Michael Heim is the chief engineer of Forever Broadcasting in New Castle, Pa. His stations have been using iPads for remote broadcasts.
Michael selected one of the popular instant messaging programs with voice capability to send audio back to the studio; he reports that the audio quality is better than that delivered over a cellphone connection.
The one problem is that the internal mic is so sensitive, it will pick up a buzzing fly at 100 yards! OK, a slight exaggeration; but the mic has been known to pick up ambient sounds, so he needed to wire an external mic for better operation.
In his application, Michael does not use the earphone connection, but there is no reason an ear bud couldn’t be used. The wiring for the ear buds is straightforward with no special accommodation required. The microphone, on the other hand, is a different story. Online, Michael learned that the mic input wants to “see” 800 ohms across the mic terminals to mute the internal mic and switch over to the external terminals.
As a test, Michael used a Shure SM58 mic. Measuring across the mic element with an ohmmeter, he saw about 500 ohms. Michael added a 330-ohm resistor in series, after which the operation was intermittent. He replaced the resistor with 470 ohms and it worked perfectly every time. There appears to be no upper limit for the resistance, and placing roughly 1,000 ohms across the terminals works well.
He constructed his cable using a four-conductor phone plug on one end and an XLR on the other. He hid the 1/4-watt resistor inside the XLR connector.
He was unable to find a four-conductor phone plug to fit the iPad so he bought a combination ear bud and mic assembly at a discount department store and wired the mic in with a piece of extension wire. The wiring diagram for the cable is shown in Table 1.
Michael put it all together and tried it with a variety of microphones — the SM58, EV 635, even an RE20. The cable worked every time. It should work with any quality dynamic low-impedance microphone. The series resistor has little or no effect on the audio level, and the iPad provides plenty of gain.
Of course, the iPad mic input is unbalanced, so you will need to short Pins 1 and 3 inside the XLR plug. Then wire the 470 ohm resistor on Pin 2 in series with the cord.
Forever Broadcasting has found the iPad to be an easy and portable way to improve remote broadcast quality. Thanks, Michael, for a great tip.
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Engineering Consultant Frank Hertel of Newman-Kees needed to refer to the newest published FCC Broadcast Rules and Regulations. He admits to having a tough time locating them.
Frank finally located the link. The content can be found at www.ecfr.gov, but here’s a direct link.
You’ll find all the various broadcast rules, listed on clickable headings. Definitely something to save in your favorites list. Thanks, Frank, for helping to keep everyone legal.
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Fig. 1: Here, our perspective is turned up to look at the ceiling, where the fan has been mounted to blow upwards. Last issue we offered a few tips for inclusion during routine inspection of your transmitter building. Consulting Engineer R. Morgan Burrow, P. E., mentions a few more:
Make sure your fire extinguisher is fully charged.
Do you have emergency lighting — or at the very least, a flashlight mounted inside the entrance, complete with fresh batteries?
Sounds can signify pending doom. With the monitors turned down, just listen as you walk around the room. A high-pitched squealing could be blower bearings or air-conditioning belts that need attention.
Outside, check the tower light photocell to make sure the lights are operational, and transmitting their status to the remote control. A towel thrown over the photocell should trigger the lights “on” after a short period of time. Do remember: Many AM towers are hot and insulated from ground; so if your tower photocell is mounted on the tower, turn the AM off while you conduct this test. No sense in getting an RF burn, or worse.
Check the building temperature and the operation of all air conditioning and/or louvers, vents and exhaust fans. This includes the dummy load cooling fan, seen in Fig. 1.
Finally, make a note of your findings, to simplify next month’s inspection.
Contribute to Workbench. You’ll help your fellow engineers and qualify for SBE recertification credit. Send Workbench tips to [email protected]. Fax to (603) 472-4944.
Author John Bisset has spent 44 years in the broadcasting industry and is still learning. He handles West Coast sales for the Telos Alliance. He is SBE certified and is a past recipient of the SBE’s Educator of the Year Award.
