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Stereo-Mono?

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Stereo-Mono?

Postby JRocker » Thu Oct 22, 2020 12:04 pm

I've got a rack mount compressor/gate (Stereo or dual-mono operation) and my question relates to understanding the unit's stereo/mono operation.

My understanding is, if I am inserting the compressor into an insert channel and since the audio track is mono, I would use the compressor in mode right?

If I'm mixing the tracks down into a mix, and since it will be a stereo mix, I use the compressor in stereo mode, right?

Have I understood it or misunderstood it?
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Re: Stereo-Mono?

Postby Hugh Robjohns » Thu Oct 22, 2020 1:18 pm

Yes.

The stereo mode links the outputs (usually) of the side-chains in each unit together, such that the greatest control voltage from either determines the amount of gain reduction applied by both channels.

In that way both channels go up and down by the same amount, and so stereo images stay stable.

If you were to run the two channels in mono mode but with a stereo mix going through them, it would be possible for one channel to apply more gain reduction than the other, resulting in central sources (vocals, kick drum, etc) suddenly and briefly wandering off to one side and back again... so you'd potentially have a very unstable stereo image.
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Re: Stereo-Mono?

Postby JRocker » Thu Oct 22, 2020 8:45 pm

Thanks for the reply Hugh, but can you explain that in more tech free layman's terms please?
And does that mean regarding the mono into insert, my understanding is correct?
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Re: Stereo-Mono?

Postby CS70 » Thu Oct 22, 2020 9:21 pm

JRocker wrote:Thanks for the reply Hugh, but can you explain that in more tech free layman's terms please?
And does that mean regarding the mono into insert, my understanding is correct?

For the mono, you are correct.

For stereo.. the way a compressor works is (very roughly) that it the signal is duplicated and one copy is routed to a parallel "side chain": a circuit outside the main path that detects the transients. The side chain is used to generate a control signal for the actual attenuation circuit, that controls the volume of the original signal, according to the attack time which is dialed in by the user.

(Just to avoid confusion: all compressors have a side-chain but a basic compressor does not expose it to the outside.. If a compressor exposes the side chain input as an physical input, maybe placing an EQ before it, this allows the original signal to be controlled by another independent, external signal.. a technique which is commonly referred to as "sidechaining".. but it's a different thing).

So basically the compressor uses a part of itself (the side chain) to decide what the other part (the level attenuation circuit) will do.

In presence of a stereo signal, most compressors will sum the L and R channel of the "copy signal" to mono and use the result to attenuate both L and R channels of the "original signal" identically. So in other words the side chain always sees a mono signal even if the original signal (and the output) is stereo. The alternative would be to have two side chains (i.e. two detection circuits) and two control signal, and potentially attenuate the L and R channels of the original stereo signal differently.

That would create the problem that Hugh mentioned: if, say, a kick hit were to be attenuated on one side and not the other, the listener would perceive the result as the kick moving to the left or the right - because our brains uses the time and level differences in what we hear with our two ears to determine position. Since drummers don't usually move the drums sideways when they play, that'd be disconcerting...
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Re: Stereo-Mono?

Postby Tomás Mulcahy » Fri Oct 23, 2020 6:56 pm

JRocker wrote:
Have I understood it or misunderstood it?
You have understood it correctly :)
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