Building Your Own KVM Switch...With Audio Connectors?

Michael B asks: "I recently bought an 8 port KVM to switch between various workstations and servers at home, and I'm realizing what I can't switch is audio! I've seen 4 port audio/video type switches (for things like switching between video console systems) but this doesn't address more than 4 ports and having the adding RCA/S-Video is a waste. I've thought of building one from scratch using Radio Shack parts, but can't find any info, such as a schematic, online. Does anyone have any ideas or sources of info for this one? Belkin seems to make a 4 port KVM with audio, which would be nice, but I need more than 4 ports. Thanks for any help!" After a bit of digging, I found this monster, which does 8-port KVM and audio, but that weighs in at a hefty $500USD, which is rather expensive. Has anyone managed to find schematics to such switchers so that folks, who aren't afraid of soldering irons, can build their own?

Comments

cheaper solution

[rumpledstiltskin]

instead of spending $500 or spending all your time building your own switch, just buy a tuner with multiple inputs. you get to have your switch (at the cost of having to push one more button, although if you use the remote it shouldn't be all that strenuous) and you could hook up some advanced (dolby, dts, whatever) audio to it if you'd like.

Re:cheaper solution

[TimeTrip]

It also allows you to separate the audio from the video/keyboard/mouse. Often times at work, i switch between machines on my KVM switch, but keep my headphones connected to just one machine because its playing my mp3's :). If the audio was run throught the KVM switch i would probably go insane.

Here's an idea

[HotNeedleOfInquiry]

If your 8-port switch has led select lights for the port that is active, bring out a wire from each of the signals that drives the leds and wire each of them to a simple buffer transistor and relay.

Each of the relays would gate the respective sound signals onto a common bus. Use a double pole relay for stereo. 2N2222 (npn) and 2N2907 (pnp) are available from Fairchild through Digikey in TO-92 packages and would be a good choice for a relay driver.

Likewise, an Omron TX2-5V is a reasonably priced DPDT telecom relay with a 5v coil, also available from Digikey.

Re:Here's an idea

[Kz]

Instead of mechanical relays, use electroinc analog switches, like the easily found 4066. cleaner, less power, no need of buffer transistors. maybe you'd have to use a transistor to match the LED output to the CMOS logic, but that's easier than driving a relay.

Don't switch audio -- mix it.

[-dsr-]

Buy a cheap audio mixer and control all your sound inputs individually.

Re:Seperate switch (Formatted right)

[GigsVT]

Note to self, PREVIEW! (Slashdot doesn't seem to do the TT tag the way one would expect!)

I built a 2X2 for switching two computers to either earphones or speakers.

Two DPDT switches, one controls input, one controls output.

(ASCII art deleted because Slashdot can't deal with it.)

1 and 2 are connected together, i and o are inputs and outputs, left and right. Tie all grounds together, switch L+R. I guess doing that could cause a ground loop hum, but it hasn't caused a problem for me. There isn't really a pop or anything when you switch mine, even with the speakers on. You need reasonably good quality switches though, if the switch feels loose or wiggly, it will probably cause a lot of static when you switch it. I found a lot of my surplus switches were not up to audio use. I eventually settled on push-on push-off switches I scavenged from an old set of speakers, they were power and bass boost switches.

This concept is the same way one would build a switch with many more inputs or outputs, you just need rotary switches, DP#T where # is the number of things to switch.

Here are some switches

The 2 pole 5 position and 2 pole 6 position from there is probably what you want. You can cascade a DPDT and two 5 position rotary switches for 10 outputs with one input or vice versa. Scavaging from manual computer switch boxes is probably not a good idea, since most of those are low numbers of positions, with high number of poles.

Re:Any recomendations for mixers?

[John Meacham]

the Samson PL1602 is a real nice cheap rack-mount mixer. I use it to mix all my audio inputs, plus it has its own headphone port of the mix, and I can send arbitrary channels back to the computer (independent of what i am listening to) for recording audio from other devices.

Not too hard...

[Tom7]

This is pretty trivial to build; you shouldn't need schematics. Audio signals are pure analog, so all you really need to do is get a switch with multiple throws (3 for stereo) and wire it up to jacks and mount it in some kind of box. If you get a rotary switch you could have a pretty much arbitrary number of computers hooked up to it.

If you care about sound quality or having the device look nice, just get a desktop mixer. That's perfect for having multiple computers driving a single stereo.

Belkin SOHO

[Sleepyguy]

buy a belkin soho .. they are cheap and they support audio and usb.

_

Re:USB KVM

[jmorris42]

Haven't actually tried it either, but I doubt you can do that. KVMs have to have smarts so as to fake out each PC to think it always has the keyboard/mouse. Some are even bright enough to hold the state of the keyboard LEDS and restore the correct pattern when you switch machines. Unless you got one that was also smart enough to handle audio in a similar way (at which point it would probably BE the audio device) you would run into trouble. You see, if the KVM isn't faking things for a device each time you switched you would generate a disconnect on the machine you just switched away from and a connect event on the one you just punched up. I doubt Linux could handle that many disconnect/connect events and I'd bet money Windows wouldn't last a day under that sort of abuse.

mixer

[g4dget]

Why do you want to switch? Why not put the audio into an analog mixer and have all of them active simultaneously?

Re:the easy way - true story

[Cecil]

For anyone who is actually considering Radio Shack as anything other than an emergency refuge for "awk, I ran out of solder and I need to finish this!"... Radio Shack does not even sell electronics stuff around here anymore. Maybe a bit of soldering equipment, and some connectors, but for actual electronics? Nah. They are morphing into a half-assed Electronics Boutique but with more annoying salespeople. I am fairly certain from other people's stories that this is a trend that is continuing everywhere in the Radio Shack chain.

