Midas M32R Output Levels

LPdan

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Joined
Apr 18, 2015
Location
NY
We have a couple M32R consoles, and recently a rental customer complained that output levels were not matching the meters.
I was able to test the second console, and based on my research, agree the output is quite high. Using the internal oscillator, I can send a sine wave at a given level to the output. Console meter matches the set point.
However, when I measure the output voltage with an oscilloscope (unloaded), RMS readings are up around 2V.
Am I doing something wrong, or not understanding something? I have not been able to get through to Midas for an explanation.
 

DrewE

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Mar 18, 2019
Location
Vermont
What are the console meters showing for the output level? What units are they (supposedly) calibrated in? A decibel is, as you probably know, primarily a relative measurement between levels, and not inherently an absolute level in itself. My very vague recollection is that the X32, at least, just meters relative to its full scale, which makes a good bit of sense in the digital domain but says little about analog output levels. (My very vague recollection may be all wrong, too.)
 

MRW Lights

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Jan 4, 2017
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NYC
.... who... who is nitpicking a digital console vu meter to the point it's an issue for them? Especially an m32r? Not that in any way am I not a fan of a Midas console for what it is... and sure the meter should be a relatively accurate depiction of signal flow, but.... what's the problem? Please tell me they're mixing by meter only and not listening? Or using an RTA? I have questions.... this sounds like a classic case of I don't know the practical application of what I'm doing so I'll complain about something irrelevant to get a lower price?
 

TimMc

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Feb 15, 2017
The X32/M32 XLR outputs are capable of about +/-12V of total swing at 0DBFS.

My guess is the client was running up to "0" and found it very, very loud.
 

FMEng

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I don't think your customer knows anything about digital audio level metering and how it relates to analog levels. They probably need some coaching on system gain structure, too.

As a broadcast engineer, I always want to know how a piece of equipment is calibrated, what the meter ballistics are (LUFS, VU, PPM, instantaneous peak) what clip level is, and how that relates to the next piece of equipment in the chain. Since I have used the both the X32 and M32 for live, remote broadcasts, I know what the output level is, and I have measured it with proper test equipment, using tone. It is +4 dBu (analog) when the meter is at -18 dBFS (digital). That conforms exactly to a longtime industry standard. The meters are instantaneous, peak reading. The first, yellow meter segment is -18 dBFS. I can say with confidence that the X32 is actually quite precise and accurate, and does what the LCD display control markings and metering says it does.

I will mention that the standard of nominal level being -18 dBFS has gradually shifted to -20 dBFS, by many manufacturers. The latter allows a bit more headroom but the change is small. What's 2 dB among friends?

dBu has it's roots from the dBm in telephony and broadcasting. 0 dBu = 0.775 Vrms, which is the open circuit voltage which matches 0 dBm into 600 ohms. 0 dBm = 1 milliWatt into 600 ohms = 0.775 Vrms. Bell Labs, Western Electric, and AT&T created the standard of power referenced to 600 ohms. Broadcast facilities in the tube and transformer era ran everything at 0 VU = +8 dBm. Once IC op-amps became prevalent in the 1970s, the standard became +4 dBu because that allowed 20 dB of headroom with a balanced, op-amp output stage, and 600 ohms became bridging, high impedance. Modern equipment is bridging, not 600 ohms.

Here's a handy calculator to between dBu, dBV, and Vrms.
http://www.sengpielaudio.com/calculator-db-volt.htm
 
Last edited:

LPdan

Well-Known Member
Joined
Apr 18, 2015
Location
NY
What are the console meters showing for the output level? What units are they (supposedly) calibrated in? A decibel is, as you probably know, primarily a relative measurement between levels, and not inherently an absolute level in itself. My very vague recollection is that the X32, at least, just meters relative to its full scale, which makes a good bit of sense in the digital domain but says little about analog output levels. (My very vague recollection may be all wrong, too.)
The console is labeled only in dB, which as you state, is technically just a ratio. The data sheet has dBu for units. It says output levels are +4 dBu nominal, +21 dBu maximum. I thought +4 dBu corresponded to 1.228 Vrms?
 

LPdan

Well-Known Member
Joined
Apr 18, 2015
Location
NY
.... who... who is nitpicking a digital console vu meter to the point it's an issue for them? Especially an m32r? Not that in any way am I not a fan of a Midas console for what it is... and sure the meter should be a relatively accurate depiction of signal flow, but.... what's the problem? Please tell me they're mixing by meter only and not listening? Or using an RTA? I have questions.... this sounds like a classic case of I don't know the practical application of what I'm doing so I'll complain about something irrelevant to get a lower price?
Honestly this was my initial reaction as well. Just turn it down. But in their defense, they are trying to interface the console to streaming gear, and are trying to set matched levels. I think their main concern was knowing what they would get on the output every time.
 

