THE BALANCED INTERFACE The purpose of a balanced audio interface is to efficiently transfer signal voltage from driver to receiver while rejecting ground noise. Used with suitable cables, the interface can also reject interference caused by external electric and magnetic fields acting on the cable. The true nature of balanced interfaces is widely misunderstood. For example “Each conductor is always equal in voltage but opposite in polarity to the other. The circuit that receives this signal in the mixer is called a differential amplifier and this opposing polarity of the conductors is essential for its operation.” [3] This, like many explanations in print (some in otherwise respectable books), describes signal symmetry – “equal in voltage but opposite in polarity” – but fails to even mention the single most important feature of a balanced interface. SIGNAL SYMMETRY HAS ABSOLUTELY NOTHING TO DO WITH NOISE REJECTION — IMPEDANCE IS WHAT MATTERS! A good, accurate definition is “A balanced circuit is a two-conductor circuit in which both conductors and all circuits connected to them have the same impedance with respect to ground and to all other conductors. The purpose of balancing is to make the noise pickup equal in both conductors, in which case it will be a common-mode signal which can be made to cancel out in the load.” [4] The impedances, with respect to ground, of the two lines is what defines an interface as balanced or unbalanced. In an unbalanced interface, one line is grounded, making its impedance zero. In a balanced interface, the two lines have equal impedance. It’s also important to understand that line impedances are affected by everything connected to them. This includes the line driver, the line or cable itself, and the line receiver. The line receiver uses a differential amplifier to reject common-mode voltages. The IEEE Dictionary defines a differential amplifier as "an amplifier that produces an output only in response to a potential difference between its input terminals (differential-mode signal) and in which output due to common-mode interference voltages on both its input terminals is suppressed." [5] Since transformers have intrinsic differential response, any amplifier preceded by an appropriate transformer becomes a differential amplifier. The basic theory of the balanced interface is straightforward. (For purposes of this discussion, assume that the ground reference of Device A has a noise voltage, which we will call "ground noise," with respect to the Device B ground reference.) If we look at the HI and LO inputs of Device B with respect to its ground reference, we see audio signals (if present) plus the ground noise. If the voltage dividers consisting of Zo/2 and Zcm on each of the lines have identical ratios, we’ll see identical noise voltages at the two inputs of Device B. 3 Since there is no difference in the two noise voltages, the differential amplifier has no output and the noise is said to be rejected. Since the audio signal from Device A generates a voltage difference between the Device B inputs, it appears at the output of the differential amplifier. Therefore, we can completely rejects the ground noise if the voltage divider ratios are perfectly matched. In the real world, we can’t perfectly match the voltage dividers to get infinite rejection. But if we want 120 dB of rejection, for example, we must match them to within 0.0001% or 1 part per million! The ground noise received from Device A, since it exists on or is common to both wires, is called the common-mode voltage and the differential amplifier provides common-mode rejection. The ratio of differential or normal-mode (signal) gain to the common-mode (ground noise) gain of the interface is called the common-mode rejection ratio or CMRR (called "longitudinal balance" by telephone engineers) and is usually expressed in dB. There is an excellent treatment of this subject in Morrison's book. [6] If we re-draw the interface as shown here, it takes the familiar form of a Wheatstone bridge. The ground noise is “excitation” for the bridge and represented as Vcm (common-mode voltage). The common-mode impedances of the line driver and receiver are represented by Rcm+ and Rcm!. W hen the + and ! arms have identical ratios, the bridge is “nulled” and zero voltage difference exists between the lines — infinite common-mode rejection. If the impedance ratios of the two arms are imperfectly matched, mode conversion occurs. Some of the ground noise now appears across the line as noise. The bridge is most sensitive to small fractional impedance changes in one of its arms when all arms have the same impedance. [7] It is least sensitive when upper and lower arms have widely differing impedances. For example, if the lower arms have infinite impedance, no voltage difference can be developed across the line, regardless of the mis-match severity in upper arm impedances. A similar scenario occurs if the upper arms have zero impedance. Therefore, we can minimize CMRR degradation due to normal component tolerances by making common-mode impedances very low at one end of the line and very high at the other. [8] The output impedances of virtually all real line drivers are determined by series resistors (and often coupling capacitors) that typically have ±5% tolerances. Therefore, typical line drivers can have output impedance imbalances in the vicinity of 10 Ù. The common-mode input impedances of conventional line receivers is in the 10 kÙ to 50 kÙ range, making their CMRR exquisitely sensitive to normal component tolerances in line drivers. For example, the CMRR of the widely used SSM-2141 will degrade some 25 dB with only a 1 Ù imbalance in the line driver. Line receivers using input transformers (or the InGenius IC discussed later) are essentially unaffected by imbalances ® as high as several hundred ohms because their common-mode input impedances are around 50 MÙ — over 1000 times that of conventional “active” receivers. Note that this discussion has barely mentioned the audio signal itself. The mechanism that allows noise to enter the signal path works whether an audio signal is present or not. Only balanced impedances of the lines stop it – signal symmetry is irrelevant. W hen subtracted (in the differential amplifier), asymmetrical signals: +1 minus 0 or 0 minus !1 produce exactly the same output as symmetrical signals: +0.5 minus !0.5. This issue was neatly summarized in the following excerpt from the informative annex of IEC Standard 60268-3: “Therefore, only the common-mode impedance balance of the driver, line, and receiver play a role in noise or interference rejection. This noise or interference rejection property is independent of the presence of a desired differential signal. Therefore, it can make no difference whether the desired signal exists entirely on one line, as a greater voltage on one line than the other, or as equal voltages on both of them. Symmetry of the desired signal has advantages, but they concern headroom and crosstalk, not noise or interference rejection.