Unicom Data Mode Interface

One of the most common problems using digital modes is improper system level. Even at best, digital modes have limited dynamic range compared to modes that closely «fit» filter bandwidths in the transmitter and receiver. When the radio filter is wider than the mode being used, the system depends heavily on having absolutely no distortion at any place in the system. Adjustments and levels throughout the entire system affect bandwidth and quality of transmitted and received signals.

When transmitting, it is extremely important to use correct gain levels. If the input of a transceiver is overdriven, the signal will contain unwanted products. Problems might not show on spectrum or IMD displays and if they do, many people do not recognize them. Excessive level into the radio can aggravate harmonic distortion (this often does not register on IMD or displays), causing transmissions on multiple frequencies. For example a PSK transmitter using 1,000 Hz offset will have some signal level at 2,000 Hz and every other multiple of 1,000 Hz. These harmonics might make it back through a narrow filter, and while they cause others interference they will not show on a spectrum display! The same is true for poor carrier suppression. Since the carrier can be outside the pass band of a receiver, it will not always show on displays of people you are working.

Problems also occur when audio levels are too low. If audio level from the sound card or interface is too low, the ratio of signal to hum and/or noise will also be reduced. The proper setting is generally one that allows the microphone and receiver gain to be set at normal operating levels for SSB. Power is reduced or adjusted by fine-tuning levels with the controls inside the Radioarena UnicomDual. The best way to check for proper transmission is to listen to your own signal on a separate receiver. Use a narrow filter, and take care to avoid overloading the receiver. If you cannot listen through a separate receiver, the best general guideline is to use normal microphone gain settings and approximately half volume on the sound card «Volume» settings. Adjust the transmitter level control in the Unicom Interface for normal transmitter drive (just at the start of ALC action) and use the microphone gain on the transmitter (or sound card volume) for fine adjustment.

Use a normal receiver volume setting, and adjust the sound card microphone level (make sure any extra gain options are off) to approximately half scale. Regardless of what you do, it is always a good idea to have someone listen to your signal when the band is empty, signals are strong, or noise is very low. They should look carefully for spurious signals, noise, and hum. When transmitting on modes like MFSK and PSK, always try to use a frequency setting of more than 1500 Hz and less than 2500 Hz. This will allow the transmitter’s SSB filter to suppress unwanted harmonics from the audio system driving the transmitter. Remember it is sometimes necessary to select the narrowest filter possible in the receiver, rather than depending on the computer to filter out strong unwanted stations. If you depend only on the sound card, you will find that even very clean strong signals overload your system. The problem is often in your receiver system, not in the offending transmitter. Many transceivers allow a selection of narrow filters while operating SSB, or include pass band tuning controls. If you have trouble with a strong station nearby causing interference, try to use more selectivity or use the receiver’s notch filter to reduce the signal level.

You must have a 16 bit sound card installed in your computer to use digital mode programs. Read the documentation with the sound card for full instructions on the use of this card in general. Find the mixer control panel, which may be a program that was supplied with the soundcard or it may be part of the Windows Control Panel. Learn how to adjust the soundcard playback output level and the record input level using the mixer panel: these controls will be used to set the levels for Digital Modes. The modem is feeding audio direct from the soundcard output to the AF input. There is an adjustable attenuator of about 100:1 ratio to reduce the 1 volt output of the soundcard down to 10 mV for the AF input. Do not use a speech processor with Digital Modes. When you have reached the correct setting, you will see that changing from the tuning tone to typing text, will make the transmitter power swing from 50% (no typing) to 100% (typing or tuning-tone). This change in power is correct. It’s quite OK to let the transmitter ALC line operate on digital modes. The ALC line will control the drive level without clipping in the same way that it does on voice operation.

To check that the receiver output is not overdriving the soundcard, tune to a strong steady carrier to give 1,000 Hz audio tone, with the RF gain of the receiver at maximum so that the receiver AGC line is fully operating. Check that the waterfall shows a single vertical white line with no red. Reduce the soundcard input gain in the mixer control panel if you see red lines on strong signals (DigiPan example). To complete the soundcard setup for use with digital modes, the soundcard sample rate needs to be set.

A key defining characteristic of any digital audio signal is its sampling rate, usually just called the sample rate. This refers to how frequently the analogue signal is measured during the sampling process. If you think about it, the reason why this is important is obvious: imagine that you had a recording of someone speaking, and you sampled the recording once per second. You’d end up with a lot of choppy noise. The more frequently the signal is sampled, the better the approximation to the original sound. However, the higher the sample rate the more memory is required to store the samples, so you don’t want to sample more than is necessary either!