Scream! 4 distinguishes between configuration and control of digitizers. The most important difference is that a digitizer may be controlled through Scream! at any time whilst it is acquiring data, whereas configuration options only take effect after a reboot (with consequent loss of data.)
To change the configuration of any connected digitizer:
Locate the digitizer you want to configure. All connected digitizers have an entry in the tree on the left of Scream!'s main window. If the digitizer is transmitting data through a remote server or DCM, you may need to “unroll” the entry for that server (by clicking on the 
icon) to see the digitizers connected to it.
Right-click on the digitizer's entry (not the icon for the server or any Comxx icon).
Click Configure.... Scream! will then contact the digitizer and retrieve its current configuration, a process which will take a few seconds. This done, the Configuration setup window will be displayed.
Once you are happy with any changes you have made in the Configuration Setup window, click UPLOAD to send them to the digitizer and reboot. This will take a short while.
To control a digitizer whilst it is running, either right-click on the digitizer's entry in the list and click Control..., or double-click the entry. In either case Scream! will contact the digitizer to retrieve control information and display the Control window. The options you can control immediately are:
the type of sensor you are using,
GPS power cycling options,
the short-term and long-term average values for triggering (but not which streams perform the trigger, or which are output by it) (see “Triggering”, page 72),
the length of pre-trigger and post-trigger periods,
calibration signal options, and
mass control functions.
Some of these options can also be altered in the Configuration setup window. For more information on the Control window, see Chapter 7, page 80.
The System ID pane gives information about the digitizer and its internal software, and allows you to change GPS timing parameters.
System Identifier and Serial Number : The digitizer type is identified by its system identifier and serial number. Every data and status block generated by the digitizer includes these two fields at the beginning, so that the block’s origin can be identified. On delivery from the factory, the system identifier and the serial number are set to the GSL works order number and the digitizer’s serial number, but any combination of letters A-Z and numbers can be used, such as an abbreviation of your institution’s name, etc. The system identifier can be up to 5 characters long, whilst the serial number cannot be longer than 4.
Sensor Type : If the sensor attached to the digitizer is a Güralp velocity sensor, useful seismometer functions (such as sensor locking, centring, and calibration) may be controlled through the digitizer. The Sensor Type you set here determines which functions will be available through the Scream! digitizer configuration set-up interface or through interactive commands.
GPS Type : The digitizer needs to be able to time-stamp accurately all data that passes through it. It can set its clock either by receiving time signals from the GPS satellite network using an attached Garmin-compatible (NMEA output) unit, or by taking time information from a central site (stream sync mode). In stream sync mode, the digitizer expects to receive two-byte packets from the central timing source, which may have its own GPS unit, or take signals from one of the radio time standards. Choose the mode you require from the drop-down menu.
Enable GPS power cycling : If you are using a GPS unit to receive time signals, but do not experience significant drift in the system's clock (for example, in a stable-temperature environment), you can save power by selecting Enable GPS power cycling. With this option in use, the GPS time is only checked at intervals of a specified number of hours. Disabling this option keeps the GPS unit running constantly; if you have ample power, this will give the most accurate results. You can choose any whole number of hours for the interval.
The Output control tab allows you to configure which data streams are sent to Scream! from the digitizer.
Güralp digitizers initially sample incoming data at a high rate (e.g. 2000 Hz for the DM24), which is then filtered and reduced to a lower rate (decimated) using an on-board digital signal processing unit, or DSP. The DSP has several filtering-decimation stages, which run one after the other. Stages which can produce output are called taps. The Güralp DM24 can output 4 taps simultaneously.
Each configurable tap can be set to a different decimation factor by choosing values from the drop-down menus on the left. Decimation factors of 2, 4, 5, 8, and 10 are available. The numbers visible in the drop-down menu of each tap are the data rates that each of the possible decimation factors will provide, given the settings of the taps above it. Only integer (Hz) data rates are allowed: thus, for example, if one tap produces data at 25 Hz, the only possible further decimation factor is 5.
To the right of each decimation factor menu is a grid of check-boxes. These boxes mark which streams of data to generate at each sample rate. The screenshot above shows a possible configuration for a triaxial instrument. Every channel of the digitizer may be output at any tap; currently, all three axes are being output at Tap 2 (20Hz).
