GB2129238A - Snapshot recorder - Google Patents

Abstract

A snapshot recorder for handling varying signal information digitises it 13 and then stores it 14. The sampling and digitising rate is determined by the rate of signal change by means of filters 10, 11 and oscillator 12. Sampling rate information from timer 16 is stored with the signal information so as to be available on read out to aid reconstruction via DAC 19. The information displayed will represent the original signal even though the original sampling rate varied. In an alternative embodiment a fixed sampling rate is employed, but some of the samples are omitted to reduce the storage requirements, when signal interest is low. <IMAGE>

Description

SPECIFICATION Snapshot recorder The invention relates to Snapshot Recorders which record incoming signal information.

Existing Snaphot Recorders take in one or more analogue signals and sample these at regular time intervals. The results are converted to a digital signal in an analogue to digital converter and are fed into a store. The digital signals can be placed in successive locations in the store which may be used in a circulating buffer mode where new information overwrites oldest information.

Such snapshot recorders are often provided with a trigger circuit that can recognise an external event, or a particular event in the incoming signals. A signal from the trigger circuit may stop the process of filling the store immediately or after a user-pre-set time delay. In the latter case signals will be written in the store relating to time before and after the trigger.

To record a series of analogue signals with high sampling rates over long periods of time requires a very large store. In many cases, however, only brief random events are relevant requiring only a small part of the store. Between the events, the signals comprise mostly low bandwidth data or low amplitude noise.

The object of the present system seeks to reduce the need for such large stores whilst at the same time ensuring that the integrity of relevant incoming information is preserved.

According to the invention there is provided a snapshot recorder comprising sampling means for sampling incoming signals, storage means for retaining at least some of the samples, sensing means for sensing a characteristic of the incoming signals, and control means for varying the rate of delivery of samples to said storage means in dependence on the sensed characteristic.

The sensing means is preferably adapted to sense the rate of change of the incoming signals, whilst the control means is adapted to increase the rate of delivery of the samples when an increase in the rate of change of the incoming signals is sensed.

The control means may either produce an output signal which controls the sampling rate of the incoming signals, or may control the passage of samples taken at a fixed sample rate.

Means may also be provided for generating a signal indicative of the rate of delivery of the samples, for transmission or storage with said samples.

Delay means may be provided for ensuring the capture of rapidly changing information prior to storage, and further means may be provided for detecting and storing background information for subsequent display.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows various waveforms which may be encountered by a snapshot recorder; Figure 2 shows one embodiment of the present invention; Figure 3 shows an alternative embodiment; and Figure 4 shows two options for varying the rate of delivery of samples taken at a fixed rate.

Snapshot recorders may receive incoming signals which display wide variations in character.

Three examples of typical analogue signals are shown in Figure 1 and are as follows:- 1. A DC or very slowiy varying signal. Such a signal can be reconstituted if it is sampled and digitised at a slow rate.

2. A DC signal or slowly varying analogue signal with low amplitude superimposed noise. If the substructure of the noise is not required, such a signal may be reconstituted if it sampled at a sufficient rate to follow the variation in the analogue level and the envelope of the noise.

3. A signal that is varying rapidly and has considerable detail structure. In this case the signal needs to be sampled rapidly if it is to be reconstituted from the samples.

The present system is designed to respond to these characteristics in such a way that the integrity of the data is retained whilst keeping storage requirements to the minimum.

One embodiment of the present invention is shown in Figure 2. The incoming analogue signal is digitised in ADC 13 and then passed to a 'duration and value' store 14. The sampling and digitising rate of 1 3 is under the control of voltage controlled oscillator 12. Filters 10 and 11 are used to derive information from the incoming signal to vary the rate of digitisation by producing the voltage for controlling the VCO.

Rate of change filter 10 produces a larger output when the rate of change of the input signal increases. Smoothing filter 11 produces an output proportional to the average rate of change. The output of the VCO 12 not only controls the sampling rate of ADC 13, but increments the input address control 1 5 for the 'duration and value' store 14 and controls a timer 1 6. A digital signal representing the time between clocks is output from timer 16 to store 14, to provide information on the sampling period on read out.

When the rate of change of the incoming signal is above a predetermined value, i.e. the signal is varying rapidly, then the digitisation rate is arranged to be a maximum. When the rate of change is low or zero then the digitisation rate is minimum. The digitisation rate can either be varied proportionally to the rate of change of the incoming signal (as shown) or can be varied by selecting from a series of discrete sampling rates, chosen according to a quantised version of the rate of change of the incoming signal.

