Internal gain structure of a console

[PoorGlory]

OK, I am having issues with my board, and I have tracked down what I think is the main problem. When I send a CD player (-10 outputs) into my tape ins (-10 input) and put the 2 channels faders at unity, I need to keep the master fader at like -16 in order to keep the master's meters around 0. Does this seem abnormal to anyone else? Like the master bus is getting too much signal? What could be causing this?

[Raw-Tracks]

Originally Posted by PoorGlory

OK, I am having issues with my board, and I have tracked down what I think is the main problem. When I send a CD player (-10 outputs) into my tape ins (-10 input) and put the 2 channels faders at unity, I need to keep the master fader at like -16 in order to keep the master's meters around 0. Does this seem abnormal to anyone else? Like the master bus is getting too much signal? What could be causing this?

If it's a CD that was mastered in the last couple of years, the hot level is due to heavy compression and limiting at the mastering stage. Modern CD's are extremely loud.

[ulysses]

Oh, there could be all kinds of reasons for this. How sure are you that your CD player's outputs are really "-10"? Have you measured them? You need a CD with some test tones on it. I use the Alan Parsons soundcheck CD a lot, but it's also pretty easy to make a test tone and burn it to a CD (there's a Mac OS program called "Make a test tone" that is very handy for exactly this kind of thing - it lets you control frequency and level, and even do sweeps). Of course, you still have to decide on an arbitrary reference level for CD output - a sine wave that peaks at 0dBFS should produce an analog signal just a couple of dB below the clip point of the analog circuitry carrying it. Typical reference points are in the -18dBFS to -14dBFS range to equal the nominal output level, which would be +4dBu (1.23Vrms) or in your case -10dBv (.316Vrms).
Once you get a reference level out of your CD player, you can feed it into the tape returns on your console and set them to unity gain and measure the output level. Is the output actually 16dB hotter than it should be, or are your meters out of adjustment? If you send a -10dBv signal into a nominal -10dBv input set to unity gain and send it to a nominal +4dBu output, you should get a +4dBu signal. Either something is poorly labelled, or something is out of calibration. Measuring it is the place to start, and to do that you need a known reliable signal source and you need a known reliable measurement device. Don't trust the meters on your console until you've measured them, and don't trust the output of your CD player until you've measured it. You can't use them to measure each other, you need a known reference. Using "Make A Test Tone" and Toast to produce a test CD should be reliable, and using an AC voltmeter known to be accurate at audio frequencies will get you the other half of the equation. This is something that every studio owner and user should have and know how to use - it's important to know the reference levels and limitations of every piece of gear you plan on interconnecting. Otherwise, you're just guessing.

[Matt Syson]

Hi
An addition to Ulysses piece. Many, certainly cheaper digital voltmeters are only accurate up to 500Hz. Making a test tone at say 400 Hz should be nearer decent calibration points of such meters. Since my Fluke that I carry regularly is actually out at 1KHz (0.775 V reads as 0.771) I noted it's reading of 1KHz levels so I don't have to carry my better gear everywhere.
400 Hz is not so piercing on the ear when listened top all day!
Matt S

[kwhaus]

If I'm hijacking this thread, I apologize. I have some questions relating to the topic.

I'm still new to this aspect of studio gear. In the past, I used a hook it up and hope it works system. I'm working to actually calibrate my gear properly and I'm not sure if I understand Ulyssey's post properly.

Starting from PT out to a 002R, I have a 1Khz sine tone coming out one of the channels. I have the 002R set for +4dBu to interface with my board which is +4. I have a fluke 179 meter I'm using to make measurements. So that 1Khz tone should measure 1.23Vrms coming from the 002R right? It isn't. I have to adjust the output on PT to +3.3 to hit 1.23Vrms. At this point I'm concerned I'm already getting off track somewhere. Going into the board, if the line trim and the level pot are set to 0, I should get 1.23Vrms coming out of the direct output which actually does measure pretty closely (0 on the fader measures just slightly below the line). Now if I send this to the master channel and the master is set to unity I should read 1.23Vrms coming out of my master outs right? I have the channel receiving the tone panned fully to the left and am measuring the left master output. At this point I'm only reading about .6 or so Vrms. Is the 1.23 that should be coming out of the master actually summed between the left and right so that each channel reads half or is my master short gain somewhere? The master left VU meter is reading almost correctly. The 1.23Vrms at the channel hits about +1 on the VU so I need to figure out how to adjust that maybe.

I haven't even gotten to adjusting for headroom yet (I don't have a scope to work with to really measure my clip point). Do I seem to be on the right track? What is the next step? If I'm clipping at a certain voltage over 1.23Vrms, I need to adjust my meters to read 0 at some voltage that is equivalent to 20dB below that clip point?

