OT - Balanced Audio
Grazie
wrote on 10/23/2003, 11:25 AM
Comments
johnmeyer
wrote on 10/23/2003, 2:08 PM
As an electrical engineer, I learned a lot about this in school (over a quarter century ago ...), but never really appreciated how ingenious the idea really is. In a nutshell, the standard unbalanced audio is designed to do everything possible to keep external noise and interference out of the cable. Thus, you end up with all sorts of shields around the cable. If you've ever cut through a standard "RCA" cable or a Cat-59 or Cat-6 coax (TV) cable, you've seen the braided shield.
By contrast, balanced audio makes no attempt whatsoever to keep out the noise. Instead, the only requirement is that both wires (you always need two wires so the current can both go to the equipment, and then return) get exposed identically to the noise. The usual way to do this is to twist the wires together. Then, at the receiving end, a "differential amplifier" is used. It only amplifies the differences in the signals appearing on each wire. Because the noise signal is present in exactly the same amount in each wire (because they have been exposed identically to the noise), the difference between the two noise signals is zero.
The reason, historically, that balanced audio was only used in expensive equipment is that differential amplifiers were expensive to produce using tubes or, later, discrete transistors. However, for the past twenty years, such amplifiers are available on a chip for pennies. Therefore, I am at a loss to explain why this superior method of moving signals around hasn't become more widely available on consumer and prosumer equipment. I guess the old way is "good enough" for most consumers, or so the manufacturers think. Ethernet, of course, is a balanced system, and look how incredibly cheap that has gotten (you can get Ethernet cards for your computer for just a few dollars).
One last note. I recently discovered very simple equipment that lets you convert balanced to unbalanced for both audio and video. These are passive devices that have RCA jacks for audio and an RCA or S-Video jack for video on one end, and then have an RJ-45 on the other. This lets you ship audio and baseband (composite) video over very long distances with no interference and very little loss, using standard Cat-5 network cable. Very cool gadget for home theater installers, as well as anyone else who has to run audio and video over long distances for presentations. The one I got is made by Unicom and is called a "Video/Audio Adapter" part number VAA-U501-VA.
By contrast, balanced audio makes no attempt whatsoever to keep out the noise. Instead, the only requirement is that both wires (you always need two wires so the current can both go to the equipment, and then return) get exposed identically to the noise. The usual way to do this is to twist the wires together. Then, at the receiving end, a "differential amplifier" is used. It only amplifies the differences in the signals appearing on each wire. Because the noise signal is present in exactly the same amount in each wire (because they have been exposed identically to the noise), the difference between the two noise signals is zero.
The reason, historically, that balanced audio was only used in expensive equipment is that differential amplifiers were expensive to produce using tubes or, later, discrete transistors. However, for the past twenty years, such amplifiers are available on a chip for pennies. Therefore, I am at a loss to explain why this superior method of moving signals around hasn't become more widely available on consumer and prosumer equipment. I guess the old way is "good enough" for most consumers, or so the manufacturers think. Ethernet, of course, is a balanced system, and look how incredibly cheap that has gotten (you can get Ethernet cards for your computer for just a few dollars).
One last note. I recently discovered very simple equipment that lets you convert balanced to unbalanced for both audio and video. These are passive devices that have RCA jacks for audio and an RCA or S-Video jack for video on one end, and then have an RJ-45 on the other. This lets you ship audio and baseband (composite) video over very long distances with no interference and very little loss, using standard Cat-5 network cable. Very cool gadget for home theater installers, as well as anyone else who has to run audio and video over long distances for presentations. The one I got is made by Unicom and is called a "Video/Audio Adapter" part number VAA-U501-VA.
farss
wrote on 10/23/2003, 3:20 PM
A word of warning.
Balanced systems are definately superior, no question. Problem is building a truly balanced system. Traditional balanced audio systems use transformers to create the balanced to unbalanced conversion. To make a transformer capable of handling the audio spectrum is expensive so over the years they have been ditched from all but the very best audio gear in favour of differential amplifiers. However this means the field wiring is directly connected to the electronics. High level unwanted signals that exceed the differential amplifiers supply voltage are clipped causing the amplifier to become very 'unbalanced' i.e. they end up in the output as who knows what.
With the increasing level of radio frequency interference in the environment the risk of things like mobile phones breaking through into these kinds of audio circuits is increasing. This is probably not such a worry in a studio situation where things are more controlled, in the field is another matter.
The other downside is that every unbalanced to balanced to unbalanced conversion as signals go in and out of gear is just that, another stage of electronics that the signal has to go through. Each one adds something to the signal that shouldn't be there. I've heard of audio guys going through their entire rig and ripping all the conversion out, saving over a dozen stages in the signal path resulting in a much cleaner signal.
Which isn't to say that everyone should be doing that!
The guy in question had a shielded studio and a very controlled environement.
The critical figure to look at when evaluating equipment that uses balanced circuits for field I/O is the maximum common mode voltage. Good transformer isolation will give a figure of 100s if not 1000s of volts. For the very best in transformers looks up Jensen, they make not only audio transformers but also ones for video both composite and component.
I know hum in video is not a common problem but when it happens it's a real disaster if it gets recorded.
Balanced systems are definately superior, no question. Problem is building a truly balanced system. Traditional balanced audio systems use transformers to create the balanced to unbalanced conversion. To make a transformer capable of handling the audio spectrum is expensive so over the years they have been ditched from all but the very best audio gear in favour of differential amplifiers. However this means the field wiring is directly connected to the electronics. High level unwanted signals that exceed the differential amplifiers supply voltage are clipped causing the amplifier to become very 'unbalanced' i.e. they end up in the output as who knows what.
With the increasing level of radio frequency interference in the environment the risk of things like mobile phones breaking through into these kinds of audio circuits is increasing. This is probably not such a worry in a studio situation where things are more controlled, in the field is another matter.
The other downside is that every unbalanced to balanced to unbalanced conversion as signals go in and out of gear is just that, another stage of electronics that the signal has to go through. Each one adds something to the signal that shouldn't be there. I've heard of audio guys going through their entire rig and ripping all the conversion out, saving over a dozen stages in the signal path resulting in a much cleaner signal.
Which isn't to say that everyone should be doing that!
The guy in question had a shielded studio and a very controlled environement.
The critical figure to look at when evaluating equipment that uses balanced circuits for field I/O is the maximum common mode voltage. Good transformer isolation will give a figure of 100s if not 1000s of volts. For the very best in transformers looks up Jensen, they make not only audio transformers but also ones for video both composite and component.
I know hum in video is not a common problem but when it happens it's a real disaster if it gets recorded.