# Noise Reduction IC???

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#### ste2006

I am trying to design a circuit to reduce background noise inside a vehicle as its driving and stopped but generally with a lot of back-ground noise. As is the audio is full of background noise and it is very hard to hear people talking,

I know there are some new IC's on the market that are supposed to assist in noise reduction and am wondering has anyone any experience with them or ever used them,

My other option is to go down the road of a second Mic and Op-Amps etc but i have heard that this is not perfect either,

Anyone any thoughts or experience as Analogue electronics is not my strongest point,

#### crutschow

I don't quite understand. You are trying to reduce the background noise for what purpose? Is this for a hands free cell phone?

#### godfreyl

He wants to reduce the background noise in a car, to make it easier to hear conversation. Unfortunately the noise can not be canceled with electronics, loudspeakers, microphones etc. The best option is to use acoustic treatment to reduce the noise getting into the car, and to absorb it in the car.

#### ste2006

sorry, I will clarify a little more, the sound is for security to record conversations and what is going on generally on security patrols and is being transmitted over GSM back to a base for monitoring. It is a security measure for patrols so that if they run into hassle more patrols will be dispatched. Problem is it works but due to general background noise it is very hard to hear what is going on so not being too effective.

What i need to do is try to cancel some of the ambient background noise to make out the voices of interest clearer.

#### ste2006

Some further clarification,

The noise cancellation is only for inside of the vehicle and to try and make some effort at taking out some of the background noise. It does not have to be perfect but as is with no noise cancellation its not great.

Seperate Headset Mic is not an option as it must be built into the vehicle and would also be cost prohibitive.

I am really just looking for advice on the best method to go about some kind of noise cancellation and whether a discrete circuit or an specially designed IC is best to go with.

Plan is to place one Mic in the cab where a near the rear view mirror and the other Mic out of the way of voices but still getting the general road noise.

Any thoughts would be great,

Thanks,

#### godfreyl

I don't think it is possible to do the sort of noise canceling you want. As mentioned earlier, you could use acoustic treatment to reduce the noise in the cab. Another approach would be to put microphones as close as possible to the voices of interest i.e. right next to people's heads, either on the side of the headrest or near the seatbelt attachment points on the side of the cab.

Plan is to place one Mic in the cab where a near the rear view mirror and the other Mic out of the way of voices but still getting the general road noise.
I think what you're hoping for is:
• Signal from microphone A = voice + noise
• Signal from microphone B = noise
• If we subtract B from A, the noise will cancel, leaving only the voice
Unfortunately, the noise is different at the two microphones so what happens is:
• Signal from microphone A = voice + noise A
• Signal from microphone B = noise B
• Any way you add or subtract them, the result is worse than just using microphone A

#### kbm10

##### Junior Member level 3
I am trying to design a circuit to reduce background noise inside a vehicle as its driving and stopped but generally with a lot of back-ground noise. As is the audio is full of background noise and it is very hard to hear people talking,

I know there are some new IC's on the market that are supposed to assist in noise reduction and am wondering has anyone any experience with them or ever used them,

My other option is to go down the road of a second Mic and Op-Amps etc but i have heard that this is not perfect either,

Anyone any thoughts or experience as Analogue electronics is not my strongest point,

If you choose to implement it using some real time dsp technique, then I can suggest a simple technique, i.e noise cancellation. Assume that people have to stop for breath after every say 3s. Then what ever is coming in is noise. It is not as simple to implement as it sounds in embedded system, but it is an interesting project to spend some time on. So cancelling this from the normal signal, you can get a better signal. Google on different noise cancellation techniques.

I am also not very strong in Analog use of second opamp can be avouided if you use a pseudo-differential technique.

correct me if i am wrig

#### D.A.(Tony)Stewart

You cant fix near field noise problem with remote mics.
The only fix is the proper noise cancelling mic.

You can tell which ones they are by the lack of sensitivity beyond one foot. When they cancel out the audio pressures on the back and front of the electret diaphram surface, you get perfect sound cancellation. i.e.e no background sound. Old telephone handsets were designed that way so that you had to speak a;most touching the mic to get perfect SNR .. meaning loud signal to background noise ratio.

I find the best consumer solution for this was the cheapest desktop mic in a handheld position about< 1cm from the corner of the mouth and not in front so no puffing is detected. For Voice recognition in noisy backgrounds, this is your best bet. THere are active and dynamic noise solutions, but remote mics can not work inside a car due to phase noise from reflections. Where they can work is near large power substation transformers that hum badly and noise cancelling audio to speakers can reduce the residential noise pollution.

This latter example works because signals direct line of sight to mic and speaker waves are equal amplitude and cancel out at a distance with the transformer noise waves from the street. Active noise cancellation works but when imbalanced can oscillate or contribute to more noise rather than cancel by being the wrong phase across the range of frequencies of the noise.
For noise cancelling ear buds it is an easier solution as the noise is usually far away from ear buds.

#### ste2006

ok so maybe a noise cancelling Mic, Sorry i should have mentioned earlier that there will ultimately be approx 500 of these units so off the shelf parts that are expensive are not going to work and putting acoustic material in 500 trucks is not going to work either,

Hence why i want to do it electronically,

Can i buy seperate noise cancelling Mic's?? I know hands free kits do something and some new mobile phones can noise cancel. How do they do it??

#### D.A.(Tony)Stewart

I dont know what phone you are interfacing to , but my favorite pop free logitech clone desk mic with the best noise cancellation I could ask for , for $5~$10 .with cable packaging and a stand. Of course it fails miserable as a desk mic. since you almost eat the mic next to THE CORNER NOT IN FRONT of your lips, otherwise the sound level drops off quickly.

Of course depending on how hard the wires are bent and yanked , they dont last that long or can last forever.

Whats your budget? I can shop around for you. When do you need it?

#### ste2006

Problem is the Mics cant be in front of the people as they are just designed to be in the vehicle mounted like a Parrot hands free set say,

Budget is as cheap as i can but its not going into a phone, It ends up going into a board i designed with a GSM chipset and another array of items,

Regarding time and quantity, Its the usual everyone wants one yesterday then maybe 10 units to play around with and test and then slowly draw down on the rest,

Important thing now is to prove the concept.

So i suppose my question at the start should have been, How do guys like Parrot hands free sets reduce background noise??

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