Speckle is the chief limitation of coherent lidar and coherent imaging in optical heterodyne detection. Coherent systems generally use optical heterodyne detection. At radio wavelengths, simultaneous spectral information is obtainable with heterodyne receivers. The signal detection scheme of the instrument is using the principle of optical heterodyne detection. The analogous diversity reception for optical heterodyne has been demonstrated with arrays of photon-counting detectors. The direct-conversion (heterodyne) detector was invented to make continuous wave radio-frequency signals audible. This improved signal-to-noise ratio makes absolute amplitude measurements feasible in heterodyne detection. Current research is extending this method to infrared and light frequencies (optical heterodyne detection). Javan carried out the first demonstration of optical heterodyne beats with lasers in 1961. In heterodyne detection mode HgCdTe can be uncooled, although greater sensitivity is achieved by cooling. The theremin uses the heterodyne principle to generate an audio signal. The following recording was made on a Duet combined heterodyne and FD detector. Frequency counters or high frequency heterodyne systems can be used. The alias is effectively the lower heterodyne of the signal frequency and sampling frequency. "Homodyne" signifies a single frequency, in contrast to the dual frequencies employed in heterodyne detection.