Heim is the chief engineer of Forever Broadcasting in New Castle, Pa.
His stations have been using iPads for remote broadcasts.
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
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.
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
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
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
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.
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.
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.
Broadcasting has found the iPad to be an easy and portable way to
improve remote broadcast quality. Thanks, Michael, for a great tip.
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.
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.
* * *
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:
Fig. 1: Here, our perspective is turned up to look at the ceiling,
where the fan has been mounted to blow upwards.
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
Contribute to Workbench. You’ll help your fellow engineers and
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firstname.lastname@example.org. 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.