Q: What do you mean by ‘normal audio’ and ‘extended audio’ with regard to radio modules?
Different builds of Wood & Douglas radio telemetry modules are available to accept different modulating audio frequency ranges. ‘Normal audio’ allows analogue waveforms with frequency components between about 10Hz up to around 2800Hz. A low pass filter attenuating higher audio frequencies.
Our ‘Extended audio’ builds have different audio filtering that allow for analogue waveform components up to around 7000Hz.
Our ‘Normal audio’ modules are suitable for low data rate modulation waveforms such as AFSK, DTMF, simple tone transmissions. Even voice could be used if regulations allow.
Our ‘Extended audio’ versions are designed to cope with complex analogue waveforms associated with higher data rate modulation schemes such as GMSK and 4LFSK. Manufacturers of these modem chips construct waveforms that fit snugly into radio channel bandwidths when frequency deviation is adjusted correctly.
Important: The occupied bandwidth of a radio transmission is a function of both amplitude of the analogue input and also the high frequency components. The equation for occupied bandwidth is: -
Bandwidth = 2[Max frequency deviation + Max modulation frequency] e.g. for a waveform that has a maximum frequency of 2400Hz and that has an amplitude that frequency deviates the radio module by +/-2000Hz maximum, the occupied channel bandwidth is 2[2000 + 2400] which equals 8.8kHz.
In our Normal Audio module builds, our circuitry controls the maximum frequency deviation and audio low pass filter circuitry limits the maximum modulating frequency. In this way the occupied bandwidth of a normal audiomodule is handled by the radio module’s circuit. In contrast, with the Extended Audio builds, because higher modulating frequencies are possible it is important that max frequency deviation is adjusted down so that the occupied bandwidth is within regulatory limits. This is something that you as the OEM customer has to take account of to ensure that any new products meet type approval specifications. Wood & Douglas offers technical support for new users to ensure that type approval occupied bandwidth and adjacent channel power limits are comfortably met.
You may wonder why we do not just make extended audio versions of our modules. Why bother with normal audio? Well the answer lies in Signal to Noise ratio. To maximise the receiver’s sensitivity with weak signals it is important to minimise the high frequency noise of the transmissions. If you are only going to be sending for example AFSK 1200Hz & 1800Hz tones, filtering any noise or audio harmonics above 2800Hz keeps the transmission crisp and clean with no added high frequency hiss components.