Use case: Synchronize data coming from multiple devices
16 Sep 2014

NOTOCORD-hem Evolution connects to different devices like telemetry systems (Data Science International, Stellar Telemetry), systems for electro-physiological applications (TMSi, World Precision Instruments), general acquisition systems (Data Translation), audio-video systems (Hikvision),… to collect record and process the input data.

Time delays between devices

We discussed previously some configurations to acquire data from multiple devices. We’ll discuss now time delays that can appear between signals acquired from multiple devices in NOTOCORD-hem. These time delays are mostly observable for continuous acquisition of long durations. To put in evidence such a time lag, the same signal (e.g. ECG) should be acquired with two different devices over the same duration (e.g. 24h). For an ECG recording, the successive QRS complexes are progressively delayed, the longer the acquisition duration, the larger the delay.

How do the time delays appear?

Each device uses a specific clock to sample the input signals. Some devices have an internal clock, while other devices use the clock of the computer (real-time clock, RTC). As these clocks are not synchronized, the device with a “slight” higher clock frequency will send more data relative to a device with a “slight” lower clock frequency in the same time interval, generating an inherent delay.  

As an example, the RTC may drift over time, so it can gain or lose several seconds a day. Considering that the relative precision of the clock Δf/f is usually of the order of 10-4, the result is a drift of the order of 10s/24h. In this case the same ECG signals acquired over 24h by the two devices will be delayed progressively reaching about 10s after 24h of continuous recording. Successive cardiac cycles’ synchronization is lost in this case, so an analysis cycle by cycle inter-devices is not possible.

Delay correction

A straightforward correction would be to use a common external clock for every device, instead of different internal clocks. Unfortunately, most devices do not propose such an option.  

NOTOCORD applies a specific software correction of the delay in the case of configurations containing the couple of servers (DSI60a, VSH10a) or (DTS60a, VSH10a). The video/audio channels recorded with the VSH10a server are synchronized with the analog channels recorded with DSI60a and DTS60 respectively (see VSH10a ref doc).

A general solution illustrated for the couple of servers (DSI60a, DTS60a) is shown in Figure 1. The configuration contains a DSI60a server (LAS30a2), recording some channels coming from an implant. In the same experimentation, one or more channels are recorded using a DTS60a server (LAS30a1). In order to synchronize the servers an additional synchronization stimulus is applied (STIM).

 

Figure 1. Synchronization of signals coming from two acquisition servers by an external stimulus.

 

The synchronization stimulus is generated externally and recorded on both acquisition systems. The stimulus is recorded either as an analog signal (e.g. by C12V converter in a DSI system) or a digital signal (time event output with DTS60). If the synchronization signal comes from an analog channel, it is converted in a time event channel by THR10a.

The time events are used for a synchronized display of the signals, based on the scope module SCP10m. Signals coming from a specific device are displayed relative to the time events coming from the same device. Figure 2 shows a synchronized display of synthetic signals that were acquired with a delay.

 

Figure 2. Synchronized display with SCP10m.

 

The current time events used by the Scope modules (output Trigger) are displayed in a control display (CTD60a) together with the input signals.

An Excel template that needs a synchronized extraction should be based on the same time event references.

 

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