Francisbr a écrit:Si ça peux aider :
http://www.audax.fr/forum/read.php?4,22374,22739,quote=1
Cordialement.
Francis Brooke
je reviens sur cette discussion de 2010 dans le forum audax .
j'y lis Francis que tu interprètes la courbe "minimum phase" comme la phase propre au HP.
il est possible que sur Arta (le soft duquel les courbes sont issues) ce soit le cas, sur REW, d'après ma compréhension de la doc, ce n'est pas le cas, sur Fuzz je ne sais pas non plus (pas plus que sur Arta).
Cependant, la notion de phase minimum et phase mixte semble assez "standard", si l'on en croit wikipedia ...
pour info, je donne ci dessous le help de REW sur ce point ainsi qu'une traduction toute personnelle, qu'il convient de critiquer
REW texte 1:
Minimum Phase
In discussions of equalisation, and particularly equalisation applied to attempt to improve the acoustic response in a room, "minimum phase" will often crop up - generally in the context of whether or not EQ can successfully be used to address a response problem. So what is "minimum phase", and why should we care?
There are rigourous mathematical and systems theory definitions of what constitutes a minimum phase system, but I will not repeat them here. In the context of acoustic measurements a system which is minimum phase has two important properties: it has the lowest time delay for signals passing through it and it can be inverted.
traduction approximative:
Dans les discussions autour de l'amélioration de la réponse acoustique d'une pièce,l eterme "minimum phase" surgit souvent - generalement dans un contexte où on se demande si une equalisation pourra ou non améliorer un problème. qu'est-ce que "minimum phase" ?
il y a une définition théorique d'une système dit "minimum phase", on ne la donnera pas ici (mais wikipedia la donne). Dans le contexte des mesures acoustiques, un système à "minimum phase" a 2 propriétés importantes: le signal a un délai (une durée) le plus petit pour traverser le système et il peut être inverser.
REW text2:
The minimum phase response is generated by using the measurement amplitude and calculating the corresponding minimum phase from it, using a mathematical relationship between the two that holds for minimum phase systems. By looking at the difference between the measured and minimum phase (the excess phase) and measuring the slope of that difference to find the excess group delay, we get this plot: "graph"
Now we have something we can work with. Anywhere the excess group delay plot is flat is a minimum phase region of the response. We can see there are regions even at very low frequencies where the response is not minimum phase, between about 44 and 56Hz for example. These will usually correspond to regions where there are sharp dips in the response, and underline the poor results which are often found when trying to lift such regions with EQ. Low frequency peaks on the other hand are usually in minimum phase regions, the plot is fairly flat in the region of the 28Hz and 60Hz peaks, which bodes well for attempts to apply EQ to them. In general, the peaks in a response are a result of features that are correctible through equalisation (speaking technically, they are due to the poles of the response and the equaliser can place zeroes that cancel the poles).
There are regions at relatively high frequencies which are minimum phase, such as 300 to 500Hz, despite the wild variations of the response in that area, so it would be possible to apply EQ there. However, we need to remember that the measurement is only valid for the microphone location at which it was made, and as frequency increases the response changes more rapidly as the microphone moves. EQ that looks good at the original measurement position may give worse results at other positions, so it is important to check wherever listeners will be. Narrow bandwidth EQ adjustments should not be used outside the modal range, the higher the frequency the broader the EQ adjustment needs to be to stand any chance of being useful ouside a very small region.
As an aside, the excess group delay plot also clearly shows there is a time offset between the subwoofer and the main speaker, the sub being about 25ms delayed, which is not so obvious from the overall group delay plot. Excess group delay is a useful plot for time aligning speakers.
traduction de certains passages:
il faut regarder le graphique 'excess phase" pour trouver les régions de type "minimum phase" (trouver les ensembles de valeurs de la fréquence pour lesquelles, la réponse en phase est de type minimum phase) car le graphe "minimum phase" est une construction mathématique à partir des mesures faites du système, "comme si" ce système était de type "minimum phase" mais il ne l'est pas partout.
PS: si vous voulez je mets la totalité du chapitre, car j'en ai pris qu'un tiers.
