![]() If the two signals are 90° out of phase, the meter will show half scale, and shows zero (no deflection) if the signals are 180° out of phase. I have no details for the Neve meter, but from what I can gather the SSL meter shows full scale when the signals are in phase - regardless of amplitude (within reason). Some people think this is completely wrong, and it's very hard to disagree. Problem is that +180 and -180 are effectively the same thing for a steady tone, so the left and right had extremes of the scale are the same! The meter also responds to amplitude as well as phase, so panning a signal left or right shows a phase change that doesn't exist. The scale is apparently marked -180° at the left side, +180° on the right, and zero (in phase) in the middle. Sony (formerly MCI) have a phase meter that cannot actually work as claimed. This is obviously a very personal issue, and there is no hard and fast 'right' or 'wrong' answer. From information gathered from forum posts, I offer the following information. Phase correlation meters are less common, and there is divided opinion about how they should display the results. The result can be confusing - especially for anyone new to this type of display. Most (but not all) vectorscopes show both the amplitude and phase of the signal. A mono signal, but with one channel 90° out of phase will display a circle, and if one channel is inverted (180° phase shift), the display is a horizontal line. ![]() The most common display is arranged so that a mono signal - where left and right channels are identical in all respects - will show a vertical line on the oscilloscope display. Depending on the way the circuit is set up, the display will change. There appears to be argument amongst studio engineers as to the 'best' display mode. One channel (left or right) is fed to one input, and the other to the second input. Not very many mixer manufacturers have even bothered making phase meters, and broadcast studios (FM radio, TV, etc.) most commonly use a 'vector-scope' - essentially a two channel oscilloscope connected in X-Y mode to display a lissajous pattern. If a channel is inverted or a stereo microphone pair has one mic out-of-phase, this will show up with the pointer in the red area. Because the audio content of the two channels can be very different, there will inevitably be times when the correlation of the two channels is less than 'perfect', and this is shown on the meter. Ideally, a stereo signal will remain in the green section most of the time - this indicates that the left and right channels are basically in phase. Just so you have some idea of what these meters might look like, see the photo below. ![]() It uses a now obsolete CMOS Schmitt trigger IC (MC14583) and appears to be wired in such a way as to be as confusing as possible. One circuit is from SSL (Solid State Logic) and dates from around 1984 or thereabouts. There are many, many questions posed on forum sites, and one schematic pops up a few times. This is one of those projects that came about from a reader's question, and it piqued my curiosity to the extent that I had to see what was available (virtually nothing for DIY) and how much interest exists for something like this.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |