10 Replies Latest reply: Oct 25, 2007 7:18 AM by Paul Bissell
Paul Bissell Level 1 Level 1 (45 points)
This is quasi-OT but since AD converters feed directly into Logic and Logic has no real say in the matter, this would made a good deal of difference...

I know that the MOTU 828 converts at 0VU (.775 not 1.23v) as -18dBFS. I was wondering if there was a website that has a listing of other AD converters and their equivalencies from 0VU to dBFS. Interestingly, I had to do a test myself as the 828mkII manual doesn't list or mention this. Doesn't that seem odd? I mean, that is EXACTLY what an AD does so it seems the scale it uses would be really important.

Paul

Mac OS X (10.4.5)
  • 1. Re: 0VU conversion information
    David_Pye Level 4 Level 4 (2,325 points)
    I know that the MOTU 828 converts at 0VU (.775 not 1.23v) as -18dBFS.


    This doesn't totally make sense. 0VU is just a figure on a particular metering standard, the 828 doesn't convert at this level, it converts ALL levels going into it, hence it works as a digital converter.
    The meters on the front would be the only debatable factor, they could display information in VU rather than dBfs, but as far as I've experienced they don't and if they did the meters wouldn't relate to what you see on screen that well. I have a MOTU 896 and I know the metering is in dBfs for sure, as in 0dB is a where the signal clips if it goes over. If it were in VU it would have a scale above 0dB, maybe up to +6/12dB.

    You may be getting confused with -10dB/+4dB reference standards, but the MOTU again is capable of both, it's switchable in the Cuemix software.
  • 2. Re: 0VU conversion information
    David Robinson9 Level 4 Level 4 (1,990 points)
    hi,
    in the good ol daze of analog, it was fairly important to match ins and outs.
    noise, you see.
    with digital however, this noise problem doesn't exist as much.
    sure you've still gotta getta good level in, but, once inside noise is no longer an issue.
    mic pres are the biggest issue with modren i/o boxes.
    most are substandard. motu's are not great.
    apogee's are much better, IMO.
    still, the best way, i think, is to use a dedicated mic-pre. then send +4 line level from it to your i/o.
    bottom line, if you are getting a distorted smeared sound out of your motu, its par for the course.
    upgrade your pre, or try Black Lion.
    DR9.
  • 3. Re: 0VU conversion information
    Paul Bissell Level 1 Level 1 (45 points)
    I'm not confused - far from it. If you don't understand what an Alignment Level between 0VU and dBFS is then just say so - if not, go ahead and Wiki it.

    Of course it converts "all levels going into it" however what level will it convert it at.
    The meters on the front of the 828 and 896 are in dBFS. That isn't the issue. When you give it a signal from your mixer or whatever into the analogs in (which have no gain adjustment minus the +4 switch in the Cuemix) then the MOTU has to use some standard to convert voltage (which is what an analog audio signal actually is) and dBFS. There are two 'standard' alignment levels EBU and SMPTE which are 0vu = -18dBFS and -20dBFS respectively. If you set up a tone coming from your source (say a mixer) and it shows 0VU on the mixer's needles or is stationary on the master PPM at 0, then you are sending (with a +4 mixer) 1.23volts out of the output. When that 1.23 volts goes into the MOTU, it has to converter that signal - well, how LOUD is 1.23 volts (aka OVU on a +4 mixer)? Again, there are 2 standards officially but the MOTU is different than those at being 0dBu (which is .775volts) = -18dBFS.
    Do the test yourself and you will understand. If you have a real +4 mixer, that means that when your VU meter or PPM is lined up on 0, it is sending 4dB over 0dBu which is 1.23 volts (that is why they call it a +4 device). If you have a Mackie mixer or some Presonus preamps, then when they show 0 on the meter or PPM, you are sending out an audio signal of .775 volts. They clearly mark on the units that 0VU = 0dBu to tell you that. Finally, if you have a -10dBV mixer or preamp, then when they show your tone at 0, they are sending out .316 volts. When you switch between -10 and +4 in the Cuemix, you are engaging a small preamp into the MOTU to bring up the level of the -10 signal (by the .914 volts they differ).
    Back to the issue at hand - the alignment level of converters is not a normal published spec as I think it should be since that is what it does. I also thought that all converters would be in the -18 or -20 standard camps. When I tested the MOTU I was surprised to find it didn't fall into either. However, since I am feeding it with a device that meters at 0vu = 0dBu, having a converter that is 0dBu = -18dBFS is fine with me. I was just looking to see if anyone has done it on a variety of devices and see where they fall.

    Paul
  • 4. Re: 0VU conversion information
    David_Pye Level 4 Level 4 (2,325 points)
    Hi Paul,
    Sorry, I didn't mean to sound patronising. A lot of people that find their way onto this board are, and often only need a simple explanation of the things they're asking. I personally don't use my 896 in conjunction with a desk using VU's, I use it with stand alone preamps with no metering. So sorry again but I can't help you.
    What you've raised is interesting though, and I'd like to know what you learn on this. I expected my 896 when the inputs are set to +4dB that 0VU would equate to -18dB.
    I do remember using a Tone from the studio's Digidesign 96 i/o to align our tape machines input and found that 0VU on the tape deck equalled -12.4dBfs on the meters in protools.
    What level are the meters in Logic reading when you send 0VU to it?
  • 5. Re: 0VU conversion information
    Paul Bissell Level 1 Level 1 (45 points)
    Hi David,
    Please accept my apology for sounding like a jerk. I too find this message board fantastic and pedantic at the same time. If my post was "Logic versus Pro Tools" how many THOUSANDS of responses would I have gotten? Again, my apologies.

