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Recording and Playback Setup



Introduction:

Setting Up A Record / Playback System

Record / Playback SetupOften, it's nice to be able to record and playback someone's audio so that they can hear what's happening at their transmitter. It seems like a simple enough task, but there are several factors that will determine the final quality of your reproductions.

The first factor to consider is how wide and flat (accurate) your receiver is. Obviously, you want to be able to capture the received signal to a recording device as accurately as possible. If it's not received accurately, it will not be recorded accurately. So, the first step to a good Recording/Playback system is to make sure that you can receive the signal as accurately as possible.
(For a discussion on Receiver Setup, see the
SSB RX Setup page.)

The second factor to consider is the method you will be using to do the recordings. There are basically three types of recording methods; Tape, Mini-Disc and Computer-based Sound Card Hard-Disc recordings.

The third factor to consider is how to reproduce or play back the recorded audio accurately so that the station you recorded will hear a near duplicate of what they transmitted. (Hint... do you have a spare equalizer laying around?)

Finally, the fourth factor and one that we have no control over is the band conditions, which will determine signal strength, phasing distortion and fading.

I will deal with each of these in the following sections including some cabling suggestions, as well as some ideas that will hopefully aid you in good reproductions.




Connecting The Rec/Playback System


Sending the Audio from the Receiver to the Recorder:
The cable you will be using for the recording system should be high quality shielded type. I recommend using "Starquad" which is a twisted, four conductor, double shielded type of cable. This kind of cable is used throughout the recording industry for its low loss, low capacitance and Hum/RFI suppressing characteristics. Since all of the connections that we will be dealing with will be unbalanced, all four wires will be used on the plus side of the connectors and the shielding to the ground lug.

We need to get the received audio to the recorder. The best place to tap the receiver's audio output for recording purposes would be from a fixed line-level output. If you are using this output already for your receiver EQ or another device, you can "Split" the signal with very little loss. Whether you will be sending the received audio to a tape recorder, mini-disc recorder or computer sound card, use the line inputs on these devices, preferably the Left Channel input.


Sending the Audio from the Recorder to the EQ:
If you will be using a computer based recording system with software that can EQ the audio, see the next section. If you will be EQing your recorded audio, (highly recommended) then you will need a cable to connect the recorder's output to the EQ's input. This is usually done with RCA connectors and should be pretty straight forward.


Sending the Audio from the EQ or Recorder to the Transmitter:
Getting the EQ/Recorder output to the transmitter is the final step in the hookup. I highly recommend sending the EQ/Recorder output to the transceiver's MIC input. Most transceiver's mic input is flatter in response than their phone patch or accessory inputs because of capacitors that are usually used.

The problem, of course, is that the mic input will already be used by the microphone audio. We could combine the output of the mic rack with the output of the recorder and combine the two, but I don't recommend it. Combining two or more output signals into one input gets messy! It effects the frequency response because of severe impedance mismatches and loading.

This is where a mixer comes in handy! If you have one, and have already used a mixer input channel for your audio rack output, then you can select another mixer input channel for the recorder output. Then you can use your mixer to balance the audio levels between your live audio and recorded audio signals and send the main mixer output to the transceiver mic input.

I
f you choose not to use a mixer (not recommended) there is a way to combine the outputs. This will involve sending an output to another output. It sounds strange, but it works! I will explain briefly how to do this...

To combine two unbalanced mono signals, connect a cable from the output of your recording device to the output of the last processor that's feeding its output to the transceiver. If the processors output uses a 1/4" Phone jack, then inset a "Y" adapter to the output. The Y-adapter will have a mono 1/4" PLUG to two mono 1/4 JACKS. (Radio Shack part #274-892) Since the output of the last processor is routed to the transceiver's mic input, then inserting another signal into the processors output will end up at the transceiver's mic input also. I would also recommend sending these combined signals to an audio isolation transformer before it gets to the transceiver mic input. This will help reduce ground-loop 60Hz hum and any RFI that may occur.

If you are using a 1/4" TRS or XLR balanced output from your last processor to feed the transceiver's mic input, combining this signal with that of the recorders output will result in an unbalanced signal feeding the transceiver's mic input. This is because the recorder's output is unbalanced, and mixing this signal with a balanced signal will unbalance it electrically because of XLR pins 1 & 3 being combined, or because of a 1/4" TRS plug being shorted between Ring & Sleeve. The only way to keep your balanced signal from becoming unbalanced when combining it with an unbalanced signal is to use an isolation transformer that can convert unbalanced to balanced. Then you may combine the two balanced signals and send them on their way to the transceiver.

I send my sound card unbalanced output signal to a Jensen 1:1 line output transformer (JT-11-FLCF) and then send the transformer balanced output to one of my mixer main inputs which I can mix with my live rack audio output that gets delivered to my transceiver. This works nicely with good RFI/Hum rejection and isolation.

It sounds complicated, but it's really not that bad and it does work. Leave me an e-Mail if you need some assistance.



Making A Recording

After cabling everything up, you're ready to make a test recording. Find a receive signal that is strong enough to record. The better the signal, the better the recording will be. Turn your RF receive gain down so that the target receive signal is only about 10 or 20dB stronger than the default meter position when all is quiet. You could pre-adjust your RF gain so that your meter shows about S9 when your receiver is quiet and just leave it there.

Adjust your AGC to its slowest position so that you will not be recording a signal that's pumping the receiver.

