Multiple Headphones

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Skyfire

New Member
I was hoping to record a reaction video with four people. Do we Headphones to record a Video where we are in a camera box? I tried it but ended up with an echo because the microphone was also picking up the output audio from the computer. I have seen others post videos like this, but just not sure how it is done.

Or if using headphones can’t be avoided, is there some sort of splitter piece that can handle four Headphones?

Thanks to anyone who can answer.
 
A

AaronD

Active Member
Skyfire said:
Do we Headphones to record a Video where we are in a camera box?
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Whaaat? I think your proofreading missed an edit.

Skyfire said:
Or if using headphones can’t be avoided, is there some sort of splitter piece that can handle four Headphones?
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Try one of these or similar:

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Or if you happen to have some audio gear lying around:

Most headphones are 32 ohms or higher, and most "live" speakers are 8 ohms or lower. So if you get an amplifier that is designed to drive a "live" speaker, then you can hook up at least 4 headphones to it and still be within its rating.

Be VERY careful of the volume! It doesn't take much! A small amp is probably a good idea.

Also, if you're driving headphones, avoid amps that have a bridged output. Also called "bridge-tied-load" or BTL. A lot of amp modules do that because it gives them effectively twice the voltage swing and 4 times the power from the same supply voltage. But it also requires each speaker to be completely independent. (*) Headphones are not independent: the negative terminals are shared, which is usually called "ground".
(*) In a bridged amp, both output terminals for a given speaker often idle around half of the supply voltage, not ground, and they *both* wiggle from there, opposite each other, following the audio signal for that speaker. Or, they do idle at ground, but instead of one terminal going negative, the other terminal goes positive. All that matters is the difference across the speaker itself.

What you might do though, if you don't mind losing a stereo field that may not have existed in the first place, is to connect the left and right channels of the headphones to the + and - terminals of a single BTL amp channel (or any amp for that matter), and leave the ground floating, or disconnected. Now you've converted two 32-ohm speakers into one 64-ohm speaker, as the amp sees it, so you can put even more of them on the same 8-ohm (or lower) amp channel, and you've halved the voltage that each speaker sees, which quarters the power (-6dB) for the same setting on the knob, and might allow that setting to be somewhat reasonable. One ear will be the "wrong" polarity and the other "right", but you can't hear that.
 
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AaronD

Active Member
If you want to use a live speaker in the same room as the mic, then the term to look for is "echo cancellation". OBS doesn't do that, but there may be a VST effect that does it. Or you might send the mic(s) through an external processor and *then* to OBS, and OBS's output through the same external processor and *then* to the speaker(s).

However it's done, it's practically a hard requirement that the same module sees both the FINAL signal that goes to the speakers and the RAW signal that comes from the mic. Immediately before the speaker amp and immediately after the mic amp. Its output is the echo-cancelled mic, ready for the rest of the mic processing. Even mute needs to be *after* the canceller (if your mics have switches on them, tape them on), so that it doesn't adapt itself to silence and then fall on its face when it turns back on again. Separate echo canceller for each mic, and ideally one speaker - not two fed the same, but literally just one speaker - so that the echo cancellers have less to be confused by.

If you can't do that, then you're effectively in the same boat as a live music gig. Look at anti-feedback techniques from that world, which include:
  • Speaker directionality and placement, keeping in mind that directionality depends on frequency and is NEVER a single number, even if that's all the spec sheet gives you.
  • Mic directionality and placement, same thing.
  • Room acoustics, including walls, floor, ceiling, furniture, people, etc.
  • Automatic notch filters, often called "feedback eliminators". These are easily fooled though, so they may not work for you. I had one in a live rig, for example, that claimed that there wasn't anything to do...while I was manually "riding the fader" to keep a feedback ring going without running off to full-system-power. Its warranty replacement did the same thing. I gave up.
  • Manual notch filters, often called "ringing out the system". You do this with a parametric EQ, by turning up the volume until it just barely starts to ring, and then sweeping a deep narrow cut across the frequency range until it stops ringing. Turn up the volume and tweak the frequency some more to find the exact right spot for it, until it rings at a completely different frequency. Keep that notch where it is, and add another, the same way, to kill the second ring, etc. When the volume gets high enough without ringing, you're done. Check the sound to see how much you've wrecked it with all those EQ notches, and maybe take some out with the understanding that you can't push the volume up as high as you did to set them.
  • Pitch-shift or frequency-shift, rarely used for feedback, though I'm not sure why. I have. The idea is to take advantage of the room's uneven frequency response, and turn the feedback "loop" into more of a "spiral", where each time through comes out at a slightly different frequency. If you set it right (doesn't take much: 17 cents in my experience for one particular basketball-court-sized gymnasium, smaller rooms probably need more), then a single trip through the loop goes from a peak in the room response to the valley next to that peak. At a volume setting where that peak would have "rung", its signal dies in the valley instead. This allows a higher setting before feedback, but only as much as the room has variations in response. Above that, it still "misbehaves", but it's more of a "weird warble" as each stimulus "spirals away" and another one starts; it's not a piercing ring anymore. Again, turn up the volume while tweaking the shift amount to find the exact right spot for it, and then check the sound to see how much you've wrecked it.
    • This trick assumes low enough latency that each trip through the loop is too short for anything intelligible. If that's not true, then you still hear it as an echo regardless, and so the pitch/frequency-shift only makes it worse.
 
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