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Limiter DSP for LFE channel
Matt:
I tested that same THX sample.
First, I added +6dB so it would actually clip.
It looks like this at 45 seconds:
Then I added a subwoofer limiter to the left and right channels (there's no subwoofer channel in this clip). Then it looks like this at 45 seconds:
See how the energy slope is much steeper at the low frequencies with the limiter engaged? A regular limiter wouldn't change the slope at all.
So it seems to be working for me.
mojave:
I had a chance yesterday to get back to this. I wasn't seeing anything happen because I didn't add any gain in.
I played around yesterday with pink noise to try to figure out what was going on. I'm trying to understand the relationship between the internal volume control, the level field in the limiter, and adding gain before the limiter.
Example A
+10 dB (Left, Right)
Subwoofer Limiter (0,Left)
Subwoofer Limiter (0,Right)
With the internal volume at -10 dB I get the first image. With the internal volume at 0 dB, I get the second image. Is the Analyzer representing the actual db levels? If so, the sub is quieter with the internal volume at 0 dB than it is at -10 dB. The low frequencies should be staying the same with the upper frequencies still increasing in gain. If the Analyzer just shows relative levels, then it makes more sense.
What exactly does the level field do? I've tried it with both positive and negative numbers with different results and don't really know what I'm supposed to enter.
Matt:
--- Quote from: mojave on September 18, 2012, 09:45:40 am ---Is the Analyzer representing the actual db levels?
--- End quote ---
The 'Options' button at the top right of Analyzer lets you pick if it should process independently of Internal Volume (-10 dB in your case). I think the default is to process independent of internal volume, or else the graph is almost invisible if you turn down the volume.
--- Quote ---What exactly does the level field do? I've tried it with both positive and negative numbers with different results and don't really know what I'm supposed to enter.
--- End quote ---
It's dB.
0.0 is the normal full level (100%).
If you enter something like -6.0, it will limit before it gets to full range, so the peak level will only go to like 50%.
mojave:
--- Quote from: Matt on September 18, 2012, 09:59:56 am ---If you enter something like -6.0, it will limit before it gets to full range, so the peak level will only go to like 50%.
--- End quote ---
The field isn't intuitive that it should be a negative number. I had been putting in positive numbers and just this morning realized it should probably be a negative number. I was thinking "I want to limit by 6 dB so I should enter 6 in the field."
--- Quote ---You take the incoming signal and split it with a low-pass / high-pass combination at like 50 Hz with 12dB/octave filters. This gives you the low and high components.
Then, when the signal crosses the limiting threshold, you run the limiter in two stages.
--- End quote ---
Is this how you ended up doing it? If so, can the split frequency be made variable? As I mentioned earlier, since I started this thread I have made 3 different sub systems. I would use 50 Hz on one, 30 Hz on one, and 20 Hz on the last. Above those frequencies I am amp limited so don't need much limiting. However, below those frequencies I am driver limited and might need limiting.
I'm sort of talking to myself here, but Matt did a good job on this. ;D
hulkss:
What would be great is if your clever limiter does not introduce group delay distortion.
For an ideal filter, the phase will be linear and the group delay would be constant. Group delay, whose unit of measure is time in seconds, can also be thought of as the propagation time delay of the envelope of an amplitude modulated signal as it passes through a digital filter. Group delay distortion occurs when signals at different frequencies take different amounts of time to pass through a filter.
Any typical high pass subwoofer protection filter has a problem with group delay.
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