Frequency Discrimination for FM Chirps of Various Bandwidths

Gated sinusoids with linearly changing frequency and essentially rectangular power spectra were used as signals in a frequency discrimination experiment. Signal bandwidths of 10, 50, 200, 500, and 1000 Hz were produced by changing the extent of frequency modulation for a fixed-duration chirp. The 100-msec FM chirps were generated digitally with 10-msec linear rise/fall times and presented at 70 dB SPL in a continuous wide-band noise of 0-dB spectrum level. One of the two observation intervals in a 2AFC experiment always contained a chirp that had a center frequency of 750 Hz. The other interval contained an identical chirp, except that its center frequency was 750+Δf Hz. The Δf required for 75% correct discrimination (Δf75) was estimated from individual psychometric functions for each of four experienced listeners. As the bandwidth of the chirp increased from 50 to 1000 Hz, the increase in Δf75 was almost proportional to the square root of the signal bandwidth. FM chirps with a bandwidth of 10 Hz are slightly more discriminable in frequency than a gated pure tone of the same duration at the same center frequency. The direction of frequency modulation, to higher or lower frequency, had only a small effect, even at the largest bandwidth.