KnowBrainer Speech Recognition
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Topic Title: Using a Noise Expander / Gate and Mixer
Topic Summary: My experience with the DBX 266XS and A&H ZED 10FX
Created On: 02/05/2012 11:03 AM
Status: Post and Reply
Linear : Threading : Single : Branch
 Using a Noise Expander / Gate and Mixer   - Amin Sabet - 02/05/2012 11:03 AM  
 Using a Noise Expander / Gate and Mixer   - phils - 02/07/2012 09:40 AM  
 Using a Noise Expander / Gate and Mixer   - Amin Sabet - 02/07/2012 06:14 PM  
 Using a Noise Expander / Gate and Mixer   - phils - 02/07/2012 06:44 PM  
 Using a Noise Expander / Gate and Mixer   - Amin Sabet - 02/09/2012 11:14 AM  
 Using a Noise Expander / Gate and Mixer   - phils - 02/09/2012 10:15 PM  
 Using a Noise Expander / Gate and Mixer   - phils - 02/12/2012 11:49 AM  
 Using a Noise Expander / Gate and Mixer   - Amin Sabet - 02/12/2012 06:05 PM  
 Using a Noise Expander / Gate and Mixer   - phils - 02/26/2012 11:41 AM  
 Using a Noise Expander / Gate and Mixer   - Amin Sabet - 02/27/2012 01:57 AM  
 Using a Noise Expander / Gate and Mixer   - phils - 02/27/2012 09:43 AM  
 Using a Noise Expander / Gate and Mixer   - Amin Sabet - 02/27/2012 10:14 AM  
 Using a Noise Expander / Gate and Mixer   - phils - 02/27/2012 10:33 AM  
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 02/05/2012 11:03 AM
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Amin Sabet
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I mentioned this in another thread, but I think it is worthy of it's own discussion. I am currently using a noise expander (gate) and find that it makes a dramatic difference in my results when dictating under noisy circumstances.  Most people doing speech recognition will never need to use an expander and would never consider spending the money on one. However those frequenting this forum are not your average use case, so I think there are others here who could benefit.

A noise expander is a piece of hardware or software which reduces the loudness of all sounds which are below a certain noise threshold. Most expanders allow you to choose a threshold as well as a ratio. The ratio determines how much sounds are attenuated when they come in below the threshold. For example, with my expander, if I set a threshold of -10db and the ratio of 2:1, then for every 1db below -10, the sound gets attenuated by 2db.

The expander I use is the DBX 266XS ($150).  The 266XS is a compressor in addition to being an expander.  The compressor attenuates sound which exceeds a certain db threshold (opposite of an expander) and is not necessary for speech recognition, so I only use the expander functionality when doing speech recognition. Under ordinary circumstances my expander threshold is set to -40db with a ratio of 1.5:1 or 2:1.  That gets rid of all background line static when I am not speaking. It is also extremely effective at suppressing any breathing noise, and once your settings are good, it is 100% effective at suppressing key clicks, chair squeaks, coffee cup clinks, and the like. 

When the environmental noise gets very loud, like my kids blasting their Xbox 360 across the room, I simply crank up the threshold on the expander to 0db, and they get blocked out. Keep in mind that when you are talking and therefore the noise going into the microphone exceeds the threshold for the expander, any simultaneous background noises will likewise be included along with your voice. The expander will only attenuate the sound in between your words. However the combination of a highly directional microphone like the Audio Technica PRO8HEX and an expander makes it possible to work under very noisy conditions with excellent results. Even without the expander, I can do well. However with the expander, those stray occasional words which pop up during my silence are suppressed.

Typically, the setup for using an expander is to use a microphone with an XLR termination (hence my choice of the PRO8HEX which has XLR vs PRO8HEmW which has 3.5mm termination) and plug that into a mixer which amplifies the signal and then sends the sound to a hardware expander.The expander does its thing and then sends the sound back to the mixer, which then sends the sound to your computer. Of note, you have to make sure your mixer is sending adequate levels to your expander.  If you are a soft talker with the mix gain set too low, the expander will squash your voice at the same time it squashes the background noise.  The solution for this is to increase the gain on your mic preamp as needed.
 
