SIM 3 TRACES OF MISWIRED AND/OR DAMAGED BALANCED (3-PIN XLR) CABLES

Transcription

1 SIM : TESTING AND VERIFYING BALANCED LR CABLES SIM TRACES OF MISWIRED AND/OR DAMAGED BALANCED (-PIN LR) CABLES Miswired and/or damaged balanced -pin LR cables can create a host of problems within a sound reinforcement system. Although dedicated cable testers should be used to check cables on a regular basis, SIM can provide a method of verifying the cables before they are used to sort good cables from bad, and help diagnose the problems with them. This application note will outline a method for verifying cables with SIM, and how to recognize the problems using the SIM traces. NOTE: When verifying cables with SIM, the results shown are in no way an indication of the effect the cable will have on a sound system. The traces shown are the result of the way the differential inputs and outputs behave under those unique conditions. These conditions are helpful to identify the problem in the cable. NOTE: All of the examples in this document are with standard two-conductor, twisted-pair shielded cable with -pin LR connectors on each end. CABLE TESTING METHOD To use SIM to verify cables, use the following steps to test each cable that will be used:. In the SIM application, go to Settings > Branches, and use the default Branch 0 ( FP / 0 in Console and FP / Line In in Processor). Make sure the Delay values for those measurement points are at In the Selector panel, select Branch 0 as the Live Branch; make sure the box is checked so that the live trace is not hidden.. Plug an LR cable between the noise generator connector and the Line input LR-F connector.. Select the Frequency Response measurement, and select Processor.

2 SIM : Testing and Verifying Balanced LR Cables 4. On the analyzer front panel, turn the generator level control up at least midway. Select noise (Nz) as your source. In the Meters tab, adjust the Console and Processor level as necessary. Select the Values tab and click on the frequency trace graph at about the khz area to show the cursor. Set the graph view to 0 db per division. 5. View the response of the noise signal through the cable in the frequency and phase graphs to determine if the cable is good or has a problem. Remove that cable and test the next, until all are tested and sorted. TIP: Press restart (the R key) after plugging in the cable to clear the previous measurement.

3 SIM : Measuring Balanced LR Cables TRACE FROM A GOOD CABLE A trace from a cable without problems will show as unity gain in frequency response and flat phase at 0 degrees. This means that, Pin and Pin are properly connected and have continuity. NOTE: An open or disconnected shield (Pin ) will not be detected with SIM, and would need to be checked separately with a continuity tester, if desired. SIM SIM Line Figure. SIM trace of a good, properly wired balanced LR- cable

4 SIM : Testing and Verifying Balanced LR Cables REVERSED POLARITY Pin to Pin Miswired The most common and serious cable problem is reversed polarity, because it can lead to difficult-to-trace audio anomalies within a multi-loudspeaker sound reinforcement system. When a cable has reversed polarity Pin connected to Pin and Pin connected to Pin the frequency trace graph will show as unity gain in frequency response, but the phase trace will be at 80º at the top or bottom of the phase graph, often accompanied by a number of vertical lines in the midband. SIM SIM Line Figure. SIM trace of a cable with reversed polarity 4

5 SIM : Measuring Balanced LR Cables Pin to Pin Miswired, with One Connection Open An additional problem will occur if polarity is reversed and only one of the two conductors is connected. The level of the SIM frequency trace will be shown approximately 6 db below the zero line, rising in the high frequency to about - db. The phase trace will approach -80º at the bottom of the graph. SIM SIM Line Figure. SIM trace of a cable with reversed polarity and an open conductor NOTE: The shape of the SIM frequency and phase traces may appear somewhat different than the examples if Delay Finder is slightly off in time. Also, phase may be erratic with some problem cables. 5

6 SIM : Testing and Verifying Balanced LR Cables OPEN CONDUCTORS Open Conductor, No Shorts If Pin or Pin is open (the conductor is disconnected or broken somewhere between the pins) but the other conductor is correctly wired, the level of the SIM frequency trace will be shown at approximately 6 db below the zero line, rising in the high frequency to about - db. The phase trace will be around the zero line, with a slight rise in the high frequency corresponding to the one in the frequency trace. SIM SIM Line SIM SIM Line Figure 4. SIM trace of cable with proper polarity and one open signal trace 6

7 Open Conductor with Short Between Pin or Pin and Pin (Shield) at Output APPLICATION NOTE SIM : Measuring Balanced LR Cables If Pin or Pin is shorted to Pin on the output side and that conductor does not have continuity, the frequency trace will be flat and 6 db below the zero line. The phase trace will be flat and on the zero line. SIM SIM Line SIM SIM Line Figure 5. SIM trace of cable with open signal conductor shorted to shield at output end NOTE: The cable s Input is the LR-F connector that is plugged into the generator output and its Output is the LR-M connector that is plugged into the Line input of the analyzer based on the direction of signal flow. 7

8 SIM : Testing and Verifying Balanced LR Cables Open Conductor with Short Between Pin or Pin and Pin (Shield) at Input If Pin or Pin is shorted to Pin on the input side and that conductor does not have continuity, the frequency response will be approximately 0 db below the zero line, showing a rise to about -6 db in the higher frequencies. The phase trace is centered around the zero line, and also has a slight rise in the higher frequencies. SIM SIM SIM Line SIM Line Figure 6. SIM trace of cable with open signal conductor shorted to shield at input end 8

9 SIM : Measuring Balanced LR Cables Short Between Pin or Pin and Pin with Non-Shorted Conductor Open If Pin or Pin is shorted to Pin on either the input or the output, and the other conductor does not have continuity, the frequency response will be approximately 55 db below the zero line, showing a slight rise in the higher frequencies. The phase trace is centered around the zero line, and also has a slight rise in the higher frequencies. SIM SIM Line SIM SIM Line Figure 7. SIM trace of cable with short to shield and other signal conductor open NOTE: A short between Pin or and Pin, but with continuity of both signal wires, will show a SIM trace of unity gain and 0-degree phase. However, it may cause noise or other problems in use. 9

10 APPLICATION NOTE SIM : Testing and Verifying Balanced LR Cables PIN AND PIN SHORTED Pin Shorted to Pin, Continuity on All Conductors When a short occurs between Pin and Pin, the results are dramatic. If these two pins are shorted anywhere in the cable and both conductors have continuity from end-to-end, the two sets of Line LEDs on the analyzer front panel will turn off, and whatever measurement is on the SIM screen will freeze. The yellow Data Below Threshold will appear. If you press restart (R), the frequency and phase graphs will be blank. SIM SIM Line Figure 8. Short between signal conductors, with continuity no signal Pin Shorted to Pin at Input, Open Conductor at Output If Pins and are shorted at the input side and one of the two conductors is open toward the output end, no signal will be seen on the SIM screen in the frequency response graph. SIM SIM Line Figure 9. Short between signal conductors, with open at output no signal 0

11 SIM : Measuring Balanced LR Cables Pin Shorted to Pin at Output, Open Conductor at Input If the Pin or Pin conductor is open and a short exists between Pins and at the output end, the frequency response trace will typically show a level of -80 db centered in the mid-band, and rising on both ends of the frequency spectrum. Phase response will center approximately 90 degrees from zero, and its + or - direction will depend on which conductor is open. SIM SIM Line Figure 0. SIM frequency response trace of cable with signal conductors shorted at output and an open at the input

12 Meyer Sound Laboratories Inc. 8 San Pablo Avenue Berkeley, CA T: F: Meyer Sound Laboratories Inc A