Deemphasis Converter

The standard preemphasis time constant for FM broadcast in North America is 75 s. In Europe and most other places, it is 50 s. It's easy to change the deemphasis-network capacitors in a conventional tuner to convert from one standard to the other. However, this can't be done for DSP tuners such as the Sangean HDT-1X or Sony XDR-F1HD that implement deemphasis in firmware. These HD Radio tuners may be of interest overseas for their performance on analog FM. There also may be times when you want to use a 75 s tuner on 50 s signals without making internal modifications.

This external circuit will change the deemphasis time constant from 75 s to 50 s. The op-amp input network undoes the 75 s response, while the feedback network imposes a 50 s response. The frequency response error will be no more than a few tenths of a dB for the component tolerances shown. The voltage follower isolates the input circuit so that the response is independent of tuner output impedance. Build one circuit for each channel. Any quad op-amp optimized for audio use, such as a TL074, will do. Power supply bypassing is not shown. The 2.2kΩ resistor provides some protection for the op-amp and duplicates the output impedance of the HDT-1X and XDR-F1HD. To convert from 50 s to 75 s, swap the capacitor values.

The solid trace is magnitude and the dotted trace is phase. The overall response is flat (gain 0 dB, phase 180) when receiving a 50 s signal with a 75 s tuner driving the deemphasis converter.

June 2, 201088108 MHz