I'm not certain if there is even a real question there. For whatever reason I'm not confident I'm really understanding this properly and I'd like to make sure I understand what I'm doing.

[ulysses]

The first question is whether or not your Fluke is accurate at 1kHz. If you have the literature it came with, maybe it'll tell you. As the fella above suggested, try a 400Hz tone and see if you get a different reading than you did with 1kHz. I would expect the 002R to have output that's reasonably flat in frequency response between 50Hz and maybe 18kHz at least, but I wouldn't expect the outputs to be very well calibrated. One feature you find in higher-grade gear is trimpots to allow calibration. A dB or two isn't a big deal, so long as you know what the actual output level is.
As for your console, you'd have to read the documentation for your console to see what the gain structure is supposed to be on the channels and on the master busses. The pan pot will typically attenutate each side by 3dB when it's in the center, but shouldn't attenuate the "on" side when it's panned hard.
But your output seems to be about half of what it should be (0.6V instead of 1.23V) which is 6dB loss. Often time when you have exactly 6dB loss, you discover it's because you're only reading half of an electronically balanced output. But there could be some other cause.
When you say your meters are reading correctly on the master, do you mean they match what you think should be coming out of the console (0VU, for +4dBu) or what is actually come out of the console (-6VU, for the -2dBu or 0.6V you're measuring)?

[kwhaus]

Ulysses,
Thanks for your reply. Your assistance is much appreciated.

Originally Posted by ulysses

The first question is whether or not your Fluke is accurate at 1kHz. If you have the literature it came with, maybe it'll tell you. As the fella above suggested, try a 400Hz tone and see if you get a different reading than you did with 1kHz. I would expect the 002R to have output that's reasonably flat in frequency response between 50Hz and maybe 18kHz at least, but I wouldn't expect the outputs to be very well calibrated. One feature you find in higher-grade gear is trimpots to allow calibration. A dB or two isn't a big deal, so long as you know what the actual output level is.

It is 2.0%+3 at 500Hz-1kHz. It is 1.0%+3 at 45-500Hz. 400 Hz should read more accurately, but this doesn't account for my 0.6V difference.

Originally Posted by ulysses

As for your console, you'd have to read the documentation for your console to see what the gain structure is supposed to be on the channels and on the master busses. The pan pot will typically attenutate each side by 3dB when it's in the center, but shouldn't attenuate the "on" side when it's panned hard.
But your output seems to be about half of what it should be (0.6V instead of 1.23V) which is 6dB loss. Often time when you have exactly 6dB loss, you discover it's because you're only reading half of an electronically balanced output. But there could be some other cause.
When you say your meters are reading correctly on the master, do you mean they match what you think should be coming out of the console (0VU, for +4dBu) or what is actually come out of the console (-6VU, for the -2dBu or 0.6V you're measuring)?

I completely forgot about the balanced out. The board is a Trident 65 and does have balanced master outputs. I don't have time at the moment to measure it, but will check again later tonight.

The meters are reading 0VU at the 0.6V I measured which made me think that I was measuring incorrectly. I figured that the meters should read -6VU based on the measurement unless something was wrong in the console. Looking at the schematic, there isn't really anything that would go wrong in between where the signal goes to the meters and the output so my guess is that I'll measure again later including the other leg and find everything good.

I'm not sure about the 002 and the availability of trim pots. I haven't opened it up to look.

I sent the sine tone from PT at -20 which give me 20dB of headroom. If I need to increase the output by 3.3dB to reach 1.23V, I then am limited to 16.7dB of headroom coming out of the 002R. If want 20dB of headroom and I want my output to equal 1.23V, I need to see if there are output trimpots and adjust each channel accordingly. On the way in, I need 1.23V coming in to hit 0VU on the meter and adjust any trim pots accordingly. This is correct right?

[ulysses]

Originally Posted by kwhaus

my guess is that I'll measure again later including the other leg and find everything good.

That's my guess too.

Originally Posted by kwhaus

I'm not sure about the 002 and the availability of trim pots. I haven't opened it up to look.

I wouldn't hold my breath.

Originally Posted by kwhaus

I sent the sine tone from PT at -20 which give me 20dB of headroom. If I need to increase the output by 3.3dB to reach 1.23V, I then am limited to 16.7dB of headroom coming out of the 002R. If want 20dB of headroom and I want my output to equal 1.23V, I need to see if there are output trimpots and adjust each channel accordingly. On the way in, I need 1.23V coming in to hit 0VU on the meter and adjust any trim pots accordingly. This is correct right?

