The Aircheck Tape

An Audio QSL & Reception Report

Paul Courson, WA3VJB


  The popularity of the "QSL card" has been declining for quite a while, partly because increasingly reliable communications make written verification less appealing. Traditionally, this post card (maybe now called hard copy) was a routine part of the exchange between hams, and allowed expanded description of station equipment lineup and reception conditions that were not covered during the actual contact.


  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.

Top of Next Column


  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.

Back to the General Technical Page


11 Feb 1997