Stereo/Mono Bal/Unbal

[ThomJ]

I have been asked the question and am having trouble with a complete answer so taking a chance on looking stupid:

 

Can anyone explain the meanings/difference between

 

mono/stereo

 

more importantly Balanced/Unbalanced lines (into and out of mixers)

 

The person I am explaining to is not stupid but is not like a computer and may need the info punching in more than once. :unsure: :(

 

Ta

 

Thom

[James]

I have been asked the question and am having trouble with a complete answer so taking a chance on looking stupid:

 

Can anyone explain the meanings/difference between

 

mono/stereo

 

more importantly Balanced/Unbalanced lines (into and out of mixers)

 

The person I am explaining to is not stupid but is not like a computer and may need the info punching in more than once. :unsure:  :(

 

Ta

 

Thom

<{POST_SNAPBACK}>

 

For stereo try reading Systems for Stereo Sound Reinforcement - Performance Criteria, Design Techniques, and Practical Examples by Jim Brown, (Publication available from the AES website,)

 

For Balanced Audio try Bill Whitlock's paper Balanced Lines in Audio Systems-Fact, Fiction, and Transformers also available from the AES.

 

James

[p.k.roberts]

Good publications, but maybe a bit heavy? At the risk of being accused of outrageous simplifications, how about these for explanations.

 

When we hear stuff in the world, the sound we hear slightly different versions of sounds in each ear. The sounds are likely to arrive at very slightly different times at each ear (they are probably travelling slightly different distances) and at slightly different levels (the one that's travelled furthest has gone through more air, so has lost more energy, so is quieter). This enables our brain to analyse the slight differences and work out where a sound is coming from.

Stereo is an attempt to recreate this. We send slightly different sounds to a pair of speakers and the sound can then be made to appear as if it's coming from any point between them, sometimes known as the 'Sound Field'. This can be achieved by using a single mic via a 'pan pot' or delaying one side (most common in 'pop situations); alternatively a pair if mics can be used, each fed to the appropriate speaker (more common in 'classical' situations).

A Mono system simply sends the same audio signal to all the speakers, so no directional information is there.

 

The balanced/unbalanced thing is to do with how we get electrical 'audio' signal down wires and sort of works like this. If you imagine a screened audio lead like, say, a guitar lead, it has a screen and one core. Any interference that is picked up by the single core is amplified along with the audio. This is clearly not a 'good thing'. With a balanced connections (such as the input or output of a good quality desk) the audio cables have a screen and two cores (hence the need for some kind of 3 pole connector). In effect the same audio signal is fed down each core but 'out of phase'. Any interference picked up by the cores will be 'in phase' and will be cancelled out by a balanced input; this clearly is a 'good thing'.

 

As I said at the beginning of this post, there are some huge simplifications here, but hopefully it gives you the idea.

[ThomJ]

many thanks, it was the balanced line explanation I needed most, but the stereo analogy was great.

 

I'll pass it on

 

:(

[Freddie]

Also try here

© BTS, Bath SU

[Simon Lewis]

Zaphod,

 

If I am reading this correcly, then your theory is wrong! Noise rejection is a function of external noise being a common mode signal. This is why the two cores are twisted - to ensure that the same level of noise is present on both legs. You can operate a mic lead with the earth wire disconnected, and the differential signal is still amplified...

 

It is not the case that the noise on the screen is 'subtracted' from the two legs.

 

Sorry to be pedantic ;-)

 

Simon

[j_b]

Zaphod,

 

If I am reading this correcly, then your theory is wrong! Noise rejection is a function of external noise being a common mode signal. This is why the two cores are twisted - to ensure that the same level of noise is present on both legs.

<{POST_SNAPBACK}>

 

Spot on. Also, each of the two signal wires (hot & cold, or +ve and -ve, whatever you choose to call them) need to have the same impedance to ground as each other, so that the external noise is coupled into the pair at the same magnituide.

 

In fact, they don't both need to be driven, so long as the impedance is balanced...dare we go there?

[paulears]

I think the BTS answer is an example of how some things get taught a little wrong, to avoid getting into areas where heavy weather is almost certainly going to follow. CMR, (as an almost permanent feature of many threads on 'professional audio forums, - where they spend 3 weeks debating pin 1 issues, which is the real problem in many cases) is a tough area.

