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A continued interest in reception quality is
part of the AM facet of the hobby. And one of the best ways to illustrate
how a station has been received is to make a tape recording of the contact.
The term "aircheck" is borrowed from broadcasting, where disc
jockeys and newspeople can hear their on-air performance with a sense of
realism not possible by simply recording from the studio microphone.
The realism comes from how someone's voice is changed by the
audible characteristics of the station's transmitter, audio chain and processing
equipment. For broadcasting, it's an absolute way to judge "loudness"
against a competitor. For high-fidelity AM on the shortwave ham bands, an
aircheck tape can be a way to troubleshoot and refine toward desired audio
quality. Some audio adjectives are universally understood -- "Scratchy
Apache" being among the most celebrated terms. But what would it mean
if someone told you that your audio sounded rather "grainy?" Or
that there was a "lump" in the audio response somewhere to the
right of midrange? After a while you might come to understand, but a recording
of your transmitter with those descriptions would let you hear for yourself.
Since most home stereos include a two channel cassette deck
(left and right), it's possible to go one step further as we shed light
on what we shall call "before" and "after" audio. "Before"
audio is what you sound like in person. "After" is what you sound
like as you transmit and are received at the other end.. It's important
to limit how much change happens on receive, since that unfairly colors
what is supposed to be a documentation of transmitted audio.
Let's use Left Channel on our home stereo cassette machine to begin setting
up an aircheck. The simplest approach would be to take a set of clip leads
from your receiver's speaker terminals into what's called the LINE level
input of the tape deck. The line level matches most closely the level of
voltage it takes to drive a loudspeaker. Feeding a microphone input would
overdrive and distort at the tape deck. Adjust the tape deck's
record control to about midrange, and then adjust your receiver audio level
to get a proper deflection on the deck's VU meter or light-bar graph. If
that's not enough to comfortably hear from the speaker, turn down the deck's
control and turn up the receiver volume to suit. Conversely, if things are
too loud by the time the tape deck is happy, consider using headphones off
the tape deck jack to monitor the QSO for which the aircheck is being made.
Let's use the Right Channel of the same tape deck to introduce
the other half of the conversation. A good quality, recording-type microphone
is indicated here. The purpose is to give your contact a sense of what you
would sound like "in person," as compared with how they'll be
hearing your station. It's the second half of a mutual exchange of aircheck
tapes, so that each station has the "before" and "after"
material for analysis.
Many tape decks allow either a mic or a line level input on
a given recording. We've got the left channel situated, now it's a matter
of mic placement and adjustment so that your local speaking voice matches
the pre-set level of the receive (left) channel. Some decks have independent
left and right input volume controls, others use a "balance" adjustment
made after initial level set.
Now, a moment of truth -- will RF
from your transmitter affect the tape deck? It's possible, and you can test
before even starting the QSO. Simply key up and talk, then unkey, rewind
the tape, and see if you sound normal (relatively speaking). If the RF has
contaminated the recording in any way, you'll need to decide whether you
want to make the deck immune to the interference, or consider a more basic
"one sided" aircheck of only the party in QSO. That can be a quick
way to dash a tape off to someone who already knows what you sound like
in person.
If all is clean, you're ready to record a QSO. A good aircheck
tape includes a list of things to check that will help shed light on how
a station sounds. Obviously, marginal band conditions are not a good time
to even consider an aircheck, since the propagation gods reign supreme.
But when signals are good and interference is minimal, you've got prime
time for a test.
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Many station ops like to
substitute among a variety of microphones. This is a chance for them to
run down the examples. Each mic under scrutiny should be adjusted for similar
audio levels at the transmit end, and the operator should use the microphone
in a manner best suited to that mic (some sound better with a "crooning"
style, while others are best at a less intimate distance from the mouth).
For your part, as the aircheck monitor, you'll do the station a favor
by limiting reception to consistant, repeatable changes, if any are made
at all during the course of the comparison. It wouldn't be helpful, for
example, to hear a carbon mic in a 16kc bandwidth, but then hear a ribbon
mic at a 4kc selectivity. Each mic should get a specific bandwidth sample
at a duration adequate to compare.
The beneficiary of an aircheck tape may have some parameters
in mind that they want to measure. Specify and agree on measuring hum, phase,
room acoustics, distortion, equalizer and compression settings, and any
other characteristic that can be manipulated on the way to better audio.
Just make sure someone has described what the change is, so that it makes
sense on later playback of the tape.
Advanced aircheck techniques involve discussion of both recording
and reception philosophies. On record, many cassette decks offer noise reduction
schemes that may or may not match the deck at the other end (beneficiary).
The best bet is to use a premium tape with no noise reduction. The aircheck
monitoring station should also avoid automatic volume control at both the
tape deck and at the receiver.
A receiver using "fast" automatic gain control will
add a certain amount of audio compression to the station recorded that will
make them sound louder than they have transmitted. That gets away from the
goal of making reception as transparent as possible, so that the transmitter's
characteristics can stand on their own. Select "slow" AGC, or
even manual gain control if signals are stable.
Receiver characteristics help qualify the aircheck tape presented
to the recipient. If you have only a narrow bandwidth receiver, you need
to tune slightly off to the sides as part of your recording, to capture
transmitted audio that goes beyond your receiver's capabilities at center
tune. You need to note how far away from center you have tuned, so that
the beneficiary can get a sense of distance for upper-frequency "treble"
response.
Receiver quality is also affected by the audio fidelity of stages
beyond RF amplification and bandwidth. Some receivers, including the Hammarlund
HQ100A, use an audio loudness curve that actually changes the proportion
of bass and treble response sent to the loudspeaker depending on the volume
level selected. This will affect any tape recordings made when hooked up
as described above.
The highest quality airchecks are made from the so-called "diode"
output of a vacuum-tube, non-synthesized, receiver with an enhanced bandwidth
that is not defined through mechanical filters. Examples include the Technical
Materiel Corporation GPR-90, the National NC-303, the Hammarlund SP-600,
and the Collins R-390.
Using the diode output can be a little tricky since it wants
to see a high impedance and offers relatively high voltage. The line level
input of a common tape deck wants low impedance and low voltage. Reasonably
good results can be had with careful adjustment of the RF gain control on
the receiver, and the tape deck input volume control, while keeping hookup
cables as short as possible. The analog "hissette" has its own
limitations. Alternatives include aircheck tapes made using the high-fidelity
helical-scan tracks of a standard VHS video tape deck. Playback taps those
same tracks for outstanding quality compared with audio cassette. Recordable
CD machines, DAT machines, open-reel machines and other recorders also offer
improved audio, so long as both parties have the equipment.
The best part about getting your first aircheck tape, is hearing
first hand how others are hearing you. This can be good and bad. If your
audio quality needs work, it can be somewhat shocking to hear why people
have been telling you that. But it's good to know what they mean, and then
do something about it. For those with improved audio, it can be a real sense
of satisfaction to hear the reward of your efforts. By the way, the Internet's
RealAudio player is a rising medium to swap audio files back and forth.
Some vintage radio pages already feature sound
clips of the festivities around the AM windows -- it's a real treat
for those who live outside the signal area to hear the goings-on.
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