“Why do most people tell me that balanced cables are only needed when you have really long cables, like 50’+?”
Balanced cables, when connected to the output of a balanced device, can carry a positive and negative version of the same signal. They are on separate wires and inverted in polarity from one another. When received at the other end, a special amplifier compares the difference in voltage between the signals in the two wires and amplifies it. Since the desired signal is completely opposite between the two it gets amplified. Any noise picked up along the cable length will be common (in phase/polarity) between the two wires. The amp amplifies the difference, which means signals that are the same end up getting canceled. This is known as common mode rejection. The degree to which these common (noise) signals are rejected is usually stated in a specification known as Common Mode Rejection Ratio (CMRR).
The longer the cable, the more likely it becomes that there will be more garbage picked up along the way, so it becomes more important that it be balanced so the user can benefit from the common mode rejection. Most people will tell you that cable lengths longer than about 20 feet should be balanced when dealing with analog audio (other forms of data transfer have their own unique limitations). This is a good rule of thumb, but in practice it really depends a lot on the environment (sources of EMI, etc.) and to a lesser extent the exact gear being used.
Related question: Is there any benefit to using balanced wiring for very short audio cable runs? In other words, is it really true that balanced cables are ONLY needed with long cable runs? The answer will be covered in a future Tech Tip. If anyone would care to venture a guess now feel free to e-mail us. A yet to be determined prize will be awarded for the first exceptionally good answer to this question.