 

The BTS answer is sort of student friendly, at the level they're at. Once they move into real engineering, then the simple (but wrong) explanation gets replaced with the real one.

 

Some concepts are just too advanced to answer correctly as it takes the learner into areas that involve MATHS - not popular. Both legs get noise on them, and if the noise is equal, but opposite polarity, then it cancels - this is true. It's just that the majority of noise induced into audio systems doesn't happen like this, but explaining the real version involves a bit more brainpower. Most of the people reading the cancel explanation, already know about phase cancellation in audio systems, and so can understand the noise analagy (even if it ain't correct!)

[Simon Lewis]

Paul,

 

Your comments are very valid. However, as I'm sure you've experienced, once a student learns the (incorrect) simplified version it can be really hard for them to take on the correct version!

 

I try and use simple analogies that will still hold water when a technical expalnation is given later - for example - balaned lines can be likened to a playground see saw. An equal signal present on both sides results in no movement...

 

Overall, I'd like to reverse this "if it's got maths in it, we'll ignore it" approach, but I doubt if I'll make too much impact ;-)

 

Simon

[Russ]

Your comments are very valid. However, as I'm sure you've experienced, once a student learns the (incorrect) simplified version it can be really hard for them to take on the correct version!

<{POST_SNAPBACK}>

 

Regardless, this is the way education works. The first thing they teach you at A-level chemistry is that GCSE chemistry was a load of [I actually can't think of an appropriate word to go here which doesn't get caught by the profanity filter]. Happens all the time - you get used to it after a while :).

 

Having said that, the real explanation in this particular case doesn't seem markedly more difficult to understand. But that's probably just me.

[paulears]

I don't think it's you at all...... I think it's just that sometimes, the little extra input then runs away and you end up going off on a huge tangent (of great, useful and often interesting material) that still leaves you the original work to do on top.

 

A great example is condenser mics - dynamics are easy to explain, using the "do you remember the big coil, meter and magnet experiment you did at school" approach. However, condensers introduces new concepts - capacitance, charge etc that can really get messed up in the student heads. You have to explain one bit, that leads to the next, and so on. Then someone mentions phantom power, or the lack of it, and you have to then explain electrostatics and pre-charged diaphragms.... aaaaagh

[Darkfold]

Made perfect sense to me the first time I read it (Physics ALevel student). I just like the Bath pages as I can point all the little techlings who come my way there first to give em all a bit of an understanding of how it works. Most kids I meet are quick enough to realise that a simple explanation is not always exactly right.

[Tomo]

I don't like the fact they give ground as being the 'reference', because it isn't.

 

I've always given the explanation that:

 

A balanced signal is one where you have two lines with the same signal on them, but one is inverted. The DIFFERENCE between the lines is what is measured to give the signal.

Because noise is introduced nearly equally on both lines, it almost vanishes when you measure the difference.

 

The grounded screen is there to reduce the total noise, because if it's too big to start with you can't get rid of it.

 

(This is still simplified, but at least this makes the real explanations merely build upon rather than replace.)

[BillSputters]

As a TV sound rec, I sometimes get involved in multicamera shoots of stage shows. Now TV being what is is, all the money goes on pictures and * all on sound. Given that my standard kit is a rifle and 2 radios, I am fighting a losing battle. Anyway, I can often persuade the PA guy to give me a feed of whatever he is doing, and use my rifle as an audience mic. Trouble is, the feeds from sound desks are often full of hum. Question is, if I just use the + and - legs of the feed, and get rid of the earth, will this reduce/get rid of the hum?

[IanG]

As a TV sound rec, I sometimes get involved in multicamera shoots of stage shows. Now TV being what is is, all the money goes on pictures and * all on sound. Given that my standard kit is a rifle and 2 radios, I am fighting a losing battle. Anyway, I can often persuade the PA guy to give me a feed of whatever he is doing, and use my rifle as an audience mic. Trouble is, the feeds from sound desks are often full of hum. Question is, if I just use the + and - legs of the feed, and get rid of the earth, will this reduce/get rid of the hum?

<{POST_SNAPBACK}>

 

'Maybe'. :-)

 

When I've done similar things I've used a passive DI as an isolating transformer, and that's solved the problem nicely.