Balanced Input/Output

[thetechnobear]

Hi,
In 1.0.7 I added a new config option in inconfig and outconfig for balanced i/o, its very simple it looks for an inverted signal on L/R channels and combines...
It works perfectly on my audio interface... but on my Virus TI it doesn't work.... the right(?) channel always shows no signal.
(both are advertised to have balanced outputs)

Im wondering if this is a 'hardware issue' and a misconception I have on balanced outputs.

I thought that if you plug a set of headphones into a balanced output, you sure hear the sound in both ears (albeit one is 180 deg out of phase).
True for audio interface , but this is not the case with the Virus... and a bit of research on the internet, I found this comment:

I find that differential, balanced outputs sound "odd" when listened to via headphones when compared to the signal from that unit's headphone output. Use the same source material for monitoring, and you should hear the difference.
Do realize: it's the device's circuitry that determines what is "balanced" or not; it just requires both legs to have the same impedance referenced to signal ground (a la JR's post above). The "differential" side of things is one (very effective) method of reducing/eliminating the effect(s) of external interference on the desired signal.

If you get sound in both ears, that is a useful positive test, but sound in only one ear, does not confirm the negative. You can still have a balanced impedance termination with only one hot leg driven with audio.

so this sounds like, its possible to have a balanced output that will not appear on both L&R because the circuitry is not 'correct'... ok, at this point my lack of hardware knowledge means I get confused about the different ways balanced signals are achieved

Is this true... is the headphone test 'flawed'? is there a better way to test?

is there anyway that I can get the Axoloti Input, to 'correct' this..

or is there some simple circuit I can add/cable 'hack', to make it work? (whilst of course keeping the benefits of balancing)

Mark

[thetechnobear]

ok, so this article is interesting...

it sounds like there are two type:

impedance balanced

Devices with impedance-balanced outputs actively drive the hot output only. The cold output is tied to ground via a resistor that matches (or balances) the output impedance of the hot signal conductor. In other words, there is no audio signal on the cold conductor, but, in a full balanced system, common-mode rejection will be maintained since the impedance is balanced between the two conductors.

active balanced

Devices with active-balanced outputs actively drive both the hot output and the cold output. The signal on the cold output is an inverted (polarity-reversed) version of the signal on the hot output. This creates symmetrical signals between the hot and cold outputs. This is the topology of all Benchmark equipment.

so I'm guess the headphone trick (and my axoloti solution) only works with active balanced, and not impedance balanced...

Is there a way to do impedance balancing with Axoloti?

I can imagine its going to be tricky converting impedance to active balanced output externally, as this implies some circuitry to create an inverted signal.

also makes me wonder, how do mixers/audio interfaces deal with this? do they support both?
how do I know my audio interface is really handling the Virus as an impedance balanced... as its seems there are multiple balancing wirings...

also another additional question for @johannes , is there any possible (electrical) issue with connecting axoloti to an impedance balanced gear (input and output) with a normal TRS cable.

doh... something I thought was a simple subject just got a whole lot more complicated

[johannes]

An active balanced output is also impedance balanced, and superior in every way except the weird inverted phase sound if you plug headphones in.

I do not see real disadvantages to "software" output balancing, except losing an output channel.

Essentially balanced outputs can connect to unbalanced input without problems.
And unbalanced outputs can connect to balanced inputs without problems.
You only get the benefit of balanced when both the input and output connected are balanced (using a balanced cable).

A good reference about interconnecting balanced and unbalanced:
http://www.rane.com/note110.html

A software-balanced input is less ideal than a true balanced input, for example the inputs could clip if there is a strong common mode signal present.

I don't expect electrical issues caused by connecting Axoloti to balanced in- and outputs.

[thetechnobear]

thanks for this... good to know there is no issues,
just seems a pity my Virus doesn't implement active balancing.

im a bit confused about the benefits of impedance balancing, I cant see how it reduces noise, or is it simply to help prevent ground loop issues...
(I don't get really the whole reason of needing to keep the impedance the same on the hot and cold... which is what apparently it does)

which I kind of leads into . why you say

An active balanced output is also impedance balanced, and superior in every way

as an aside, for those interested, Ive actually found another articles which gives a reasonable explanation - http://www.soundonsound.com/sos/oct13/articles/qanda-1013-4.htm

[johannes]

With the same impedance on a balanced output, the same amount of interference noise will appear to hot and cold at the receiver.
One difference between electronics and programming, is that in electronics there is no strict separation between output and input, connecting an output to an input affects the output... how much? - that's where "impedance" is involved...

[thetechnobear]

With the same impedance on a balanced output, the same amount of interference noise will appear to hot and cold at the receiver.

ah... so with impedance balancing the cold signal is pure noise... so this can still be inverted and added to the hot to cancel the noise - the difference being, you wont end up with double the amplitude in the 'real signal'.

hmm, so my current implementation will still work... but seems like the reducing halving the amplitude should be optional. as this is only needed on active balanced signals and not impedance balanced.

[johannes]

ah... so with impedance balancing the cold signal is pure noise...

Pure noise (thermal noise) will be uncorrelated between hot and cold, that will not cancel out with a balanced connection, but relatively reduced because the difference amplitude is doubled. Adding (or subtracting, its the same) two uncorrelated noise sources does not double the amplitude.
More relevant is ground loop noise and electromagnetic interference noise, those will be very correlated and will cancel out significantly.

but seems like the reducing halving the amplitude should be optional.

I'd not halve the output level in balanced mode, that leaves the most significant bit of the DAC unused, it is better to reduce the input gain of the mixer by 6dB. And in case the input does not handle that level it is still better to reduce the analog output gain of the DAC.
For the input side, it is more logical to halve the difference to preserve the maximum signal level of audio/in object, but you'll get only -6dB peak if the Axoloti input is connected to an impedance balanced output.

[shihweichieh]

Hi guys, I am new to Axoloti, my question is very basic... do you know where to have the balanced signals? from here? and what is the corresponded object for this output pins? I was looking for DC coupling outputs for galvanometers controlling.

wei

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[nicotep]

Does that mean that I can wire TRS jacks to the balanced onboard pins for both input & output in case that I'll be using balanced jacks, but won't have any trouble using mono jacks as it will "convert" to unbalanced ?