LPdan

Well-Known Member
Joined
Apr 18, 2015
Location
NY
I don't think your customer knows anything about digital audio level metering and how it relates to analog levels. They probably need some coaching on system gain structure, too.

As a broadcast engineer, I always want to know how a piece of equipment is calibrated, what the meter ballistics are (LUFS, VU, PPM, instantaneous peak) what clip level is, and how that relates to the next piece of equipment in the chain. Since I have used the both the X32 and M32 for live, remote broadcasts, I know what the output level is, and I have measured it with proper test equipment, using tone. It is +4 dBu (analog) when the meter is at -18 dBFS (digital). That conforms exactly to a longtime industry standard. The meters are instantaneous, peak reading. The first, yellow meter segment is -18 dBFS. I can say with confidence that the X32 is actually quite precise and accurate, and does what the LCD display control markings and metering says it does.

I will mention that the standard of nominal level being -18 dBFS has gradually shifted to -20 dBFS, by many manufacturers. The latter allows a bit more headroom but the change is small. What's 2 dB among friends?

dBu has it's roots from the dBm in telephony and broadcasting. 0 dBu = 0.775 Vrms, which is the open circuit voltage which matches 0 dBm into 600 ohms. 0 dBm = 1 milliWatt into 600 ohms = 0.775 Vrms. Bell Labs, Western Electric, and AT&T created the standard of power referenced to 600 ohms. Broadcast facilities in the tube and transformer era ran everything at 0 VU = +8 dBm. Once IC op-amps became prevalent in the 1970s, the standard became +4 dBu because that allowed 20 dB of headroom with a balanced, op-amp output stage, and 600 ohms became bridging, high impedance. Modern equipment is bridging, not 600 ohms.

Here's a handy calculator to between dBu, dBV, and Vrms.
http://www.sengpielaudio.com/calculator-db-volt.htm
This is awesome, thank you so much for the info!!!
 

DrewE

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Joined
Mar 18, 2019
Location
Vermont
The console is labeled only in dB, which as you state, is technically just a ratio. The data sheet has dBu for units. It says output levels are +4 dBu nominal, +21 dBu maximum. I thought +4 dBu corresponded to 1.228 Vrms?
4 dBu does correspond to 1.228 Vrms. Since the maximum output is 21 dBu, that means the nominal output should occur at about -17 dB on the meter (assuming I'm remembering properly that it's relative to full scale, i.e. 0 dB is at the absolute maximum signal level) when the meter is showing the proper output signal after all faders and trims and whatnot.

If they're streaming, it might make a lot of sense to see about connecting the console to the streaming system digitally and not have to worry one bit about analog levels. That makes things somewhat simpler for setting levels; full scale on the console output is guaranteed to be full scale on the streaming system's input, and your nominal operating level is simply that less whatever headroom you're aiming for. There's also at least theoretically better audio quality since you aren't converting from digital to analog and back to digital, with the resulting noise and distortions and so forth--although in most cases that would be very much a negligible (likely inaudible) change in the grand scheme of things.
 

FMEng

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More than likely, the streaming encoder is a PC with unbalanced inputs that clip at absurdly low levels, or some kind of semi-pro mini box with equally crappy inputs.

The quick and dirty way to match levels and align meters is to assign the LR Main buss to a matrix output, then lower the level of the matrix, to act a pad. That way, the main buss can run at nominal levels, but the output signal can adjusted to what the encoder wants.

The proper way to do it is to use some sort of interface between that attenuates the levels and unbalances the connection. There are several out there. Morley (formerly Ebtech) comes to mind. If it's a PC, the best way would be to use a USB audio interface, with balanced, line level inputs.

When I stream church video, the console feeds my pro grade camera, which has XLR inputs, set for line level. The camera feeds an HDMI to USB converter. (HDMI includes digital audio.) The USB converter goes into the encoding PC. It works great because the A to D is happening in the camera, and then it stay digital from then on. I align the console level to the camera using tone, and then everything agrees.
 

TimMc

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Premium Member
Joined
Feb 15, 2017
This is awesome, thank you so much for the info!!!
The answer is the free version of the YouLean Loudness Meter. It can run as a hosted plug in (almost any DAW, Audio Hijack on Mac) or as a hostless application.

Each streaming service has a different input level and dynamic range expectation. It's not as simple as setting a peak level.
 

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