If you want to change the names used for the channels, click in the white box containing a Z in the above picture, and type a letter or number. It will name the channels with a sequence of letters or numbers beginning with the one you choose (e.g. A, B, C; 2, 3, 4; 9, A, B), unless you type Z in which case they will revert to Z, N, and E.
Each combination of channel and tap has two check-boxes. The upper check-box of each pair activates continuous output, whilst the lower activates triggered output. In the example above, the digitizer will output data continuously for all three channels at Tap 2, but never for any other taps. You cannot check both continuous and triggered output for the same channel and tap.
When you enable a triggered stream, the digitizer will output data in that stream only when a particular set of trigger criteria are met. This is pictured in the window as a switch. For example, to generate high-rate data from Tap 0 only when an event registers at some other tap, the lower check-boxes of Tap 0 should be ticked.
With this configuration uploaded, Tap 2 will continue to produce output at all times, but Tap 0 will also emit data whenever the trigger criteria are met. The Triggering button is now shown in red to remind you that the trigger is active.
Every checked box in this window will give rise to a data stream coming from the digitizer, which will be displayed in Scream!'s main window when Scream! first receives some data from it.
In its standard configuration, the digitizer outputs continuous data at a sample rate you specify. In addition to this, Güralp digitizers can run a triggering algorithm on the data they acquire. This allows you to record data continuously at a relatively low sample rate, but record at a much higher sample rate during short periods when the trigger is active. The parameters controlling the triggering algorithm, and controlling the data output once the system is triggered, are all selectable by the user, permitting maximum flexibility of operation and the most efficient use of available storage space.
The digitizer can be set up for triggered output, that is, to output certain data streams only when a particular trigger criterion is met. The trigger criterion can be tested with data from the same or some other stream. For example, you could use a later tap (with a lower sample rate) as a trigger for output from an earlier, more detailed tap. Scream! 4 also allows you to configure each digitizer to receive triggers from other digitizers.
To create a new stream with a trigger, open Scream!'s Digitizer configuration window for the relevant digitizer, and click on the Output control tab. In the Output control pane, a tap which gives rise to a triggered stream has a tick in the lower row of its grid of check-boxes. You cannot configure the trigger criteria until you have selected at least one stream to be affected by the trigger.
Once you have decided which streams should be output when the trigger is activated, you will be able to click on the Triggering button to describe the trigger condition. Alternatively, click on the Triggering tab at the top of the window. Either action will open the Triggering pane:
There are two triggering algorithms which Güralp digitizers can use. However, not all models can use both methods. Scream! will find out from the digitizer whether its on-board software supports each method.
In addition, some digitizers support external and software triggering.
The STA/LTA algorithm applies a simple short-term average – long-term average calculation to the triggering stream. It works by identifying sections of an incoming data stream when the signal amplitude increases. The purpose of taking a short term average, rather than triggering on signal amplitude directly, is to make it less likely that spurious spikes will trigger the device. Averaging also introduces an element of frequency selectivity into the triggering process.
You can select which tap is tested for the trigger from the Data source drop-down menu. The tap does not have to output data to Scream! for you to be able to use it here.
Any or all of the channels available at that tap may be used to determine a trigger. You can select which channels are considered by checking the boxes in the Channel column of the table. If any of the checked channels passes the trigger condition, the trigger will activate, and will not detrigger until all of the checked channels have fallen below their respective ratio values.
The STA and LTA columns allow you to set the intervals over which the two averages are calculated, in seconds. Typically, the time interval for the short term average should be about as long as the signals you want to trigger on, while the long term average should be taken over a much longer interval. Both the STA and LTA values are recalculated continually, even during a trigger.
The Ratio column determines by what factor the STA and LTA must differ for the trigger to be passed. Finding the ratio most suited to your needs is best done by experiment. Too high a value will result in events being missed, while too low a value will result in spurious non-seismic noise triggering the system. Like the averages, their ratio is continuously recalculated for all components. Note that none of the boxes are allowed to be empty, and so you will need to enter the new value before removing the old one. Alternatively, you can use the up and down cursor keys to change the values.
