Video Transcript
Best Practices in Q-SYS Gain Structure (Part 4)
5m 48s
00:07
Welcome back! Let's pick up where we left off.
00:10
Alright, so far we’ve talked about INPUT gain structure and processing,
00:14
now let’s talk about the other side of this equation.
00:17
Typically, there is going to be some EQ, some other user level control,
00:22
and peak limiters to ensure we don't clip anything on the output,
00:26
and then perhaps you also add some delay in if you are dealing with a larger distributed audio system.
00:32
There are varying thoughts on how to equalize a given system.
00:35
Some EQ in the output signal chain is typically for “loudspeaker EQ”,
00:40
which are filter sets typically recommended by the manufacturer, and then there's Room EQ,
00:45
which accounts for the size, shape and reflectivity of the room.
00:49
This work can get cut in half if you are using QSC loudspeakers in the installation
00:54
because you can enlist Intrinsic Correction blocks for your loudspeaker EQ.
00:59
These are custom voicing for QSC loudspeakers.
01:03
For user gain controls,
01:05
you should generally aim to allow front end control of the individual sources rather than an overall gain.
01:12
If the user adjusts each source before the mixer, it keeps the ratio of each input in the mix relatively constant.
01:18
Allowing the user to control the output level to boost a quiet source
01:22
will cause all the other sources to become incredibly loud.
01:26
And almost equally important is to properly set reasonable upper and lower limits for those gain blocks.
01:33
No user needs -100 to +20, period!
01:37
You want to avoid giving the user license to put the system in feedback everyday.
01:42
In addition, output limiting will prevents a signal from going beyond a predetermined level,
01:47
it'll prevent clipping, and prevents overloading the amplifiers.
01:51
The peak limiter block default threshold is zero.
01:56
We did a few experiments and we found what would happen if you do use zero as that threshold level.
02:02
Below the threshold, the signal is unaltered and everything is fine.
02:06
However, beyond the threshold, the limiter will start to take effect.
02:10
As you can see, while the 0 dB threshold seems like a good number to start, 0 dB cuts it a bit too close.
02:18
So we recommend you change the threshold level to -3 dB to allow for a bit more margin on the limiter,
02:25
which should help keep you out of clipping.
02:27
However, when you are using QSC amplifiers,
02:30
you don't really need any sort of limiting circuitry on the output side.
02:34
QSC amplifier limiters are automatically set if the QSC loudspeaker blocks are configured correctly.
02:40
There are a few types of limiting.
02:42
The “DAC limit” prevents clipping at the input stage of an amplifier.
02:47
Next is “Protect”, which will provide Limiting due to driving low impedances.
02:52
And lastly you have “Limit” which is the computed limiting based on load time.
02:57
However if you are not going to use a Q-SYS network amplifier,
03:00
then eventually you need to convert the digital audio back to analog.
03:04
In this limiting example, we saw clipping at 0 dBFS (as you would expect).
03:10
In keeping with our -20 dBFS gain structure, the digital ‘Gain’ should be left at 0dB.
03:17
If we look at our controls here,
03:20
this is perfect for a standard professional downstream audio device (+4 dBu nominal, +24 dBu Max).
03:28
However, what if the next device in line was a consumer level device?
03:32
Ideally, we’d like all the components in the chain to clip at the same point.
03:38
Assume that Consumer line level is about -8 dBu, and you still want to leave 12 dB of headroom.
03:45
You need to also assume that the maximum input level is +4 dBu (RMA).
03:50
To scale the output at this level, you would use the ‘Output Gain’ control below to top meter.
03:56
This is an analog gain stage after the Digital to Analog convertor.
04:01
Sometimes we must drive a mic level input on another device from Q-SYS.
04:05
There are two major concerns here.
04:07
1) Phantom power is not enabled on the input Q-SYS is feeding. That will damage the Q-SYS output circuit.
04:14
2) The ‘Output Gain’ will need to be adjusted to reduce the max output to mic level.
04:20
As you can see in this diagram,
04:22
moving to Consumer or Microphone level greatly degrades the signal-to-noise ratio.
04:27
So whenever possible, use Professional Line Level.
04:30
Let’s wrap up everything that we’ve learned, shall we?
04:33
For your input calibration, make sure that you bring in everything at -20 dBfs.
04:39
Then trim high and low frequency noise using low-pass and high-pass filters.
04:44
Parametric EQ can be used for manufacturer suggested curves.
04:48
And then Notch Filter or Parametric EQ can be used to trim out feedback frequencies.
04:53
Automatic Gain Control or Compressors can be used to get consistent levels from a wider range of talkers.
04:59
Next, don't place user level controls or gates ahead of the automixer because that's just terrible.
05:06
And then, of course, use the fewest user gain controls possible to get the job done
05:11
and manage the range to avoid letting users ruin the gain structure.
05:15
For Output limiting, using QSC amplifiers and loudspeakers together
05:20
will get you limiting automatically when those blocks are configured properly.
05:23
We always recommend -3 dBFS as that safe limiter threshold.
05:29
And then, finally, you're going to scale the analog section of the output circuitry
05:34
to match your downstream device.
05:36
Ideally, you want Q-SYS to clip at the same point as the next device downstream.
05:41
And that's it for Gain Structure through Q-SYS
Lesson Description
Best Practices in Q-SYS Gain Structure (Part 4)
5m 48s
(Part 4 of 4) Learn to do's and don't for setting good gain structure within Q-SYS, particularly within a meeting room scenario.
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Best Practices in Q-SYS Gain Structure (Part 4)
5m 48s