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Author Topic: Lost of performance when mono mic feeds two recording channels?  (Read 931 times)

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Offline gunk

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Lost of performance when mono mic feeds two recording channels?
« on: January 11, 2018, 04:21:42 PM »
How is the performance of a mic affected by making it feed two channels?  For example, a mono mic is wired to feed left and right recording channels on a portable digital stereo recorder.

Presumably the signal level to each channel would be reduced making S/N worse.  Would the difference be noticeable?

What about electrical matching?  My electronic theory is too poor to know exactly what happens here or how it would affect the audio but I get a feeling impedance matching would be affected in some way.

I ask this because my new mono mic is pre-wired with a 3.5mm stereo jack plug (TRS) which feed both L and R channels with the mic signal.  Is it worth making an effort to rewire this so the mic feeds only one channel or is the difference minimal?

Offline EmRR

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Re: Lost of performance when mono mic feeds two recording channels?
« Reply #1 on: January 11, 2018, 04:39:43 PM »
It really depends on the mic and preamp in question.   A passive split feeding two channels halves the impedance seen by the mic.  If that value is still 10x the stated impedance of the mic, there should be no appreciable change. If it becomes significantly less than 10x (like 2x-5x), there may be some loss of level, increase of distortion, and in severe cases some loss of treble. Any of those things are probably very minor in most cases. 

Offline gunk

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Re: Lost of performance when mono mic feeds two recording channels?
« Reply #2 on: January 15, 2018, 02:05:51 PM »
Would the signal going to each of the two channels be half what it would be to a one channel?

In other words, 3 dB worse for the S/N value?

Offline DSatz

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Re: Lost of performance when mono mic feeds two recording channels?
« Reply #3 on: January 18, 2018, 09:05:20 PM »
> Would the signal going to each of the two channels be half what it would be to a one channel?

With equal load impedance on the two inputs, the power to each would be cut in half--but the interface between a microphone and a preamp isn't based on power transfer; it's based on voltage. So there's no set rule; it would still depend on the specific impedances of the microphones and the recorder inputs. Still, impedances aren't always constant across the frequency spectrum, so the amount of the signal loss could vary at different frequencies, altering the balance of the frequencies in your recording. That's something you'd normally want to avoid.

Another issue that you didn't bring up is microphone powering, which can get messy in this type of situation.

You're doing something that professionals generally try to avoid doing. If you really need one microphone to feed two inputs, you could use a high-quality microphone "splitter" (basically a transformer with two independent secondary windings) but it would be better if you fed the signal into one channel of a good preamp (or into a good mono preamp such as a Sound Devices MP-1), then split the line output of the preamp. Otherwise, what you're doing would tend to reduce the quality of any recording that you make that way.

Just what do you hope to gain with this arrangement, anyway? Does your recorder only work in stereo, and it just bothers you to make a stereo recording with one channel unused? Or is there some other reason (e.g. you don't know what levels to expect, so you want to set one channel's gain 6 or 10 dB higher or lower than the other one)?

--best regards
« Last Edit: January 18, 2018, 09:15:11 PM by DSatz »
music > microphones > a recorder of some sort

Offline gunk

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Re: Lost of performance when mono mic feeds two recording channels?
« Reply #4 on: January 22, 2018, 04:59:12 PM »
Another issue that you didn't bring up is microphone powering, which can get messy in this type of situation.

....

Just what do you hope to gain with this arrangement, anyway? Does your recorder only work in stereo, and it just bothers you to make a stereo recording with one channel unused? Or is there some other reason (e.g. you don't know what levels to expect, so you want to set one channel's gain 6 or 10 dB higher or lower than the other one)?

I have a Rode SmartLav+ mic which comes with a 3.5mm stereo jack plug.  The mic capsule is wired to both left and right channel contacts on the plug.  So I am wondering what affect this has if I record either in stereo or in mono (jack plug tip only).

I hadn't thought about powering issues that you mention.  This is a Plug-In Power mic so presumably the recorder applies the same voltage twice over (so to speak) on the mic capsule and when the mic is working it sends its signal to be split across two channels.

It seems to work okay but I am interested if another recorder might not be so happy with such an arrangement. 

Your transformer solution sounds a bit elaborate for such as simple cheap mic.

 

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