FM Acoustics technical
CMRR
Not everything that has XLR connectors, 2 signal lines and a shield is balanced.
The basic goal of signal transmission is to transfer the signal from a source to a load with no loss and with maximum fidelity.
Major aspects for optimal signal transfer are:
(a) minimize signal losses in the line
(b) perfect shielding
(c) elimination of hum, noise and interference signals that could be induced into the interconnect cables
(d) minimize crosstalk
(e) adequate surpression of ground potential errors
The idea of balancing sounds simple: use two opposing polarities of the original. The positive polarity version is sent through the positive line and the opposite is sent through the negative line.
When these two signals are recombined in a precision differential summing input stage, the original full level signal is recovered while common mode noises, interferences, hum, etc. are surpressed (these unwanted
signals enter both lines symmetrical and therefore can be rejected).
In additon crosstalk will be reduced dramatically because:
(a) only half of the original peak level signal is sent down each line and
(b) there is self-cancellation of two closely coupled but opposite polarity signals.
A truly balanced circuit must be totally symmetrical ("balanced") in relation to ground.
The technical term that describes the accuracy of the balancing is the "Common Mode Rejection Ratio", in short "CMRR".
This value indicates how well the circuit will reject unwanted signals that enter both lines symmetrically, (e.g. hum or noise).
The higher the CMRR value, the better the balancing of the circuit and the less negative effect interference signals can have.
The CMRR values of existing so-called "balanced" equipment is as low as 30-60dB, a disappointing result not really warrantying the terms "balanced" or "symmetrical".
Such low CMRR values are not acceptable as the circuit and the lines are not well balanced at all. For decent performance a CMRR of 80-100dB is required and this must be so over the full frequency band.
However in reality many of the existing circuits show poor performance at the critical low and high frequencies.
What also must be avoided are signal currents flowing in the ground paths of the various units.
The voltages generated by these ground currents must be common mode voltages. Only then can it be guaranteed that and they have no negative influence on a balanced signal.
A balanced circuit must provide two equal but opposite polarity signals, which must be symmetrical to ground. The impedance from positive and negative signal lines to ground must be equal for common mode signals and for normal audio signals.
CMRR
Not everything that has XLR connectors, 2 signal lines and a shield is balanced.
The basic goal of signal transmission is to transfer the signal from a source to a load with no loss and with maximum fidelity.
Major aspects for optimal signal transfer are:
(a) minimize signal losses in the line
(b) perfect shielding
(c) elimination of hum, noise and interference signals that could be induced into the interconnect cables
(d) minimize crosstalk
(e) adequate surpression of ground potential errors
The idea of balancing sounds simple: use two opposing polarities of the original. The positive polarity version is sent through the positive line and the opposite is sent through the negative line.
When these two signals are recombined in a precision differential summing input stage, the original full level signal is recovered while common mode noises, interferences, hum, etc. are surpressed (these unwanted
signals enter both lines symmetrical and therefore can be rejected).
In additon crosstalk will be reduced dramatically because:
(a) only half of the original peak level signal is sent down each line and
(b) there is self-cancellation of two closely coupled but opposite polarity signals.
A truly balanced circuit must be totally symmetrical ("balanced") in relation to ground.
The technical term that describes the accuracy of the balancing is the "Common Mode Rejection Ratio", in short "CMRR".
This value indicates how well the circuit will reject unwanted signals that enter both lines symmetrically, (e.g. hum or noise).
The higher the CMRR value, the better the balancing of the circuit and the less negative effect interference signals can have.
The CMRR values of existing so-called "balanced" equipment is as low as 30-60dB, a disappointing result not really warrantying the terms "balanced" or "symmetrical".
Such low CMRR values are not acceptable as the circuit and the lines are not well balanced at all. For decent performance a CMRR of 80-100dB is required and this must be so over the full frequency band.
However in reality many of the existing circuits show poor performance at the critical low and high frequencies.
What also must be avoided are signal currents flowing in the ground paths of the various units.
The voltages generated by these ground currents must be common mode voltages. Only then can it be guaranteed that and they have no negative influence on a balanced signal.
A balanced circuit must provide two equal but opposite polarity signals, which must be symmetrical to ground. The impedance from positive and negative signal lines to ground must be equal for common mode signals and for normal audio signals.
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