    Through the 828, a 0VU tone is showing -18dBFS in Logic's meters - BUT that tone is 0dBu and not +4dBu since I don't have a +4 source. I would like to say that a 0VU tone at +4dBu would be -14dBFS but since I don't have a 'real' +4 mixer or preamp, I can't.

    I can say however that pushing my master fader to make the needle read +4VU (again on a 0VU = 0dBu mixer) does make Logic read -14dBFS. That I can prove and thus can infer that your 896 would be reading -14 and not -18dBFS if presented with 1.23 volts of love. I was surprised when I did my first test - I totally thought that the MOTUs would be 'standard'. This new finding has led to all types of research on this and that explains my initial post. I am going to have a few of my students bring in their M-Boxes, Firepods, etc and other portable devices so I can do the test on those converters even though I don't own them. I figured all the US manufacturers would go -20dBFS and the Europeans would take -18dBFS as per SMPTE and EBU respectively but obviously that isn't the case.

    I can confirm that the meters on the front of the 828 and logic's meters (measured on the record engaged channel) are within .1dBFS of each other in both record and playback.

    Paul
  • 6. Re: 0VU conversion information
    David_Pye Level 4 Level 4 (2,325 points)
    I thought, I may have this wrong, but when operating at +4 0VU should equal -18dBfs?
    Not +4VU = -18dBfs??

    If you think about two digital devices operating at +4 0dBfs should equaly 0dBfs, you shouldn't have to push one up by 4dBfs to make the other equal 0?
  • 7. Re: 0VU conversion information
    Paul Bissell Level 1 Level 1 (45 points)
    Like I mentioned, EBU says -18dBFS and SMPTE says -20dBFS. Like any standard, I guess there is wiggle room and reasons for not complying' with an unenforceable rule.
    Anyway - I don't have a +4dBu device to send a 0VU signal from. I do have two 0dBu devices however - a Mackie 3208 mixer (real VUs and PPMs) and a Presonus Eureka (real VU). For them, a VU setting (aka the meter says) 0 would send out 0dBu. Thus if one pushes up the master fader to make the signal read +4 on the meters, it would be the same as 0VU on a +4dBu device. Either way you are sending 1.23volts. Converters don't know what they are hooked up to and they don't care. They just get a voltage signal and are converting that to digital. They have some sort of reference they are going to use to assign a voltage reading to digital. I am saying that the 828 doesn't use the two 'standard' references, but its own 0dBu = -18dBFS. I sent it .775volts and it gave Logic digital bits of -18dBFS.

    I'm sorry, I don't know what you are saying with your second sentence.

    Paul

    My DBX 386 (micpre with A/D conversion) is a -20 device.
  • 8. Re: 0VU conversion information
    mattrixx Level 3 Level 3 (940 points)
    Just thought I'd share my little story about metering. It's not that exciting really.
    I used to work in a production house that had all these analog VU boxes floating around. It was great, as I was able to hook one up to some spare outs of my Fireface800 and route my program signal to it using the Fireface mixer software. Great, it gave me the metering that I was used to.
    Anyway, I moved out of there some time ago and was without my meters....
    The next step was to get a PC box and stick a spare RME multiface in it.. This then allowed me to run, in much the same fashion, the fabulous RME Digicheck software. Among the many metering options in it, is 4 bar meter that has PPM's in the middle and VU's on the outside. It has a switchable K referencing system that is great for the various level formats that have been discussed in the thread here already. (you can also run multiple instances of Digicheck, setting up all sorts of other meters as shown below)

    In short, I personally believe that good metering is really important. It can greatl assist in consistancy and energy.

    !http://www.users.on.net/~mattrixx/Digicheck.jpg!
  • 9. Re: 0VU conversion information
    David_Pye Level 4 Level 4 (2,325 points)
    Although much smaller, Roger Nichols Inspector give some of the same information if you don't have a spare PC anywhere nearby.
  • 10. Re: 0VU conversion information
    Paul Bissell Level 1 Level 1 (45 points)
    Ok - end of the line. Thanks for the responses. Here it is:

    According to the EBU and SMPTE guidelines 0dBu = -18 or -20dBFS respectively. That would mean .775 Volts.

    The start of my search for knowledge was in error as I read 0VU = -18 or -20dBFS. That of course got me started on my 'whose ZERO are we using? 0vu=0, +4, +8, -10?? +X???

    So the MOTU 828 is right with the EBU standard with 0dBu = -18dBFS. Still curious why not the SMPTE standard since that is the American version. Oh well.

    However this does mean that those with +4 mixers or preamps etc that are connected to converters will get 0VU = -14 or -16dBFS. I am not one, so no problems here but -14 seems really tight. Whatever.

    Here however is my final chunk of grey matter. My DBX 386 Preamp A/D converter has a row of LEDs showing printed dBu values on the top and dBFS on the bottom (they share the same LEDs ). Over one of the lights is 0dBU and below the same light is -20dBFS. BUT, printed in the manual itself on the last page it states "0dBu = -18dBFS". So...either the unit's silk screening or the manual stating the alignment level is wrong.
    Curious.

    Thanks again all!

    Paul