Microphone inside car.

[andyx]

First post so be gentle. I have some experience with electronic builds but unfortunately it's all digital stuff, no experience with analogue.

Background: I have a camera connected to a dvr inside my car, like a dash cam but no ugly thing stuck to my windscreen. This works great but I can't get sound to work very well. I've tried a number of cctv microphones but they all have the same problem, to much background noise.

I need a microphone that'll work inside a car so that it'll pick up speech and everyday driving sounds. My dvr has RCA connector for audio input so I think the microphone needs to be self powered.

Any help would be greatly appreciated.

 

[Tha fios agaibh]

Welcome Andyx. It's important to use high quality shielded cable for the microphone, or your bound to pick up nasty emi interference.
The RCA audio jacks don't always have to be powered, but I think your right in this case. Another thing I'd do is add a ferite choke in series with the power supply. This will help filter out noise from the ignition system. Add a choke to the power supply of the powered mic too.
Again, that cctv microphone should work fine provided you change the cable to a good shielded cable. The foil inside the cable helps to keep noise out.

 

[shrtrnd]

Tha fios agaibh gives you good guidance for electrical noise.
My experience with your issue, is the ambient noise inside a car itself when driving. (Which I think is your primary bugaboo)
I can't think of a good way to shield from this if you're expecting to pick-up several people in different areas of the passenger compartment.
If it's just you or a particular passenger location, you can check into directional microphones with the foam-cover shields that are common for many audio applications.
Other possible options are multiple inputs interface with the old-style Telex headset microphone that the speaker would wear (but not feasible if you're doing something impromptu),
or directional microphones (multiple input again) mounted in the passenger compartment for each rider.
If you're just looking for simple, you'll have to experiment with different mics, paying close attention to those made for noisy environment applications.
Otherwise you're going to be looking into some type of recording quality noise suppression unit. (Like an audio 'equalizer' that has slide switches to dampen specific frequencies)
Good luck.

 

[Martaine2005]

Hi Andy,
Some good advice given by the above two members!
However, when you say you have tried CCTV mics, do you mean the bullet type electret mic?
CCTV (electret) mic's are usually amplified because they need to pick up sound from a distance.
This would obviously pick up every sound in confined spaces with lots of noise.
You may be better to make your own single stage amp with electret mic. Three or four components might solve your problem.
Have a look at some simple electret mic circuits.

Martin

 

[andyx]

Hi Andy,
Some good advice given by the above two members!
However, when you say you have tried CCTV mics, do you mean the bullet type electret mic?
CCTV (electret) mic's are usually amplified because they need to pick up sound from a distance.
This would obviously pick up every sound in confined spaces with lots of noise.
You may be better to make your own single stage amp with electret mic. Three or four components might solve your problem.
Have a look at some simple electret mic circuits.

Martin

Thanks to all for your advice, you all sound experts. I've now tried the choke, replaced the cable to the mic with shielded cable. I connected the shield to -ve and fitted choke as close to DVR input as possible. Still terrible sound.

I've tried three different electret mics, all specifically for CCTV, all with the same results. I think Martin is spot on, these things are for picking up faint sounds over a large area whereas I'm trying to pick up sound within a confined space.

I'm up for building my own mic, there are plenty of circuit diagrams available on the Internet however because analogue is not my thing I'm not sure what to try. Anyone got any suggestions?

Thanks again guys (girls), I appreciate your input (bad pun).

 

[Martaine2005]

(girls)

The other two are!!
You can start with a simple electret mic, resistor, capacitor and rca connector.
The electret mic has already got a built in transistor (amplifier) of sorts.
A resistor approx 10k to 20k from the 12v to load the mic's + terminal. Negative to Grnd.
A 0.1μf ceramic to 1μf capacitor leg between the resistor and mic, the other leg to your RCA connector centre pin. And GRND to - side.
The resistor and capacitor values need to be played with to get your ultimate sound preferences.
So far, you have spent about £1. Unless you use Maplins..

Martin

 

[Tha fios agaibh]

Sorry Andyx, I was thinking your problem was emi induced noise (ignition system noise)
I think Martaine is onto something with adding the cap. You might want to try a variety of sizes up to 10uf.

Despite Martins juvenile demeanor, he's quite a smart guy.

 

[Martaine2005]

Despite Martins juvenile demeanor

It's only a small "childish" side..

Yes, the caps can be played with as with the resistor too.
It could mean adding a first stage amp. Don't worry though, just a transistor and two more resistors. Still under a £1.

 

[andyx]

It's only a small "childish" side..

Yes, the caps can be played with as with the resistor too.
It could mean adding a first stage amp. Don't worry though, just a transistor and two more resistors. Still under a £1.

Thanks guys.

This maybe a stupid question to you guys, but here goes: Do I need to worry about input impedance to the DVR?

I'll give Martin's simple circuit a try. I have a test breadboard I can use for ease.

My type of electronics is a lot easier, on-off, hi-low - goes with my simple brain I suppose.

 

[Martaine2005]

Hi Andy,
No, don't worry about it for now.
Your audio in may require a further stage of pre amp.
But that is more than likely your problem with the other mics. Over modulation.
So keep it simple to start with. Then we can add to it to get it just right.

Martin

 

[CDRIVE]

Sorry Andyx, I was thinking your problem was emi induced noise (ignition system noise)
I think Martaine is onto something with adding the cap. You might want to try a variety of sizes up to 10uf.

Despite Martins juvenile demeanor, he's quite a smart guy.

Hey! Don't call my nephew juvenile! Only I can do that! If I do, it's always affectionately.

Marty's Uncle

 

[Martaine2005]

Hey John, be careful!
I have Uncle Chris looking after me....

Marty

 

[Tha fios agaibh]

Yes, he might joust me with that pipe.

Andyx, you might want to experiment by using a pair of headphones directly with the microphone circuit, instead of going thru the dvr.

 

[andyx]

Hi Andy,
No, don't worry about it for now.
Your audio in may require a further stage of pre amp.
But that is more than likely your problem with the other mics. Over modulation.
So keep it simple to start with. Then we can add to it to get it just right.

Martin

Tried the simple circuit, with the engine and radio off, just a complete drone on the recording. Tried talking, picked me up very very faintly so it's picking up something.
Any other ideas?

 

[Martaine2005]

Hi Andy,
Ok, you have the first stage of powering the mic with resistor and capacitor.
Try adding these 4 components to make a one transistor mic pre-amp.
Use C2 and GRND for the RCA in.

Martin

 

[Tha fios agaibh]

Good call Martin. Microphone levels are usually around -60dbm, and a pre-amp should boost to "line" levels around -10 to +5dbm.

 

[CDRIVE]

Good call Martin. Microphone levels are usually around -60dbm, and a pre-amp should boost to "line" levels around -10 to +5dbm.

Yes, there are those that may get the impression that Marty is here on EP for comic relief but as you can clearly see he's actually a jewel in the rough.
I love him!

Chris

 

[Martaine2005]

you can clearly see he's actually a jewel and a thief.

I didn't design that circuit!!!
It works on the breadboard though.

Martin

 

[CDRIVE]

I didn't design that circuit!!!
It works on the breadboard though.

Martin

I didn't think you did but that's not important. You picked up the ball an held on to it!

Chris

 

[Tha fios agaibh]

A "Jule and a thief"