Technical Details Request: "Mono Button"

[Iceking007]

Hello,

I am considering/planning to build a speaker controller and it would be nice to add a hardware "Mono" mode button.  In my research I have learnt that "Mono" mode doesn't just sum the left and right channels together (ie you cannot simply wire the leads together with a switch); although often seen, this is an inaccuracy since true mono is a mathematical calculation of the channels divided by two.  Forgive me if that is wrong but I do recall a YouTube post where the host gave an example and demonstrated this by db drop when panning hard L/R.

So, is anyone wise enough to be able to explain how I could construct an analogue circuit to truly switch between 2.0, 3.0, 5.0, 7.0 and mono (I don't envision LFE playing much of a role).

Thank you for your time, meanwhile I will continue my research.

[SteveG_AudioMasters_ · Accepted solution]

Iceking007  wrote

In my research I have learnt that "Mono" mode doesn't just sum the left and right channels together (ie you cannot simply wire the leads together with a switch); although often seen, this is an inaccuracy since true mono is a mathematical calculation of the channels divided by two.  Forgive me if that is wrong but I do recall a YouTube post where the host gave an example and demonstrated this by db drop when panning hard L/R.

So, is anyone wise enough to be able to explain how I could construct an analogue circuit to truly switch between 2.0, 3.0, 5.0, 7.0 and mono (I don't envision LFE playing much of a role).

Since the concept of 'mono' is about the sum of the two content channels rather than the level of them, then in practice it's fine to just join the two channels together. But as a look at the pan settings in Audition will reveal, summing to mono isn't as simple as just picking a number:

The reason that the -3dB is adjustable is because there's a strong argument to be made, certainly when listening on speakers, that it's the combined power that should be considered, not just the level - especially when it comes to summing the bass. Which means that it should drop to -6dB, not -3dB, which is why the control has a range from -6 to -2dB (although -2dB doesn't really make any sense to me, I must say). So even if you only consider stereo, there isn't a clear-cut answer to the question when it comes to real-world summing.

When it comes to summing to mono from any form of surround, it gets even more complicated, because of the presence of the C channel, which may or may not have its own content - in other words, when you add LF and RF together, that's fine - but it won't include any content that's unique to the C channel. Since this content is generally added so that it sounds okay when mixed, it's not really possible to come up with a definitive answer to what level it's added back in as. You could make an argument that the level of the C channel that has to be added back in is 0dB if the LF and RF are simply summed, since that combined level will be 3dB higher than the C channel already is, but the results of doing this don't always work out at all, and it sounds 'wrong' - simply because the C channel was always intended to be heard only from the centre, which the LF and RF weren't. And all that's before any consideration of the rest of the channels, if they actually contain unique content. And at this point it gets a lot more complicated, as you find if you read threads like this: Bass Management Testing.

What it comes down to is that there isn't really an absolutely 'correct' way to sum to mono - because mono isn't the way our ears work, and therefore everything you do is a compromise based on a listening situation. It has always been considered to be important to check the mono compatibility of stereo mixes, primarily out of consideration for mono radio listeners, but this situation doesn't occur with surround mixes as a rule, so I'm not really sure what the fuss is about from that POV.

Which means that the answer to your question is quite simple...   You have anything up to eight signals going into a box, and you take a feed each of them to eight calibrated pots, feed all the pot sliders to a virtual earth mixer, and call what comes out of it 'mono'. Then you have a switch that switches between the incoming signals and the mono - which I'm going to assume you only sent to the C channel in a surround system, or L and R in a stereo one. And then you set the pots to whatever signal levels from each channel that you feel are appropriate...

Now that might sound a bit weird, but if I was building one, that's what I'd do!

[Iceking007]

Yeah I think you'd need some sort of complicated circuit if you want it analogue; digital... well you'd have to program in a whole bunch of 1's and 0's.

I see that Audition has a mono feature so I can use that if I need precise mono metering.  If I sum the channels up with a hardware button, and then route everything to the center channel?  Would that be good for mixing?

I'm trying to follow everyone's guidelines, it is said to do your mixing both in stereo/surround and in mono to ensure nothing is clashing; that's what I'm getting at with building a button in so I can select it on the fly.

Another option perhaps if available is to integrate a USB hardware controller and have the button activate Adobe's mono feature.  The downfall to that (outside of fiddling with a card and programming) is that would be a software feature, so lag and processing power may be a problem compared to a hardware selection that would be instant and consistent.

What are your thoughts on that?  Perhaps I could build in a pot variable limiter to adjust the gain for mono?

Thank you.

[Richard M Knight]

My sound card surround outputs go in to 8 channels of my Soundcraft mixer, the direct outs from these channels go into the multi channel input of my av amp. This gives me easy level control using the channel gain controls to set the speaker levels. I then use the mixer to sum the surround to stereo using the faders and this goes into the av amp stereo ip. The mixer also has a mono output this goes into another ip on the av amp. I can fine tune all the outputs to match volume wise and it is easy to switch modes using the amp selector.

[Iceking007]

Hmm, that's an interesting way of doing things, have a mixer on the output.

My question would be then what is your mixer doing when you select mono.  Is that analogue hardware or some distal processing?

What output card are you using that has 8 outputs?  I'm considering the Zoom UAC-8 it's a USB 3 device.

[Richard M Knight]

The mixer is completely analogue (Soundcraft Spirit FX8) the mono output, I'm sure, is only a combine of the stereos. As I only use the mono for phase cancellation checks the absolute level for me in unimportant.

[Iceking007]

Right, phase issues that's what it's used for; that's what I'm looking to do.  So I can just create a switch function that sums all of the channels and I could even add a selector switch that'll send the output to either the center or L/R channels.

Thank you.

[Richard M Knight]

I find an audio vectorscope is more efficient in detecting phase errors

[Iceking007]

Okay, so what is a phase error, how do you get it, and how do you prevent it.  Also probably most important other than knowing if you have one, but how do you fix it?

I could just Google all of this but if I ask I also get more tailored answers to Adobe and my specific situation.

Thank you.