FB to control audio switching?

**Iamthehor** · 05-22-2009, 05:18 PM

While in theory you are correct, it's not that simple.

I know of no device that can:
1 "Intercept" or Split/switch 2 signals with different # of channels (2 & 6) & match output to whatever you are connecting to (this is the really complicated part).
2 Handle both low (RCA) and High (amplified) signals separately or simultaneously
3 Have correct wiring to hook up to multiple wiring systems.
4 Do it without screwing up your sound quality.

You could roll your own, but I'm thinking your development cost & time are gonna be outrageous. The only thing i can think of that would work would be a DSP like the RF 360.2 that I have or the discontinued Alpine 701 (can't remember if it has multiple inputs).

Audio signals aren't like 12Vdc - you don't just put a switch in the line & expect it to not affect the sound. You're dealing with AC voltage, Impedance & signal degradation on low-level is a major concern (computer line out or RCA for example).

A frequency modulator or using the Aux input of the H/U is the most effective, safest & easiest method your going to find. It's 1 audio connection & will not require any electronics knowledge (the freq. modulator will either plug in between the antenna & H/U or transmit on specified freq.) it will also compensate for the different # of channels & still allow processing to be done by the computer (albeit in stereo only). If you were dealing with all the same type of equipment (all H/U driven or all external amp driven), I might make some more complicated suggestions, but there is no good way to deal with both. It would be VERY expensive to build a switch system capable of handling both & that will hold sound quality (impedance matching, etc).

Whatever you decide to do DO NOT just tap into the wires behind the radio (either RCA or Speaker) you WILL cause damage to some component - which one depends on a lot of factors. You have to Isolate the 2 inputs (a switch/relay on a speaker-level line will cause problems as well (impedance, resistance, etc)). Just as an example, if you connect the line out of your computer to the speaker wires, you will fry the line out of the sound card - the Voltage of the speaker-level output is ~10X that of the computer output (different components will be different).

I won't be staying at a Holiday Inn Express until next week (business trip) but I am an Engineer & work on automotive wiring systems. Not trying to be a dick, but you don't sound like you understand what you are trying to do. Like I said, what you are wanting can be done BUT it's going to be expensive & will need to be custom-tailored to each vehicle you want to install in. WAG, but i bet it would be cheaper to install a CarPC in each vehicle than to design & build your adapter (if done electrically correct, not even thinking audiophile-approved) Something like this will give Hi-Fi audio-snobs hives.

**JonnieBlaze** · 05-22-2009, 10:25 PM

I don't think you're being a dick at all. And I really appreciate you taking the time to talk me through this and help me understand it. You're correct when you say I dont know much about what I'm trying to do. I have a computer science degree and the only audio knowledge I have is what i've learned installing the few car systems I've done and working with home audio equipment. My knowledge range on this subject is extremely lacking.

Where my little brain is getting hung up is the fact that I feel what I need to accomplish is sooo simple yet I can't figure it out. You're showing me that intercepting the audio will not work easily/cost efficiently. So maybe I need to approach this from a different angle. Maybe I just need to use the forbidden Y splitter backwards, but physically cut the connection to the headunit when I'm using the PC and vice versa when using the headunit. I'm not really sure yet. The last day or so my mind has been on how to split the audio signal. I have to sit down now and try to think of some other method that prevents signal outputting from the headunit and damaging the soundcard. I just can't believe its that difficult.

If there was a headunit that would input 4 channels as AUX (dont need left and right I guess, just highs, mids, bass, and sub) and pass them straight through to the amp (whether it be via RCAs to an external amp or through its own built in amp then straight to the speakers) without making adjustments to the processing...that'd be perfect. Or a digital input that would except any number of inputs then pass through without using its own processing. I'm going to go do some reading on headunits and see what i can find.

**Curiosity** · 05-23-2009, 01:07 AM

Iamthehor definitely has his shtuff together. Listen to him.

Since you have experience with home audio you should think of car audio somewhat the same way. You have speakers connected to the amp with lots of power going to them. All the other components: CD, DVD, cable, sat, VHS, tape, phono... (I'm old) which are line level and the only things ever switched. You can't wire all the inputs together or bypass the amp, or even bypass with just one. There's no 5.1 into a 2.0 amp. The source needs to be in 2.0 mode (just like a 5.1 PC with 2 speakers), and 2.0 into 6.1 just goes to the front 2 speakers and maybe delayed to the back. Every home is different though, and that's your dilemma. There's no one black box solution for every situation. Most cars are like a boombox with 4 speakers/no inputs and you want to run an iPod through it, others have aux, some have line level to amps, ... you get the picture.

**Iamthehor** · 05-23-2009, 08:49 AM

With a DC signal (digital) or just plain power, your Y-splitter & cut the other input will work. It will do what you expect with an audio signal BUT there is a huge chance of un-intended consequences. There's a HUGE about of "data" in an audio signal - think about it, 20-20k hz worth of freq + volume of each freq. You corrupt even a little & you will hear it. You want as little molestation of the signal as possible (short, straight wires with quality connections).

