Commercial digital audio recorders: a new life for portable Lennartz PCM 5800 seismic stations
In the last two decades the use of mobile digital seismic stations has become more and more widespread both for active and passive seismological studies.
The PCM 5800 Digital Seismological System has been produced by LENNARTZ ELECTRONIC GmbH (Tubingen, D) since the middle Eighties. It became the first digital seismic equipment for many european and worldwide seismological institutions and laboratories, in many of which it is still in operation.
The PCM 5800 System
The PCM 5800 System is based on a 12 bit Analog-to-Digital Converter reaching a dynamic range of 120 dB by means of a gain ranging amplifier. This seismological system has been produced in three configurations: Encoder (4-to-16 input), Mixer (up to 63 telemetered digital data streams from Encoders) and Decoder (for playback with analog output and/or IEEE-488 digital output). The most widespread acquisition system is the first one in four-channel configuration (for a three-component seismometer plus one free channel) recording on UHER 13cm magnetic tape recorder and equipped with a Decoder unit for the data playback (Figure 1).
In this paper we refer to this configuration as the standard equipment for portable use.
Software programming is realized through an alphanumeric display terminal (Figure 1). The acquisition is based on the classic STA / LTA (Short Term Average / Long Term Average) triggering algorithm. The software is easy to use and very adaptable. Standard time code is DCF-77 (77.5 kHz), however a GPS-DCF Time Code Receiver is also available. The PCM output of an Encoder acquired by an UHER recorder set at 4.75 cm/sec tape speed is 10kbits/sec. The playback of the data is performed by means of a Decoder PCM 5800 unit linked to a REVOX B-77 MK II tape recorder. An IEEE-488 interface allows the transfer of the recorded signals on a personal computer for analyses and storage.
Figure 1. - A standard 4 channel PCM 5800 Encoder in a 19” rack (from Lennartz Electronic, 1983).
As mentioned before, the UHER recorder has been a weak point of the system because of its high maintenance costs. After about eight to ten years of intensive use, the motor of the recorders crashes and it can not be repaired! To solve this problem LENNARTZ ELECTRONIC GmbH introduced the so called “Digital UHER”, that is based on a MarsLite/HD system recording on a 4.3 GB hard disk. Although this system allows the continuous data acquisition in a modern format, it forces to “leave” the existing and working instruments such as PCM 5800 Decoders, playback recorders and IEEE-488 interface cards. Moreover, the software for data playback and analysis must be changed.
We suggest to use MiniDisc digital audio recorders as a simple solution to substitute the UHER recorders. These recorders are designed by SONY CORPORATION and also produced by other commercial firms.
The MiniDisc System
The SONY CORPORATION produced the first generation of MiniDisc recorders on 1992. This system allows to record the same amount of audio signals of a CD (650-700 MB) on a 2.5” magneto-optical disc of about 160 MB of capacity with practically the same audio quality.
Figure 2. - Block diagram of ATRAC encoder (after Yoshida, 1994)
The ATRAC3 coding achieves twice the frequency resolution of ATRAC, splitting the signal in four sub-bands and using longer MDCT conversion blocks. This allows us to increase the output frequency domain signals (Sony Products, 2000).
Hi-MD and Electronic controls
Not all the SONY Hi-MD models currently on the market are useful to record and reproduce PCM 5800 signals. In order to record, an instrument with LINE-IN input and remote control is enough, whereas to reproduce the data an apparatus provided with LINE-OUT output, characterized by an output level of 194 mV, is necessary. The only headphone output is not enough to decode PCM signals due to its too low output level (~ 1.4 mV).
Table 1. – Audio encoding and capacity on Hi-MD. (after Sony Corporation 2004, modified).
Among the SONY Hi-MD production, we have selected, according to price-performance ratio, the MZ-NH700 and MZ-RH910 models to record data and the MZ-NH900 model to reproduce them linked with a Decoder PCM 5800 unit.
Circuit for the Recording Control
The PCM signal is characterized by a 0 / +5V level. In order to record data on the Hi-MD recorder, the signal must be characterized by a 0 centred level. Therefore, the PCM signal is high-pass filtered (0.1 Hz) before the Hi-MD LINE-IN input.
Table 2. – Block diagram of the control sequence and Hi-MD functions. Red controls are related to manual commands, the green ones are related to electronic circuit instructions. If the power supply decreases under 10V, the STOP command is enabled by the electronic circuit.
Using the keys of the wired remote control, the electronic circuit enables the PAUSE command in correspondence with the TRIGGER ON and TRIGGER OFF impulses resulting from the trigger algorithm of the PCM 5800 station. The electronic circuit diagram for the recording control is shown in Figure 3A.
Note that the recorded data will be saved on disk only after the STOP command. To avoid that a too low power supply could cause the unexpected recorder switching off with the loss of the data, a voltage comparator is introduced in the electronic control circuit. If the power supply decreases under 10V, the circuit enables the STOP command on the wired remote control and the data will be saved on disk. The diagram of the voltage comparator is shown in Figure 3B.
Figure 3. - A) Diagram of the electronic circuit for the recording control, B) Diagram of the voltage comparator circuit
In Figure 4 the BKS (Mt. Vesuvius) PCM 5800 station equipped with a SONY Hi-MD (MZ-NH700 model) is shown. The power supply for the Hi-MD is taken from the DC/DC converter module of the PCM 5800 station.
Figure 4. - The PCM5800 station BKS (installed in a concrete bunker on Mt.Vesuvius) in 19” rack, equipped with SONY Hi-MD MZ-NH700. The black box on the left contains the circuit for the recording control.
Circuit for the Decoder Link
The output signal of a Hi-MD is an audio signal characterized by variation both in amplitude and frequency. A simple electronic interface that maximizes and normalizes the signal amplitude at a level of +5V is necessary to connect the Hi-MD output to the Decoder unit. In this way the original squared shape of the PCM signal is reconstructed and the Decoder unit is able to elaborate it.
Figure 5. - Diagram of the electronic circuit for the Decoder Link.
Discussion and Conclusions
The PCM 5800 is an “old” system compared with the present available seismic instruments. Nevertheless, many seismological research institutes have a lot of these stations still working.
Figure 6. - M 3.3 regional earthquake (D=50km) of October 14th 2004 recorded at Lennartz PCM5800 station BKS (Mt.Vesuvius) equipped with Sony Hi-MD MZ-NH700 recorder and Lennartz Le-3D/1s seismometer. Trigger parameters: STA=1s, LTA=51s, STA/LTA=8.
SONY Italia is greatly acknowledged for its cooperation. We are grateful to F. Bianco for her continuous incentive to experiment with new technological solutions. Many thanks to C. Buonocunto and A. Caputo for their collaboration. All marks belong to lawful owners.
Lennartz Electronic (1983); Manual for PCM5800 Encoder. Tübingen, Germany, 189p.
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Sony Products (2000); ATRAC3 High-Quality Audio Encoding Technology. CX-NEWS, vol. 22.
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