decibels with zero about two thirds of the way to the right. This is measuring level with respect to the nominal operating level. So if you have a mixer that has a +4dBm reference level as its standard, you will see +4dBm when the meter reads 0; drop the level so the meter reads -10dB, and the output voltage will drop to -6dBm. Raise the level to +6dB, and the output becomes +10dBm. Where analog VU metering falls short is in representing peaks; very few analog meters go anywhere near the typical clipping point of +20dB VU. But in the analog world, it’s pretty unusual to spend any real time there; moreover, a lot of analog devices can handle these overloads fairly gracefully.
Digital equipment, on the other hand, does not suffer overload well at all. There’s no room for “just a little bit more”; once the numbers hit the maximum value, that’s as far as things will go. And voila: instant clipping. This is such a serious issue that a great many digital devices measure audio in terms of that brick wall limit: and the measurement is dBfs, decibels with respect to Full Scale. Since the reference is the maximum possible value, that makes all dBfs measurements a negative number (which is the first thing that tends to hang people up). As a general case, most digital gear makes the assumption that the normal operating level is 20dB below the maximum, so for these devices -20dBfs will show as 0 VU, and will produce +4dBm on an analog output.
So… how does this play out in the real world? If you have a typical system with a mix of analog and digital equipment and throw tone on your mixer so that the VU meter shows 0dB, you should expect to measure a voltage of +4dBm on an analog output; feed it into a digital recorder, and you should see a reading of -20dBfs. This is entirely normal, and is exactly what you should be shooting for.
I could go on about the differences between average versus peak metering, but that’s more intuitive and not directly pertinent to the issue you’re having.
Engineering Project Manager
Newport Television, Northeast
- December 23, 2011 at 4:18 pm
Thanks for the in-depth explanation Jeff. I ran tone and that is what I was getting. I think I have narrowed the problem to the MX02 Mini/Flash Media Encoder Live combo. Somewhere there the audio is getting mismatched.
By the way, if anyone wanted to know Blackmagic said the 1 M/E is balanced +4DB audio via the XLR cable.
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