Liquid Crystal Display of Time Dependent Signals

The invention relates to the design of a liquid crystal display (LCD), for displaying the shape of repetitive signals. The device is particularly suited for displaying the signals from a depth sounder. The standard practice for displaying repetitive signals is to use cathode ray tubes. Depth sounders, on the other hand, more often use lamps that revolve in a circle, the instantaneous brightness of the lamps being proportional to the signal being displayed.

Neither of these systems can easily be read at high ambient light levels or low repetition rates. Instead of displaying all the echoes, many depth sounders therefore electronically select only one, the corresponding depth of water being displayed either digitally or on an analog meter. Such an electronic selection may result in erroneous readings due to spurious echoes from fish or similar objects. On the other hand, with a complete display of the echoes, spurious echoes can be recognised as such.

The invention displays all the echoes in such a way as to be readable even at high ambient light levels.

According to the present invention, the display device uses an LCD with a first system of horizontally elongated electrodes. Each horizontally elongated electrode corresponds to one interval of time and is driven by a voltage derived from the input signal during that time. This is achieved simply by connecting each horizontally elongated electrode to a capacitor, which in turn is connected to the signal source through an electronic switch over the corresponding, interval of time interval. Preferably, to display the strength of the signals clearly, a second system of vertically elongated electrodes is also provided crossing electrodes in the first system. The vertically elongated electrodes are biased stepwise each stripe being at a higher voltage relative to the next one. Thus, when an electrode in the first system is at a low voltage, then only those areas overlapping with the most strongly biased electrodes in the second system are excited to threshold. This generates a histogram-like ("bar graph") display of signal strength against time.

The invention will now be described by way of example with reference to the circuit diagram.

The horizontal electrodes of the LCD (LCD) are connected to storage capacitors C and are sequentially connected to the input signal source through the CMOS analog multiplexer. As LCDs should not be operated on dc-voltages, the input signal is inverted by an operational amplifier (OP) Amp and this inverted signal is then used during alternate sweeps. Due to the operation of the diodes, peak values are stored in the capacitors. A binary counter provides the addressing for the analog multiplexer. The output of its first stage (LSB), which is a square wave, is voltage divided to drive the vertically elongated electrodes at different voltage levels. The output of the last binary stage (MSB) effects the inversion of input signal polarity. Synchronisation with the input signal being displayed can be achieved in various ways and has not been shown.