Every so often on indietalk, when advice is sought about microphones, the issue of balanced and unbalanced is brought up. The advice is always; to get a balanced mic and recorder. This is because a balanced connection greatly reduces interference. In this post I'll try to explain simply what balanced audio signals are, how they work and why they are so much better.
When recording on set the audio signal captured by the mic has to be transferred to a mixer, audio recording device or the camera through a connecting cable. There are two ways of doing this, through a balanced line or through an unbalanced line. The most obvious difference visually is that a balanced signal requires three wires whereas an unbalanced signal only requires two. The industry standard for balanced signals is the XLR connector, which has 3 pins (or 3 holes in the female connector). Here's a picture of XLR connectors.
The 3 wires inside a balanced audio cable are called Hot (red wire), Cold (blue wire) and Earth (bare wire). An unbalanced cable only has Hot and Earth. The audio signal in an unbalanced cable is simply passed down the red wire to the receiving equipment, say the mini jack input on a DSLR. The balanced signal is a little more complicated and requires additional circuitry on the transmitting equipment (say a mic) and additional circuitry on the receiving equipment (say an audio recorder). The circuitry in a mic with a balanced output copies the signal captured by the mic, inverts it's phase and passes this (out of phase) signal down the Cold (blue) wire. The original (in phase) signal is passed down the Hot (red wire). Here is a diagram showing an example of an original analogue audio signal, which would be passed down the Hot (red) wire and the "out of phase" signal created by the balanced circuitry which would be passed down the Cold (blue) wires.
Notice how the Cold (blue) signal is out of phase with the Hot (red) signal. For example; where the hot signal has a value of +50 the cold signal has a value of -50. Now let's introduce some interference across the cable, marked as green on the diagram below. The interference could be from any number of electrical devices or devices which transmit radio frequencies (such as mobile phones) or electro-magnetic interference (such as a power line). Bare in mind, we are talking about interference through the cable (connecting mic to recorder), not about electrical type sound (such as the hum of a fridge) which is picked up by the mic itself.
The balanced circuitry in the receiving equipment (say a recorder) takes the cold signal and inverts it's phase (so it's now back in phase with the hot signal) and then sums together the Hot and Cold signals. The diagram below shows the state of the signals after the Cold signal has been phase inverted by the recorder's balanced circuirty, just before the Hot and Cold signals are summed together.
I've drawn a black line, purely as a reference point. At the black line, the value of the red (original) signal is +50, the value of our blue signal is also +50, sum these together and the result is a combined signal with a value of +100. Now do the same with the 2 interference (green) signals. What is the result? ......
In fact, wherever you draw the black line, the result of summing together the two interference signals is always zero. This principle is called Common Mode Rejection (CMR). Now, imagine an unbalanced signal by just ignoring the bottom half of the diagram. Our wanted (red) signal value is +50 and our unwanted interference (green) signal is +25, compared to balanced values of +100 and zero!
The longer the cable the more likely we are to get interference, exactly how long depends on the environment and how contaminated it is with RF (Radio frequency) or EM (Electro-Magnetic) interference. However, due to CMR the balanced cable rejects this interference. In practice an unbalanced cable should never exceed 5 meters (about 16ft), anything over 3 meters (about 10ft) is taking a risk and you could run into problems with just 2 meters (roughly 6ft) of unbalanced cable. In theory (given good quality cable and a strong enough original signal) a balanced cable could be up to 7 miles in length!! I'm sure you'll agree that CMR is very effective!
From all this you will have noticed a couple of things: First of all, just having an XLR cable will not give you a balanced signal. You need the balanced circuitry in both the mic and the recorder to employ the CMR technology, so you can't just use say a mini-jack to XLR converter. Secondly, equipment which has this balancing circuitry obviously costs more money than audio equipment which doesn't. Particularly with regards to mics and recorders, only having unbalanced connectors is indicative of the very lowest budget/quality audio equipment.
Hope this is useful,
G
When recording on set the audio signal captured by the mic has to be transferred to a mixer, audio recording device or the camera through a connecting cable. There are two ways of doing this, through a balanced line or through an unbalanced line. The most obvious difference visually is that a balanced signal requires three wires whereas an unbalanced signal only requires two. The industry standard for balanced signals is the XLR connector, which has 3 pins (or 3 holes in the female connector). Here's a picture of XLR connectors.
The 3 wires inside a balanced audio cable are called Hot (red wire), Cold (blue wire) and Earth (bare wire). An unbalanced cable only has Hot and Earth. The audio signal in an unbalanced cable is simply passed down the red wire to the receiving equipment, say the mini jack input on a DSLR. The balanced signal is a little more complicated and requires additional circuitry on the transmitting equipment (say a mic) and additional circuitry on the receiving equipment (say an audio recorder). The circuitry in a mic with a balanced output copies the signal captured by the mic, inverts it's phase and passes this (out of phase) signal down the Cold (blue) wire. The original (in phase) signal is passed down the Hot (red wire). Here is a diagram showing an example of an original analogue audio signal, which would be passed down the Hot (red) wire and the "out of phase" signal created by the balanced circuitry which would be passed down the Cold (blue) wires.
Notice how the Cold (blue) signal is out of phase with the Hot (red) signal. For example; where the hot signal has a value of +50 the cold signal has a value of -50. Now let's introduce some interference across the cable, marked as green on the diagram below. The interference could be from any number of electrical devices or devices which transmit radio frequencies (such as mobile phones) or electro-magnetic interference (such as a power line). Bare in mind, we are talking about interference through the cable (connecting mic to recorder), not about electrical type sound (such as the hum of a fridge) which is picked up by the mic itself.
The balanced circuitry in the receiving equipment (say a recorder) takes the cold signal and inverts it's phase (so it's now back in phase with the hot signal) and then sums together the Hot and Cold signals. The diagram below shows the state of the signals after the Cold signal has been phase inverted by the recorder's balanced circuirty, just before the Hot and Cold signals are summed together.
I've drawn a black line, purely as a reference point. At the black line, the value of the red (original) signal is +50, the value of our blue signal is also +50, sum these together and the result is a combined signal with a value of +100. Now do the same with the 2 interference (green) signals. What is the result? ......
In fact, wherever you draw the black line, the result of summing together the two interference signals is always zero. This principle is called Common Mode Rejection (CMR). Now, imagine an unbalanced signal by just ignoring the bottom half of the diagram. Our wanted (red) signal value is +50 and our unwanted interference (green) signal is +25, compared to balanced values of +100 and zero!
The longer the cable the more likely we are to get interference, exactly how long depends on the environment and how contaminated it is with RF (Radio frequency) or EM (Electro-Magnetic) interference. However, due to CMR the balanced cable rejects this interference. In practice an unbalanced cable should never exceed 5 meters (about 16ft), anything over 3 meters (about 10ft) is taking a risk and you could run into problems with just 2 meters (roughly 6ft) of unbalanced cable. In theory (given good quality cable and a strong enough original signal) a balanced cable could be up to 7 miles in length!! I'm sure you'll agree that CMR is very effective!
From all this you will have noticed a couple of things: First of all, just having an XLR cable will not give you a balanced signal. You need the balanced circuitry in both the mic and the recorder to employ the CMR technology, so you can't just use say a mini-jack to XLR converter. Secondly, equipment which has this balancing circuitry obviously costs more money than audio equipment which doesn't. Particularly with regards to mics and recorders, only having unbalanced connectors is indicative of the very lowest budget/quality audio equipment.
Hope this is useful,
G
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I can even tell you electronically why it works.
thanks alcove for taking the time to break it down in a tangible way! Stuff like this makes this site great.
psst -- it wasn't alcove... 