For the cleanest possible signal, use a mixer that has a USB or FireWire output your computer. There are inexpensive mixers with USB/firewire out from Alesis, Yamaha, and Behringer which probably work well, but I have not tried them. I have tried Mackie and Allen & Heath mixers, both of which have superb microphone preamps and are highly recommended. Expect a good mixer with USB or Firewire out to cost somewhere between $100 and $400.

As an aside, the Samson Airline 77 has a built-in noise gate which does the same thing as my 266 XS, except that the latter is more configurable and will work with any microphone and preamp.

When using multiple hardware components, there is the possibility of introducing noise anywhere in the system. For what it's worth, my signal using the above components is exceedingly clean, far cleaner than when I run my microphones via XLR to 3.5 mm adapter into an Andrea USB pod.

The downsides to using a mixer at hardware expander are cost (total cost of $250 and up) and portability. There is no way I would take this system on the road. However, for my home and office, I wouldn't go without it.
 02/07/2012 09:40 AM
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phils
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The form factor is more daunting than the price. Update: this stuff will fit if I mount a shelf below my special ergonomic chair. I'll give it a try to see if it can kill the slap echos off my screens when I pause my dictation  Besides the rack mount gate a 20 lb one foot by two foot mixer will be hard to fit into my already cluttered office. Do you have a recommendation for a high quality small form factor mic amp.

Phil Schaadt



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 02/07/2012 06:14 PM
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Amin Sabet
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Phil,

I think you'll be very pleased with the results.  Let me know if you have any questions, and don't get discouraged if the results aren't good at first.  It takes some playing with the preamp gain and the expander settings before you optimize for a given situation.

Best,

Amin 

 02/07/2012 06:44 PM
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phils
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Thanks for the offer of help. I have had quite a bit of pro-audio, recording studio and broadcast experience so I understand that playing around with this kind of stuff is a necessity. I will start with your suggested settings and ask for help if I get stuck.

Thanks again.

Phil Schaadt



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 02/09/2012 11:14 AM
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Amin Sabet
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Phil, out of curiosity, which equipment did you get?  I am thinking of picking up another set for work since this one is working so well at home.  I'd love to find a single unit that combines a pre-amp, expander/gate, and USB out, but so far no luck.
 02/09/2012 10:15 PM
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phils
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Quote:
I'd love to find a single unit that combines a pre-amp, expander/gate, and USB out, but so far no luck.
 

I do not think that such a thing exists. I talked to my long time pro audio shop folks and I ended up with the Allen and Heath ZED 10FX mixer and the DBX 266XS gate. it took about two hours from the time I opened the boxes to when DNS was working well. Most of that time was just peering at the ZED 10FX manual and buttons to get the signal path right. the gate really squashes the slap echo from my wife's monitor as well as the classical music (which would often be turned to without strange Dragon da-da misrecognition).

I am using it now with the Airline77 in the same DNS profile as I had with my USB pod. My overall accuracy does not seem to be quite as high. Some of that is from my voice being squashed by the gate if I pause or trail off. But there does seem to be an acoustic difference from the standard USB pods. I will probably create a new user profile tomorrow when I have time to try the AT mic. threshold is set at -30 DB and the ratio was 1.5. I going to have to fiddle with this.

it is a very large pile of equipment for dictation but all of this stuff is still smaller and lighter than the old Webcor wire recorder dictation machine one of my uncles gave me as a kid when his company converted to tape Dictaphone.

 thanks for the tip. As you said this is a specialty case but it really does help in my office because of the bad acoustic layout.

Phil Schaadt



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 02/12/2012 11:49 AM
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phils
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I think I have the settings on the gate working reasonably well so that it does not squash my voice when I dictate normally.

For anyone other than Amin who is reading this, the amount of extra equipment required is not necessary in almost all cases. In my office I have incredibly bad background noise geometry such that music comes from behind me and I am sitting in front of multiple 30 inch monitors so that when my office mates are on a WebEx their voices reflect off of their 30 inch monitors and bounced right into my microphone from my right.