You're better off finding a real electronics store, or going mail-order. DigiKey is a decent mail-order place for USA/Canada/UK. You will find their selection very refreshing.

Audio, Video, Keyboard and Mouse Switching

[Anonymous Coward]

What you are trying to switch is the red, green and blue video signals, which are analogue and have several MHz bandwidth; the horizontal and vertical scan timing signals, which are lowish-speed digital (a few kHz and a few Hz respectively); the keyboard and mouse clock and data, which are low-speed digital (c.1200 baud) but two-way; and the left and right hand audio, which are analogue without much bandwidth (20kHz tops).

Just how much bandwidth is in a video signal? Well, if you have a screen of 1024x768 and it refreshes at 70Hz, that means the electron beam has to draw 1024*768*70 = 55050240 dots a second, each dot being either a crest or a trough leaving a frequency of 27.525120 MHz in the worst case, when rendering alternate dim and bright pixels. Bear in mind also that the device has to work into a low impedance of 75 ohms, so the on resistance - and the switching time - are as critical as the propagation delay.

The bidirectional digital signals are not a problem. Analogue switches will handle them. Think: switch off = different voltages both sides, switch on = same voltage both sides.

You will need some really high bandwidth analogue switches for the video, but you'll get away with 4066-type for the audio and data and maybe the timing. Take a look at http://www.maxim-ic.com for parts and schematics. The basic principle, though, is to connect all the switch ICs to switch one device at a time onto a set of common rails for R, G, B, HS, VS, KC, KD, MC, MD, LA, RA.

Another option would be to mod an existing unit with some 4066-type analogue switches. You'll need a +-5V supply from somewhere. Drive the switch gates from the switching signals in the first unit. A really impressive feat if you can manage it would be to construct a simple audio mixer (use a 2904 op amp) and rig it so the unselected channels are still fed into the mix, but at a lower volume. Then you can still hear death screams :-)

Don't even *think* about using any kind of mechanical switch or relay. Moving parts suck bigtime and will distort any analogue signal you try to pass through them.

Re:USB KVM

[MrResistor]

I think Linux would be just fine. I have 2 USB mice, an Intellimouse Explorer and a little IBM optical travel mouse (for my 3 year old, the MS mouse is huge and the side buttons are positioned so that she can't grip it and reach buttons 1 and 2 without hitting the side buttons, the other one is the perfect size for her hand). I hotplug them all the time with no problems, on SuSE 8.1 Pro with the default kernel, if you care.

I know how KVMs work, or at least how they are supposed to work ;-) but I don't think that would be an issue, unless the sound stream dies every time every time you unplug the audio device. I suspect that could largely be solved with scripts, though, at least on Linux.

Re:Any recomendations for mixers?

[Aczlan]

on the low end:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewIte m&item =2528831537&category=23785
http://cgi.ebay.com/ws /eBayISAPI.dll?ViewItem&item =3023072453&category=3319

on the high end: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item =2529625961&category=41473

DJ Aaron Z

Found a 2-port KVM switch with audio!!

[shdragon]

I found this at newegg.com

It's 2 ports, supports audio & is only about 90$.

Do it in software

[Anonymous Coward]

How about that:

Machine you work on:
cat /dev/audio | nc 192.168.100.1 48050

Machine you plug your speakers in:
nc -l -p 48050 > /dev/audio

I built my own passive selector using ....

[Abm0raz]

3x female 1/8" stereo jacks

4x female RCA mono jacks

4x SP/DT slide switches (3/16" x 1/2")

2' jumper wire

1x plastic electrical junction box

1x clear plastic picture cover from picture frame. (3"X5")

First 4 parts are available from RadioShack for a total less than $10. The junction box is available at Lowe's for $1.66. The picture cover you can get with any cheap picture frame.

step 1: lay the junction box upside down on a piece of paper and trace. Cut out the outline.

step 2: lay out the jacks and switches on the paper. the 4 jacks go with the 4 outputs (or inputs as you see fit) and the odd jack out is your static out/in.

step 3: glue the paper to the plastic

step 4: drill and dremel the plastic according to the paper pattern.

step 5: screw/glue all audio parts in.

step 6: flip over and solder all left channel and right channel negatives together.

step 6: solder wires from your static jack right channel to the middle-left side of each switch and the left channel to the top left of each switch.

step 7: solder the middle right of each switch to the CORRESPONDING input/output jack's right channel and the top right to the jacks left channel.

step 8: place the plastic on the junction box and seal with glue or screws

To use as a multi-out: place the output from your computer into the static jack. Run lines from the other jacks to your computer speakers, stereo, tv, headphones ... When a switch is in the on position, that jack will get signal. When it is off, it won't.

To use as a multi input, connect your speakers to the static jack. Connect all inputs to the other 4 jacks. When the switch for a jack is on, it's signal will goto the speakers. When it is off, it wont.

I built this cause I was sick of switching my sound cables everytime I wanted to switch from my computer speakers (when playing games) to my TV (when watching DVDs on my DVD out) to my headphones (when listening to music while coding and my roommate was sleeping).

CAUTION: this is a PASSIVE system. IF you are using it as a multi-out, there isn't much danger. You will just get decreased volume as you turn on more outputs because the signal will be split. If you are using it as a multi-INPUT system and try to input several signals to the speakers at once, you can run the risk of blowing them.

If you have any questions, reply. I'll give you my ICQ and write up some clearer diagrams.

-Ab