For example, setting the STA to 1 second, the LTA to 10 seconds and the Ratio to 4 would give rise to the trigger behaviour depicted below.
Usually, the values of the STA and LTA periods, and of the Ratio, will be the same for all checked channels. For convenience, Scream! will automatically fill in other values to match ones you enter. If you want to use different values for some channels, you should uncheck Common values before altering them.
Once you have enabled the STA/LTA triggering method on a particular channel, you can use the Control window to change the values of the STA and LTA periods, together with the Ratio, without restarting the digitizer (see Chapter 7, page 80.)
Since it is not generally advisable to trigger from broadband data, the digitizer provides a set of standard bandpass filters to apply to the data streams before they are tested for the trigger condition. This filtering serves to maximise sensitivity within the frequency band of interest, and filter out noise outside this band. You can select which bandpass filter to use from the Bandpass filter drop-down menu. The corner frequencies of the pass band of the filter are determined by the Nyquist frequency, which is given by the sampling rate of the triggering data. The three filter options have pass bands between 10% and 90%, between 20% and 90% and between 50% and 90% of the data’s Nyquist frequency, respectively.
The possible filter configurations are:
Tap # | Rate (samples/s) | Bandwidth 1 (Hz) | Bandwidth 2 (Hz) | Bandwidth 5 (Hz) |
0 | 200 | 10 – 90 | 20 – 90 | 50 – 90 |
1 | 100 | 5 – 45 | 10 – 45 | 25 – 45 |
| 50 | 2.5 – 22.5 | 5 – 22.5 | 12.5 – 22.5 |
| 40 | 2 – 18 | 4 – 18 | 10 – 18 |
| 25 | 1.25 – 11.25 | 2.5 – 11.25 | 6.25 – 11.25 |
| 20 | 1 – 9 | 2 – 9 | 5 – 9 |
2 | 50 | 2.5 – 22.5 | 5 – 22.5 | 12.5 – 22.5 |
| 25 | 1.25 – 11.25 | 2.5 – 11.25 | 6.25 – 11.25 |
| 20 | 1 – 9 | 2 – 9 | 5 – 9 |
| 10 | 0.5 – 4.5 | 1 – 4.5 | 2.5 – 4.5 |
| 8 | 0.4 – 3.6 | 0.8 – 3.6 | 2 – 3.6 |
| 5 | 0.25 – 2.25 | 0.5 – 2.25 | 1.25 – 2.25 |
| 4 | 0.2 – 1.8 | 0.4 – 1.8 | 1 – 1.8 |
| 2 | 0.1 – 0.9 | 0.2 – 0.9 | 0.5 – 0.9 |
3 | 25 | 1.25 – 11.25 | 12.5 – 11.25 | 6.25 – 11.25 |
| 10 | 0.5 – 4.5 | 1 – 4.5 | 2.5 – 4.5 |
| 5 | 0.25 – 2.25 | 0.5 – 2.25 | 1.25 – 2.25 |
| 4 | 0.2 – 1.8 | 0.4 – 1.8 | 1 – 1.8 |
| 2 | 0.1 – 0.9 | 0.2 – 0.9 | 0.5 – 0.9 |
| 1 | 0.05 – 0.45 | 0.1 – 0.45 | 0.25 – 0.45 |
As can be seen, the filter you choose defines the set of permissible sample rates.
Using the Level triggering method, a trigger is generated whenever one of the checked components reaches a certain level above the baseline. You can select which tap is monitored from the Data source drop-down menu, and the channel(s) to be considered from the Channel column of the table. The values in the Level column are the number of counts above the baseline that channel must reach before a trigger is generated.
As with the STA/LTA method, the values of the Level will often be the same for all checked channels. If you want to use different values for some channels, you should uncheck Common values before altering them.
Once you have enabled the Level triggering method on a particular channel, you can use the Control window to change the level at which the system triggers without restarting the digitizer (see Chapter 7, page 80.)
When a digitizer or digital sensor triggers, it sends the trigger itself to connected devices, as well as any extra data that it has been configured to record. You can configure other digitizers to respond to this signal by triggering in turn. This is an option which you can specify at the time of manufacture.