Adjust your recording level appropriately and start the recording. If you will be using a sound card and software that can Post-EQ (FFT) the recording, I recommend keeping the recording levels very conservative to about -9dB because of an FFT amplitude increase when EQ'd later. Otherwise you can record up to 0dB.



Equalizing For Playback


"Why?"...You may ask...
If a recording device is accurate, (most are) and the receiver is flat, then why EQ the playback? If your transmitter is perfectly flat and has the same bandwidth as the receiver, or you will only be playing back a station who's audio is only 2.4kHz wide in frequency response, then my answer is simple...
You DON'T need to EQ at all. Just remember however, the criteria for NOT needing a playback equalizer are:

1
Your transmitter MUST be able to receive AND transmit a bandwidth that is at least equal to, or in excess of, the bandwidth of the transmitting station being recorded.
2
Your transmitter AND receiver MUST be truly "FLAT" throughout its passband. (Equal amplitude from lowest to highest available frequencies within your transceiver's passband)

I have NEVER found these criteria met in any transceiver! Plain and simple! I have seen a few transceiver's who's receiver was very flat and wide enough, but they fail the transmitter criteria for reproducing enough flat frequency response. This is why we all NEED a playback EQ system if we really want to transmit a faithful reproduction of the original recorded source.

Even with all of this taken care of and in the best of scenario's, the signal to noise will be doubled as any QRN, QRM and band phasing distortion present in the original recording, will be added again during the playback to the receiving station that you recorded! Recordings made during stable band conditions, low QRM / QRN and at high signal levels are the perfect time for a great recording session. If these conditions are not met, you may want to put your recording on hold for better conditions or a different band.




Equalizer Settings:
Figuring out how to set up the playback equalizer will require some simple transmitter tests. The most quantitative and scientific approach would be to measure the power output of the transmitter when excited with different frequencies. For these tests, you will need a tone or audio sweep generator. I use the one provided in my recording software (Cool Edit 2000). Many software based recording and analysis programs have these generators built in.


Here is an outline of how to test your transmitter frequency response:

1
First, make sure you use a dummy load with the following tests!
Select a 1000Hz tone as a reference with your external tone generator or computer soundcard software. Make sure that your transmitter power output control is at full so that it is capable of its full output. Then, adjust the tone output from the source, not the transmitter, to produce 25watts from the transmitter. The 0dB reference has now been set up and should remain there for the rest of the tests.
2
The next step is to find the plus/minus dB points of where the transmitter power either increases or decreases with certain frequencies. Begin by measuring power output when 25 Hz is applied. Write down your findings. Next measure the output at 100Hz. Then 200Hz, then 300Hz, etc... all the way up to about 4500Hz. To find the +1dB, +2dB, +3dB and -1dB, -2dB, -3dB, etc... points in terms of power drop or gain compared to our 1kHz reference tone of 25w, consult the chart below:

POWER in watts = Resulting Gain/Attenuation
0dB Reference - Power = 25Watts @ 1kHz

Power Output
Resulting
Attenuation
or Gain
200w
+9dB
156w
+8dB
125w
+7dB
100w
+6dB
80w
+5dB
64w
+4dB
50w
+3dB
40w
+2dB
32w
+1dB
25w
0dB REF
20w
-1dB
16w
-2dB
12.5w
-3dB
10w
-4dB
8w
-5dB
6.25w
-6dB
5w
-7dB
4w
-8dB
3w
-9dB
2.5w
-10dB
2w
-11dB
1.5w
-12dB
1.25w
-13dB
1w
-14dB

3
After finding the dB points with your transmitter tone tests, apply these numbers to your equalizer. Say, for example, that your transmitter was only producing 12.5 watts at 80Hz. This means the power was down by -3dB in comparison to our 0dB reference of 25 watts. Adjust your equalizer for +3dB at 80Hz to make sure that your power at 80Hz is producing 25 watts, our 0dB reference power level. Go through all of the deficient frequencies the same way and apply the necessary corrective EQ measures. When you are done, check the frequencies again to make sure that ALL frequencies within reason are producing 25 watts. Obviously, there will be frequencies beyond your transmitter and EQ's ability to compensate for. If you can squeeze a flat response from 50Hz ~ 4kHz and/or beyond, you're doing great! You should be able to reproduce most stations with good accuracy!

If you have set up your playback equalizer properly, and you have sufficient bandwidth that equals the original audio bandwidth from the transmitting station, you should be able to play them back with deadly accuracy! Just remember, your receiver MUST record them flat and wide enough to begin with or the show is over!



Playing Back The EQ'd Recording


Now that your recording EQ has been adjusted, it's showtime! The only comments I have is regarding playback gain levels. An EQ'd playback can test the limits of your transmitter. So, be very careful with your playback gain so that you do not cause transmitter distortion. You may notice that you cannot DRIVE your transmitter's ALC as aggressively as you can with your live mic audio. I find the opposite, but every circumstance is different. You will need to experiment with levels that work the best in your environment.



Summary

If you need some assistance in determining if your playback system is working correctly, or if you would like to test it out with someone who has a spectral analyzer on the air, I would certainly be willing to make a schedule with you and assist in any way I can. Drop me an e-Mail and we'll set something up.

One final note regarding on-air playbacks; The FCC requires that we get permission from the amateur station that was recorded to retransmit their audio on-the-air. Also note that the FCC forbids retransmission from any sources other than that of amateur radio, including commercial broadcasts and music.

Have fun and may your recordings and playbacks accurately reflect the spirit of those you have recorded!

-John (NU9N)

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John M. Anning - NU9N
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