Like i said - the Frequency modulator (preferably wired in-line with the antenna) & using Stereo from the computer is the only way I can think of to keep both H/U & computer & work in any vehicle. If the vehicle has an aux input on the H/U, even easier (you could Y this behind the radio to hide the wires, just DO NOT connect anything else to the Y while the computer is connected). Don't put switches, relays, etc in an audio cable- Period. Giving up either multiple vehicles or multichannel is about the only way you're going to get this to work. Just a thought, but how are you going to control the CarPC anyway? If you're installing a screen in each vehicle, you can do a lot more "custom" jerry-rigging to make this work. I think the 6&2 channel for each vehicle is out, but if you got the cash, this can be done.

I may know what I'm doin with this stuff (which is why I'm typing volumes) but I'm a little envious of the CompSci degree - I know enough VB to be dangerous which has left me beating my head against the wall programming for the fusion brain lately. I'm much better at taking stuff apart & making it work better (what I do all day). Car audio is a hobby (but i don't have a carPC, too cheap & I don't want to re-install everything). I do connect my laptop to my 360.2 on long trips - MP3s + GPS & I don't have to worry about it getting stolen - You can't tell i have anything but stock (my sub box is even somewhat camouflaged)

**rodweb** · 05-23-2009, 10:39 PM

Guys, I am not an electronic person, but I am thinking of something similar with a 4016. The circuit on the right seems very simple and should do the job with 2-3 components. I thnk a suitable NOT gate might be a 4069 or 4049.

See : http://www.doctronics.co.uk/4016.htm

I am asuming you have an amp in your car.

I will be using a different controller to FB but the same concept should work. The link says the circuit is bidirectional so it should also switch two inputs to the one Output. In other words, let A1 and A2 outputs be the left and right channel on the HU and the B1 & B2 outputs be the left and right channels on the Car PC. The A1 & A2 Inputs will go to the L& R inputs on your amp. The earths on the RCA connectors could be wired common. We only care about the signal (centre wire) to switch the audio.

So on the FB, we need to use one input and one putput. Let the FB monitor the Amp turn on wire on the HU. When the FB sees a voltage on this wire, it signals the HU is turned on. When this signal is seen, the FB can apply a voltage to the enable wire. Voila! the HU signal goes to the amp and Bob is your uncle, the HU will play to the amp. Turn the HU off and the Car PC will play again.

The only issue I can see will be that the line out voltages will be quite different between the PC and the HU so volume will be quite different but you should be able to compensate for this with the HU volume control.

**Curiosity** · 05-23-2009, 11:02 PM

Ooh, bilaterals would work great. Cheaper than a bunch of relays for sure. Some audio pots could be used for gains for balancing. I think maxim has something like that, but serial controlled. Looks like some low cost mixer/switcher chips too, but you might need a PIC to control them.

**rodweb** · 05-23-2009, 11:08 PM

Originally Posted by Curiosity

Ooh, bilaterals would work great. Cheaper than a bunch of relays for sure. Some audio pots could be used for gains for balancing. I think maxim has something like that, but serial controlled. Looks like some low cost mixer/switcher chips too, but you might need a PIC to control them.

Curosity, glad you think this is on the right track, I have one of these I was thinking of using: www.arduino.cc rather than the FB

**Curiosity** · 05-24-2009, 12:02 AM

Well, I think the bilaterals paired for stereo and some logic to tie it all together would be better if you want to share with FB users or even manual switched, but it's kind of limited to simple A/B logic. But using using a uC would sure open up the possibilities, like with all the I2C chips on maxim. I saw a Mux chip on there too. I think that could mix and/or switch 4 inputs. here:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2081
http://focus.ti.com/docs/prod/folder...v4052a-q1.html

There are a few of them like that. I thought I saw something from TI once with gain controls built in, but can't remember.

**rodweb** · 05-24-2009, 12:17 AM

Originally Posted by Curiosity

Well, I think the bilaterals paired for stereo and some logic to tie it all together would be better if you want to share with FB users or even manual switched, but it's kind of limited to simple A/B logic. But using using a uC would sure open up the possibilities, like with all the I2C chips on maxim. I saw a Mux chip on there too. I think that could mix and/or switch 4 inputs. here:

http://www.maxim-ic.com/quick_view2....v4052a-q1.html

There are a few of them like that. I thought I saw something from TI once with gain controls built in, but can't remember.

That link may not be right. sort of took me nowhere. Which Chip are you suggesting?

**DVLRCR** · 05-24-2009, 12:20 AM

I didnt read everything, but this is a simple crude way to do it. just wire it up and it'll work without further input from PC etc.

EDIT: works by using two DPDT relays to switch the audio for you from Radio to PC using the remote output wire of the radio. Hope that helps you. If you want, i could probably do a board design etc as well but i'm in NZ so may not be too useful for you

also, it would be better not to use relays etc but its simple to setup and simple to use. May not work too well as i'm not sure how well the audio quality keeps when going through a relay :S