As an example, to instruct a stand-alone digitizer with digital inputs to respond to triggers generated by an attached digital sensor:
Open the Configuration setup window for the digital sensor, and check Enable External Trigger Output to make it send triggers to connected devices.
UPLOAD the new configuration to the digital sensor.
Open the Configuration setup window for the digitizer, and check Enable External Trigger Input to make it listen for triggers coming from the digital instrument, and record data from attached analogue instruments when it receives one (depending on its Output control configuration.)
UPLOAD the new configuration to the digitizer,
If a digitizer has both Enable External Trigger Output and Enable External Trigger Input selected, it will record data when it receives an external trigger as if it had triggered itself, but it will not send that trigger on to other digitizers. It will only send a trigger message if its own triggering criteria are satisfied.
For full details on external triggering, see the manual for your digitizer or the technical notes available on the Güralp Systems website.
Pre-trigger and post-trigger recording
In order to capture all of a seismic event, it is often useful to be able to record data immediately preceding the trigger. Güralp digitizers have an internal buffer of some seconds which allows this data to be added to the triggered stream. Pre-trigger data is particularly useful for emergent-type signals, where the system does not trigger until one phase after the first arrival. In addition, to ensure that the coda of each event is included, some seconds of data are recorded after the system detriggers.
The two boxes at bottom right of the Triggering pane allow the user to set the pre-trigger and post-trigger data intervals, in seconds. These values determine the minimum length of time during which data will be saved before the trigger condition occurs, and after it has lapsed. Regardless of the intervals chosen, the data in the triggered streams will begin on a whole second.
Güralp digitizers provide a range of slow-rate auxiliary channels for reporting the system's state of health and other diagnostic information, known as multiplexed (“Mux”) channels. The number of Mux channels depends on the model and configuration of your digitizer. Generally, three channels are used to report the sensor mass position, and another measures the internal temperature of the digitizer. In addition to these, depending on the digitizer options selected, up to 12 Mux channels may be configured for the user's own purposes.
Some digitizers have a separate AUXILIARY port which can be used to access these channels.
The collection and transmission of Mux channels is controlled using the Mux Channels pane:
If a tick is placed in the box next to a channel, its data will be collected and transmitted as a data stream in GCF format, just as with the normal data channels. To indicate that the data comes from a Mux channel, the Stream ID will take the form ****Mx, where M stands for Mux and x is a hexadecimal integer (i.e. 0 – 9, and A – F for 10 through 15).
Depending on the type of digitizer you connect, Scream! may be able to replace the legends with descriptions appropriate to the digitizer. For example, on DM24 digitizers, the Mux channels M8, M9 and MA appear with the legends Z, N/S and E/W Mass Position respectively, as shown above.
The Baud Rates pane of the Configuration setup window allows you to program the baud rate and stop bits for the digitizer's output port.
The baud rate you choose must satisfy two conditions:
It must be high enough to allow all the transmission of all data generated by the digitizer at the sampling rates you have chosen. For three streams of data at 100 Hz, for example, 9600 baud will usually be sufficient. If you wish to transmit 200 Hz data, however, the baud rate must be at least 19200.
It must be low enough to fit within the operating range of the telemetry equipment you are using. While modern modems often offer transfer rates up to 56 kbaud, the telephone or transmission lines may not support these rates. The same holds true for radio telemetry.
Usually, the transmit and receive rates of the data port will be the same. If not, you may select different data rates by removing the check in the box marked Identical TX/RX rates.
The Stop Bits option allows you to choose whether the serial link uses 1 or 2 stop bits. In most cases this can be left at 1, although 2 may be required if you are sending data over ‘difficult’ transmission lines (for example, some types of radio link.) Using 2 stop bits will add a 10% overhead to the data.
You will also need to set the data rate for Scream's local serial port, as well as for the SAM/DCM or other communications device (if you are using one). In Scream!, you can configure a serial port by right-clicking on its icon (not that of the digitizer) and selecting Configure... from the pop-up menu: see Section 2.1, page 9. If you are using an additional communications device, you should consult its documentation to learn how to set its baud rate.