birdie

From a Pfeifstelle is when in a radio receiver annoying and unwanted signals are received apparently that does not come from the antenna, but come from the demodulator of the receiver itself.

While turning the frequency scale of a Einseitenbandempfängers unmodulated signals are audible as whistling. The pitch obtained from the difference between the set frequency and the frequency of the signal.

When spin is thus only a high tone heard, the deeper is the frequency to set and received frequency are the same. If you turn in the same direction further, the pitch rises again.

Since in each receiver ( apart from very simple architectures) oscillators are included, the harmonics of these oscillators may fall into the reception area and cause birdies. If several oscillators available, including mixed products can produce the birdies. Modern receivers with microprocessor control generated by the cycles of the processor, display, keyboard, scanner, etc. always have a number of undesirable high-frequency signals, which appear due to insufficient shielding than birdies.