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Re: Temporal distortion - measurable?

Posted by john curl (M) on November 21, 2002 at 09:41:30

In Reply to: Temporal distortion - measurable? posted by RandyB on November 20, 2002 at 21:30:23:

This is an interesting point, and it has been researched over the decades.
First, there is no significant delay in feedback circuits associated with amps or preamps. If you look at a model of a normal circuit with negative feedback, you find that the output is connected to the input with a single resistor. This means that the propagation delay of a single resistor would be the extra delay of the circuit, beyond that associated with the finite closed loop bandwidth.
Now, WITHOUT the negative feedback resistor added, there is a significant delay in the circuit. Where did the delay go? Well, the delay is compensated for by an overshoot of current or voltage in the input stage generated by the virtually instantaneous difference seen by the input stage between the input and the output voltage. This is the root cause of TIM distortion in many cases.
At a deeper level, there are other problems with negative feedback, including generation of FM distortion, harmonic multiplication, etc, but the delay problem is pretty small.
For the last 30 years, we have been working on the delay problem between speaker drivers. This was first brought forward by the late Richard Heyser. He stated at one AES meeting that the delay caused by a path difference of 2 feet might be audible in some cases. He was openly criticized by Paul Klipsch at the time for saying this. This is because Paul was taught in school that the ear was phase insensitive, which is not strictly true. This was known as OHM's LAW OF ACOUSTICS. Later, this was shown to be inaccurate, and this OHM's LAW was put to rest.
Since those early years, 30 years ago, FFT techniques have allowed us to phase align loudspeakers to within 100's of microseconds or even better. Most hi end loudspeakers attempt to get things lined up fairly well.
Finally, it must be understood that all music signals picked up by a microphone have a finite rise time. The best practical risetime is around 10uS This is a long time, compared to how fast a well designed feedback loop can respond, and of course,the marginal digital sample rates used in CD's , slow it further.

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