Deemphasis Converter

The standard deemphasis time constant for FM broadcast in the U.S. is 75 µs. In Europe and many other places, it is 50 µs. It is easy to alter the capacitor value in an analog deemphasis network to change 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. 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-µs tuner on 50-µs signals without making internal modifications.

This external circuit will change the deemphasis response from 75 µs to 50 µs. The op-amp input network undoes the 75-µs response, while the feedback network imposes a 50-µs response. The conversion is exact to within component tolerances. The frequency response error will be no more than a few tenths of a dB for the tolerances shown. It may be easier to just measure loose-tolerance parts and select. The voltage follower provides a very low drive impedance and makes the response 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 µs to 75 µs, swap the capacitor values.

The solid trace is magnitude and the dotted trace is phase. The net response is flat (gain 0 dB, phase 180°) when receiving a 50-µs signal with a 75-µs tuner using the deemphasis converter.

More is here.

Updated May 11, 2008