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Hi I have a problem that I am not sure I can fix. I have in-ceiling speakers that are very difficult to get to. When I had them connected initially, the company that installed them used speaker wire where the negative and positive wires look exactly the same as the outer sheath is the same. I recently purchased a new receiver and when I disconnected it I dropped the cables accidentally and now I do not know which is positive and which is negative. The grills on the speakers do not come off so I cannot see which way the cones are moving. I have watched YouTube videos on it but no one explains if / how you can check phase if you cannot see the speaker end of the connections. Any help would be greatly appreciated. Cheers
(edit 21/4/17) - I've made some edits below based on finding some errors - noting each as an edit just makes it messy, I've left errors due to my lack of understanding - these are addressed by Dave's comments. ========================================================================== I've posted similar information a couple of times recently, but thought it worthy of its own thread. Tools like REW, ARTA, Holm Impulse can show the phase response of speakers, rooms etc. Often phase plots look like this (my tapped horn sub measured indoors at the listening position) The dotted vertical lines do not represent instantaneous changes in speaker phase - they represent a way to keep the graph on the page known as "wrapped phase". The left axis scale is dB and is irrelevant for this graph - phase is on the right axis in degrees When the graph hits (in this case) +180 degrees on the right hand scale, the dotted vertical line drops to -180 degrees to keep it on the page, and the curve continues to rise. The peak around 35-38Hz, and the peak just under 60Hz are 360 degrees apart (ie 38Hz is 180 degrees phase lead, and 60Hz is 540 degrees phase lead) - the graph is "phase wrapped" to keep it on the same page for easier viewing. This allows more detail to be shown rather than having a phase plot covering over 1000 degrees - to know the actual phase you need to count the vertical lines - each one represents 360 degrees. The same graph shown "unwrapped" below same measurement, just unwrapped shows a steadily rising phase from 30Hz - note the scale on the right hand side- 1080 degrees - REW wouldn't let me zoom out further. Research has demonstrated that smoothly changing phase is in-audible - lucky really, if we could hear the phase change above we wouldn't have hi-fi as we know it. However phase changes stop being smooth when we add multi-way speakers, crossovers and rooms Lots has been said about "minimum phase" and I'll attempt to add some value to the discussion on this - but some of my measurements confuse me - so hoping the SNA community can help me also A dead flat frequency (amplitude) response across the audible range (say 20Hz to 20kHz) is an unachievable target. Anytime you have a wiggle in the frequency response, you have phase changes. Look at measurements of speakers - where their response deviates most from flat is where the phase changes most. outdoor close miked measurement of my sub apologies for similar colours - top trace is FR (dark blue) and light blue trace is phase. Note the phase plot (light blue) is more steep where there are larger changes in FR. Minimum Phase - what is it? I still grapple with an easy to understand definition...and always fail Consider our stereo systems: You can define them in the time domain (we listen to our systems in the time domain) - the normal measurement is an impulse response - this describes the system in a single measurement - any ringing in the time domain will cause wiggles in the frequency domain You can define them in the frequency domain based on amplitude and phase - the classic Frequency Response curve (more correctly known as Amplitude Response), only describes part of the system. To properly define a system in the frequency domain requires both an amplitude and a phase response. You can convert between the time domain and frequency domain using a Fourier Transform. From an impulse response you can calculate both the frequency (amplitude) response and phase response using the Fourier Transform. To calculate the impulse response you need both the amplitude response and the phase response (still using a Fourier Transform). In a Minimum Phase system (and only in a minimum phase system), you can calculate the Phase Response from the Amplitude Response, and you can calculate the Amplitude Response from the Phase Response. Remember you need both Amplitude Response and Phase Response to calculate (via the Fourier Transform) the Impulse Response. So with minimum phase systems: if you have either the amplitude response or the phase response you can determine the impulse (time) response With Minimum Phase systems, a peak in the frequency response, can be EQ'd by an opposite "cut" using minimum phase EQ (Parametric EQ - PEQ) and correct both amplitude and phase. If treating minimum phase effects, dips can also be boosted via PEQ and correct both the amplitude and phase response. BUT is a room minimum phase? Unfortunately in normal rooms, regions where the response is "minimum phase" is unpredictable. Tools such as REW can measure minimum phase. Read here : https://www.roomeqwizard.com/help/help_en-GB/html/minimumphase.html Excess Phase is a measurement in REW that calculates the difference in phase between the calculated phase of a minimum phase system based on the measured Impulse Response and the actual phase response of the measured system. Excess Phase shows which areas are "minimum phase" - flat areas indicate "minimum phase". Below is an outdoor measurement of my sub - black is "Excess Phase" - not flat is non "minimum phase" My observations: I would have expected a flat area of minimum phase between 35-55Hz based on the flattish FR - not the case. I did not expect the area between 70 and 80Hz to be flat for minimum phase in the middle of a dip in FR the only relatively flat areas of minimum phase are below 25Hz and between 70-80Hz Bringing the sub into the room gives different results Setup: in room measurement at the listening position sub + mid bass low pass Xover on sub around 50Hz or so no high pass on the 18" mid bass (but loads of linear phase speaker correction to push the 18's lower) Dark blue is FR, Light Blue is phase, black is Excess Phase Measurement is mic at the listening position, sub and left main 18" driven together (in their range) Observations: phase changes most when FR wiggles most (see below 20Hz, 90-100Hz, big dip around 250Hz based on Excess Phase, EQ should work well <100Hz, but not above 100Hz All graphs are of measurements taken some time ago - and I did some cherry picking to best describe what I was trying to show. I particularly don't understand how an outdoor measurement of my sub could demonstrate less minimum phase behaviour than the indoor measurement of the sub integrated with my mid bass speakers - would be great if the SNA community could help me understand why??? cheers Mike
Just stumbled onto this page that demonstrates a very comprehensive objective (as in measurements not as in audibility) comparison of a variety of upsampling filter implementations. http://src.infinitewave.ca/ This is probably of more interest to graph nerds but I think it's pretty interesting and wanted to share. It also gives some insight into vendor claims about the filters used in their software/hardware. It does only demonstrate non-integer multiple upsampling which represents a worst case, for most of us here it's unlikely we'd ever do anything other than a direct multiple resample. Chris