Using the airline 77, I have now tested the gate with loud conversation, operatic sopranos and dueling WebEx presentations. The gate has completely eliminated all of the "???" or "Dragon da da" mis-recognitions that happen regularly without having all the stuff in line. the gate has also eliminated all "him him him". I also do not need to physically turn off the microphone while I am contemplating my screen.

Thanks Amin, and now I just need to figure out where I can permanently mount all of this equipment.

Phil Schaadt



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 02/12/2012 06:05 PM
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Amin Sabet
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Phil, I'm glad you're finding the gate to be worthwhile.  I find it helpful both at home and at work, and while I'm sure that 90% of people don't need one, I'm guessing that 5-10% of people could benefit.  

I decided to pick up a $150 Alesis mixer for the office, which is half the cost of the Allen & Heath while still offering inserts for the gate and a USB out.  I'll post back later in this thread once I see how well it's working for me.

----

Edit: So far so good with the Alesis (Multimix 8 USB FX).  I prefer the Allen & Health for build quality, tone, etc, but the Alesis works very well with the DBX 266XS for speech recognition and is much less expensive than the A&H mixer.

 02/26/2012 11:41 AM
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phils
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I have now been using  an ART microphone amplifier with the Audio Technica microphone. I have the gate settings cranked up more aggressively that I found was optimal with the airline 77. With the multiple 30 inch screens sitting in front of me I am now getting slightly better performance (especially with commands) than with my Airline77.

interestingly I am using the Audio Technica mic on top of my airline 77 profile.

Phil Schaadt



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 02/27/2012 01:57 AM
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Amin Sabet
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Phil, are you using the A-T PRO8HEX or another model?  Also, am I correct that you are going mic -> ART preamp -> A&H mixer?

I'm now using a Heil PR40 on a boom stand along with the A&H mixer and dbx gate.  I'm dictating in a room with kids/TV so loud that I wear one of these > http://www.amazon.com/3M-90561-80025T-Protection-Professional-Protector/dp/B00009LI4K/ref=sr_1_2?ie=UTF8&qid=1330325839&sr=8-2 < just so I can concentrate, and the results are truly great.

 02/27/2012 09:43 AM
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phils
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Did you get to try the Alesis Multi-Mix 8 USB?

My audio chain is: PRO8HE>ART-OR-Airline77 >> DBX>ZED10FX>USB>Win7 i7-3960x.

I think I'm getting more consistent performance with the DBX before the mixer. However, maybe I just didn't get all the settings completely dialed in with the DBX in the mixer loop.

I use Channel 1 on the DBX for the Airline77 and Channel 2 for the AT. Each goes to its own channel on the ZED10FX with matched output levels. I turn off whichever mic amp I don't use. yesterday we were playing Opera, which had been overwhelming the Airline77 alone at medium volumes, at very loud volumes with no effect on DNS using the DBX gate.

As I said, because of my room/multi-monitor/speaker geometry the AT is marginally better for me than the Airline77 but really only noticeable in commands with periods of silence in between.

When I get caught up from traveling last week, I will try the Sennheiser MD431 II on a boom but I think my room geometry is such that it probably points the wrong way.

Phil Schaadt



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 02/27/2012 10:14 AM
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Amin Sabet
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Ah, very interesting.  I haven't tried a dedicated mic preamp.  I've only used the preamps in my mixers.

I am using the Alesis Multimix at work, and it seems to work very well with the dbx and all of my mics.  Currently I use the Shure Beta58A on a boom at work, the Heil PR40 on a boom at home, and the PRO8HEX in both locations.

Compared to the A&H mixer, the Alesis is smaller, lighter, cheaper, has poorer build quality, and I don't think the preamps are as good in terms of the tone of my voice for podcasting.  However, for speech recognition purposes in conjunction with the dbx gate, I think the Alesis mixer works beautifully and is probably the one I would recommend (since it is cheaper) to folks looking for this kind of solution.  On the other hand, the A&H seems much more solid and likely to last longer. 

 02/27/2012 10:33 AM
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phils
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I went with the dedicated preamp up front because I think I am getting more consistent results from the gate. I could be wrong.

Phil Schaadt



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A proud member of the local "Peanut Gallery"

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