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I was just reading a few reviews of sound cards.
Confused authors seem to want to explain IMD as distortions of sums of input waves of different frequencies.
They are confusing the issue, not realizing the source of IMD is the time phase shift from negative feedback loops, in particular global negative feedback. A negative feedback loop is a way of reducing harmonic distortion. The method is invert the output stage waveforms and mix it with the input wave form signal to allow the two wafevorms to cancel. The theory is any signal left over is a distortion signal created during the amplification process. This resultant difference signal is then mixed into the input signal 180º out of phase, so that it will cancel out most of the distortion as it is amplified. It works for the most part, but has a very big flaw, intermodulation.
The problem causing intermodulation is each amplification stage introduces a small amount of time delay. It is the time delayed sound signal that is not cancelled by the input and output signal feedback loop mixing. In effect, the time delayed signal resulting is re-introduced into amplification by the feedback loop itself and creates a very noticeable source of non-harmonic distortion. This is defined as intermodulation distortion.
The only way to reduce IMD is to not use global negative feedback (in favor of using discrete negative feedback loops in each amplification stage) to reduce distortion and to design with high speed high quality electronics. The reason global negative feedback is used with such popularity is it is very easy to implement, stage by stage feedback requires analysing each stage and determining the correct amount of feedback, which depends on the gain and speed of each component, and that is something that changes in each batch of parts.
Now...I bet someone reading this for the first time is finally realizing what the big deal is in the difference between cheap amplifiers and esoteric amplifiers.
Ok...I'm off my rant now.
Confused authors seem to want to explain IMD as distortions of sums of input waves of different frequencies.
They are confusing the issue, not realizing the source of IMD is the time phase shift from negative feedback loops, in particular global negative feedback. A negative feedback loop is a way of reducing harmonic distortion. The method is invert the output stage waveforms and mix it with the input wave form signal to allow the two wafevorms to cancel. The theory is any signal left over is a distortion signal created during the amplification process. This resultant difference signal is then mixed into the input signal 180º out of phase, so that it will cancel out most of the distortion as it is amplified. It works for the most part, but has a very big flaw, intermodulation.
The problem causing intermodulation is each amplification stage introduces a small amount of time delay. It is the time delayed sound signal that is not cancelled by the input and output signal feedback loop mixing. In effect, the time delayed signal resulting is re-introduced into amplification by the feedback loop itself and creates a very noticeable source of non-harmonic distortion. This is defined as intermodulation distortion.
The only way to reduce IMD is to not use global negative feedback (in favor of using discrete negative feedback loops in each amplification stage) to reduce distortion and to design with high speed high quality electronics. The reason global negative feedback is used with such popularity is it is very easy to implement, stage by stage feedback requires analysing each stage and determining the correct amount of feedback, which depends on the gain and speed of each component, and that is something that changes in each batch of parts.
Now...I bet someone reading this for the first time is finally realizing what the big deal is in the difference between cheap amplifiers and esoteric amplifiers.
Ok...I'm off my rant now.