Analogue: What is flat audio and emphasis?

In the early days of two-way radio, it was discovered that it is possible to reduce the RF bandwidth required, yet transmit readable voice, by only passing audio frequencies below 3000Hz. Later, with the introduction of CTCSS (Private Line), it was necessary to also filter out audio frequencies below 300Hz (this was also needed to filter out any potential noise from the AC mains).
This became known as filtered audio. All two-way radios use filtered audio, however some models allow this filtering to be disabled.

The first FM two way radios used phase-modulation (PM or ΦM) in the transmitter and FM in the receiver. Sending a phase modulated signal to a FM receiver, results in a 6dB per octave frequency curve so to counteract this, the receiver employed a de-emphasis circuit that canceled out this 6dB/8ve curve - thus providing a flat frequency response up to 3000Hz
As FM-modulated transmitters were introduced, they too needed to be compatible with the PM/FM two way radios - hence the practice remains to this day.
Also, using pre- and de-emphasis has an effect of reducing system noise.

The above works well when one radio is talking to another over the air. However, when a receiver of one radio is connected to the transmitter of another radio, the above needs to be disabled. This would occur in a repeater or when the radio is being used as an analogue donor radio connected to a gateway. Not disabling pre and de-emphasis will result in tinny or mufffled audio. And not enabling flat audio, will result in muffled or distorted audio (audio that sounds like its being passed through a notch filter).

The requirements for filtered audio and pre- de-emphasis are defined in TIA-603-E and ETSI EN 300 086. The below graph shows the requirements a transmitter needs to meet in order to be compliant with the standard. The red line shows the upper control limit and the green, the lower control limit.

TIA-603-E preemphasis and audio filtering specification plot (TX).

TIA-603-E deemphasis and audio filtering specification plot (RX).

The above graph shows the receiver de-emphasis (with the UCL and LCL line colours reversed).

Why is this important?

• When using a mobile radio as an AFSK data modem, you need to take this into account as both of the above settings can have an impact on packet loss and BER.
• When using a mobile radio in a repeater, emphasis and filtered audio must be disabled.
• Some gateway devices need audio with or without filtering and/or emphasis.

Knowing which to use and when will save you from unhappy customers and unnecessary service calls.