It's kind of hard to follow without a visual aid, but yes I think you're on the right track. Except I don't expect you to find any trimpots on the 002R. I guess the one thing to discuss is the topic of how much headroom you need, and how much you can expect, between digital full scale and nominal line level. It looks like Digi decided they'd allow 16.7dB of headroom in their gear. Typical "standards" (although there don't really seem to be any standards) are in the 14-18dB range, so they're at least in the ballpark. If your console has a clip point of +24dB, then yes it would be nice to have about 20dB of headroom in the digital gear too. More or less depending on which gear tends to misbehave when you get close to the clip point.

Another thing you may want to consider for a moment is whether or not you've been consistently comparing RMS levels of sine waves, as opposed to peak levels. For example, the maximum level of a sine wave in the digital domain is called -0dBFS. But it's the peak that reaches -0dBFS, and the RMS value of that sine wave will be 3dB lower than that. I don't use ProTools, or any modern DAW software for that matter, enough to know how they decide what a "-20dB" signal is, but I would expect it to be 20dB quieter than a full-scale signal. Likewise, in the analog domain, suppose you have +/-18V rails and the signal clips when it comes within a diode drop of that, or +/-17.35V. This would be a peak level of just about +27dBu, but a sine wave that's just on the edge of clipping would be about +24dBu RMS. So there can be a little ambiguity there, if you get confused about clip points (analog or digital) vs. a sine wave that reaches the clip point. I hope that confuses you less rather than more.

Before you try to perform any calibration, you should compare all the channels of the 002R to each other and see how much they vary. If they're all the same, that tells you something important about what Digi was trying to do. If they're all over the place (more than 0.5dB) you know something's up. You'll also want to compare the 002R's inputs, as well as the console's inputs.
Here's an easy little test to try out: If you take that +4dBu output from the 002R and loop it back in to one of the 002R inputs, does it record at the same level it was played?
You're probably starting to see why a calibrated signal generator and AC voltmeter are valuable tools to have in the studio. But like I said, don't worry about trying to change anything yet. Just measure things and see where you're at.

[kwhaus]

Everything "should" be fine I think with my console. Here is the situation:

I've got my 400Hz tone coming out of my converter at approx. 1.23V. It is going into a channel on the console with the trim and the fader set to 0 and I'm reading 1.23V coming out of the direct output. The channel is routed to the master output panned hard to the left. I measure the output coming out of the L master insert send (with master fader set to 0) and read 1.23V on the single leg since it is an upbalanced send. So far, everything is on track. At the L master out (which is balanced), measuring on the each leg separately I read 0.728V on + and 0.735V on -. If I put the probe across both at the same time, I get 0.1V. What is going on, are they cancelling each other. I need a refresher on how the electronically balanced circuit works. Signals come out half of the total on each leg in opposite polarity. On the balanced input, one leg is flipped to the opposite polarity. The noise cancels since one leg is 180 degrees out of phase while the desired signal sums to the total voltage of each leg. Is that right? If I measure both legs together I should get pretty close to 0V if it is working correctly?

I'll get to the other checks later. I haven't been able to find any info on the 002R having any adjustment pots. I'm not gonna crack it open to look. Thanks again for the help.

[ulysses]

Originally Posted by kwhaus

At the L master out (which is balanced), measuring on the each leg separately I read 0.728V on + and 0.735V on -. If I put the probe across both at the same time, I get 0.1V. What is going on, are they cancelling each other.

Yes, they're cancelling each other. That's not the way to measure (or use) a balanced output. You need to measure the voltage BETWEEN pins 2 and 3 of the XLR connector. Pin 1, which is chassis ground, is not involved with the measurement (or the balanced audio output) at all. Connect one probe to pin 2 and the other to pin 3, and measure the voltage between them.

When you tried to put one probe on both pins 2 and 3, they were shorted together and forced to sum. Since they're out of polarity with one another, they cancel out. There's nothing that says a balanced output's + an - signals have to be exactly equal amplitude with respect to ground, so they don't have to cancel perfectly. But shorting them together with your test probe will make the signal go away, since the signal is the voltage difference between the two of them.

[kwhaus]

Originally Posted by ulysses

Yes, they're cancelling each other. Connect one probe to pin 2 and the other to pin 3, and measure the voltage between them.

Thanks. I just had a major duh moment there. What you wrote makes perfect sense and I wish I had thought of it. I kept thinking to myself "what the heck does 'measure between them' mean?" I don't think I'll forget that anytime soon.

I'm still learning and aside from internet sites, I'm not learning from anyone directly. The texts I have are great guides to understanding signal flow and calibration, but it certainly doesn't explain the practice of measuring anything.