Thus when the rate of change, which is the characteristic of interest, in the incoming signal increases, so does the sampling rate. The samples are stored in store 14, together with signals from timer 1 6 indicating the period of the samples.

This information allows reconstitution of the information. Conversely when the rate of change of the incoming signal is low, the sampling rate is reduced, so that the demands on expensive storage capacity are reduced. Reconstitution is again possible because both of the signal samples and the times since the last sample have been recorded.

On read out, the signals indicative of the period of the samples are received by timer 1 8 which runs for proportionate times. The signal samples pass to DAC 19 for conversion into analogue form. After each period of the timer 1 8, the output address supplied to the store 14 from block 1 7 is incremented and a further sample and period signal are made available from store 14. The information on display 20 (e.g. oscilloscope or chart recorder) will represent the signal applied to the ADC 13 even though the recorded samples of the signal did not represent equal increments in time. The information (signal data plus time data) may be applied to a computer or other digital processor.

Where rapidly changing information of short duration occurs, the sampling rate may not increase rapidly enough, due to time delay in the filters 10 and 11, to capture the start of the transient data. The Figure 3 embodiment provides enhanced operation to reduce this difficulty. In this embodiment, the incoming signals are sampled and digitised at a fixed high rate and by manipulation of these samples, the rate at which samples are delivered is reduced where the level of interest (that is rate of change) is low. Thus the ADC is clocked at a fixed high rate by clock generator 24. A delay 25 is provided to allow sufficient time for the output of the filters 10 and 11 to respond to changes in the incoming signal.

The voltage output from filter 11 passes to subsample control 27 which enables a iogic system within sub-sample block 26, so as to modify the passage of samples from delay 25 to the store 14.

When the voltage output from filter 11 is high then the control 27 allows all the samples to pass to storage. The timer 1 6 is restarted every time a new sample is stored in store 14, and input address block 1 5 is incremented. When the storage output from filter 11 is low, only selected samples are fed to the store. Dependent on the logic configuration employed in the block 26 it is possible to omit N-1 out of every N samples where N is inversely related to the filter output.

This is illustrated in Figure 4 'Omission Mode'.

Alternatively every N samples can be averaged to produce a single sample, where N is controlled in the same way by the filter This average sample is then fed to the store (see Figure 4 'Averaged Mode').

An additional feature may be provided in Figure 3 (or Figure 2), represented by the broken lines and by the noise envelope filter 30 and ADC 31.

The filter 30 produces an output dependent on the noise envelope, and the output can be sampled at a fixed low rate via ADC 31, and an identification tag added to each sample so that it can be identified on read out. The value read out can be synthesised into a voltage by DAC 1 9 and displayed as the second trace on the oscilloscope or chart recorder 20 to give a background or reference signal. This is useful for giving information on a received radio signal, for example, where no audio is added but the noise envelope gives useful reference data.

The advantage of the new snapshot recorder described is that it can record a steady signal containing occasional transients over the long period of time without the need for a large store.

Differential time information is alway retained so that the time between two events held in the store may be determined.

Claims (8)

Claims

1. A snapshot recorder comprising sampling means for sampling incoming signals, storage means for retaining at least some of said samples, sensing means for sensing a characteristic of the incoming signals, and control means for varying the rate of delivery of samples to said storage means in dependence upon the sensed characteristic.

2. A recorder according to Claim 1, wherein said sensing means is adapted to sense the rate of change of the incoming signals, and said control means is adapted to increase the rate of delivery of the samples when an increase in the rate of change of the incoming signals is sensed.

3. A recorder according to Claim 1 or 2, wherein said control means controls the sample rate to control the rate of delivery of the samples.

4. A recorder according to Claim 1 or 2, wherein said control means is arranged to control the passage of samples taken at a fixed rate, to control the rate of delivery of the samples.

5. A recorder according to Claim 1, including means for producing a signal indicative of the rate of delivery of the sample, for transmission or storage with the samples.

6. A recorder according to any preceding claim, in which means is provided for delaying samples, to ailow for time delay in the sensing means.

7. A recorder according to any preceding claim, including means for detecting and storing background information associated with said incoming signals, for subsequent display.

8. A snapshot recorder, substantially as herein described with reference to any embodiment illustrated in the accompanying drawings.