Popular Post asdf1000 Posted May 16, 2020 Popular Post Share Posted May 16, 2020 Hi @mitchco Stupid question here. But are the terms 'time domain correction', 'impulse response correction', 'time alignment', 'phase correction' all talking about exactly the same thing, when talking about digital room EQ with speakers? mitchco and AnotherSpin 1 1 Link to comment
Popular Post mitchco Posted May 16, 2020 Popular Post Share Posted May 16, 2020 Hi @asdf1000 Not a stupid question at all. It is a complex subject area and sometimes manufacturers use the terminology in different contexts or in marketing material that can also cause confusion. Time domain correction and impulse response correction are the same thing. Time alignment and phase correction are aspects of time domain correction. Lengthy explanation ahead. Time alignment generally refers to all speakers, and the drivers in each speaker, are "time aligned" so that all of the direct sound from the drivers arrives at our ears at the same time. There are a couple of ways that is implemented. The ultimate way is to create a linear phase digital crossover for each driver in a system, including sub(s). The DSP software will digital delay the signal for each driver relative to a reference, like the tweeter for example, so that the acoustic centers of each driver are perfectly time aligned. Here is an example of that in my 3 way triamp system dual subs: The delay in milliseconds is what the DSP software measured for each driver in the system. The DSP correction will ensure all drivers are time aligned in the digital XO and correction FIR filter. For passive XO systems, the DSP software will apply an overall or global time alignment correction to align the speakers/drivers as best as possible, within a certain time window that the DSP software can do (or set by the user). An "ideal" loudspeaker is a minimum phase device. Flat frequency response and flat phase response. However, when you place speakers in rooms, we get reflections, standing waves, room resonances, etc. Because of this at low frequencies, we no longer get the ideal minimum phase response, but rather areas that are mixed phase or non minimum phase behaviour. So using long, low frequency excessphase time domain correction windows (like 600ms at 10 Hz), the DSP software will correct all channels towards the same time and phase behaviour as defined by "target" response one defines in the DSP software. We are restoring the minimum phase response of our ideal speaker at the listening position. For example, once an acoustic measurement is made, the DSP software extracts the minimum phase response. Then by inverting the amplitude response and applying it as a filter to the measured response, the result is a flat frequency response. By EQ'ing the amplitude response, the phase response is also adjusted, as it is a minimum phase system. Then the DSP software, independently of the minimum phase correction, also corrects the excess phase response towards the minimum phase response so that one ends up with the ideal low frequency and phase response across a listening area. Important note: The excess phase is the phase difference between the real signal and the minimum phase response. To learn more about minimum phase John Mulcahy, author of REW has a good explanation. To learn more about time alignment and excess phase correction, these articles using Acourate and Audiolense shows what is possible. And this article talks about an ideal speakers low frequency and phase response and what happens when placed in a room. Hope that helps. ShawnC, asdf1000, The Computer Audiophile and 2 others 4 1 Accurate Sound Link to comment
asdf1000 Posted May 17, 2020 Author Share Posted May 17, 2020 WOW. Thank you so much @mitchco If not already, you need to be inducted into the Audio Hall of Fame for your incredible contributions. I'm going through your 2 Audiolense articles and I'll ask specific questions in the comments sections of those articles, so to keep the discussions there, in case it helps others that are reading those articles. mitchco 1 Link to comment
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