4 Recorders and Recording System II

Vinay Gupta

    Learning Objectives

In this module we will study about modern digital systems.

1. First we will study about digital display devices
2. In detail we will look into magnetic recording systems
3. Finally we will look into few applications of the modern recoding systems in with biomedical applications like EEG and ECG.

   Introduction

In this module we continue to look into various recording devices that can store data long after the signal has ceased to exit. Earlier we had discussed Graphic and Oscillographic recorders. In the 21st century, most of the analog systems have been replaced digital data recording and measurement systems. The main component in most digital measurement system is an amplifier along with a feedback system to monitor range and frequency of the system under observation. Besides the recording set-up in most digital systems, real time monitoring of the measurement is equally important. This helps adjusting the parameter before one may start to record the data. At present most digital recording systems have computer interface to monitor measurements. During developing of digital recording devices in late 20th century real time monitoring was done via digital display systems.

Let us start with

Digital Display and Data Recording System

Digital Instruments. An analog instrument, displays the quantity to be measured as deflection of a pointer i.e. an analog displacement or an angle corresponding to electrical quantity. Whereas, a digital instrument indicates the value to be measured as a decimal number. A digital meter work on the principle of quantization and in analog, measurable quantity is first subdivided or quantized into smaller intervals up to many decimal places. Here, the objective of the digital instrument is then to determine in which portion of the subdivision the measurand can thus be identified as an integral multiple of the smallest unit called the quantum, for the chosen subdivision. The reading accuracy can further be increased by increasing the number of decimal places, i.e. by increasing the quantization levels.

Advantage of Digital Displays

1. Reading indicated is in decimal numbers, therefore errors due to parallax and approximation are eliminated
2. The reading can be carried to any number of significant figures
3. Since the output is in digital form, it may be directly fed into memory devices like tape recorders, printers or computers for storage and for future computations.
4. Power required to operate digital instruments is considerably smaller.

    Digital Display Units

Digital devices can be broadly classified as planar and non-planer, based on whether the displayed characters are in the same plane or on different planes. The segmental displays like LED, LCD, dot matrix displays and segmented gas discharge displays are Planar whereas non-planar ones include nixie tubes and gas discharge tubes.Examples of segmented and a dot matrix array is shown in the figure 1.

Figure 1. Digital Display Device (a) 7–Segment Array (b)5 x7 dot array(c) 16 – Segment array

   Dot matrix displays are generally used to display alphanumeric characters. LED i.e. a light emitting diode is p-n junction semiconductor device that gives light as output when current is fed in its forward bias configuration. A liquid crystal display (LCD) is made up of liquid crystal cells, which either transmit or reflect light. They do not generate light on their own instead require an external source of light to produce an image. Both nixie tubes and segmental gas discharge display work on the principle of gaseous discharge glow where a discharge glow can be observed when the gas in the tube breaks down. Both types of gas discharge displays are built-into seven or 14 segments to display numeric and alphanumeric characters.

Magnetic Tape Recorders

The recorders discussed until now are low frequency recorders and its not possible to retrieve or reproduce data in electrical signal form. With Magnetic Tape Recorders, the recorded data or the information to be retrieved can be converted to electrical form again. It is possible to record data at much higher frequency of the order of MHz, plus larger volume of data can be stored and they also give fast access to the stored data. Different types of recording systems like magnetic tapes, discs, stripes, drums or loops use the same kind of transducers and magnetic medium to record or reproduce the data.

There are 3 methods to record data on magnetic tape – (1) direct recoding (2) frequency modulated recording (3) pulse duration modulation

But before we look into these 3 methods let us discuss the basics of the Magnetic Recording System.

Figure 2. Simple Magnetic Recording System

Construction– A magnetic tape recorder consists of magnetic tape, recording head, reproducing head, tape transport mechanism and conditioning device such as amplifiers and filters. Figure 2 illustrates a simple magnetic recording system.

• Magnetic tape – It has plastic base which is typically polyvinyl chloride (PVC) or polyethylene terephthalate, for e.g. mylar. This thin sheet of dimensionally stable plastic, on one side is coated with a magnetic material. This magnetic coat consists of very small iron oxide (Fe2O3) particles mixed in a plastic binder. The needle shaped iron oxide particles are about 0.6 micron long and 0.1 micron in diameter, occupying 50% of the coating volume and oriented in the direction of their long axes along the length of the tape.
• Recording head – It is a device that leaves a residual magnetic pattern in response to an amplified input signal.  Its basic construction is shown in figure 3. It consists of a toroidal core of high permeability material with a coil and fine air gap of 10 micron. This air gap gets shunted by passing magnetic tape and coil creates a flux of the similar shape to bridge the air gap. The iron oxide particles  get  magnetized  when  they  leave  this  air  gap  and  so  the  actual recording takes place at the rear edge of the gap. Thus the magnetic pattern that is spread along the tape is similar to original coil current variation in time.

Figure 3. Recording head

• Reproducing Head – The design of the reproducing head is similar to the recording head. When the magnetic tape passes over the head the resulting output voltage is proportional to the magnetic flux in the tape, developed across the coil of the reproducing head. Thus, the magnetic pattern stored in the tape gets converted into original electrical signal by the reproducing head.
• Tape Transport Mechanism – It moves the tape over the recording and reproducing head at a constant speed without any strain, distortion or wear. This mechanism guides the tape near to the magnetic heads with great precision and maintains proper tension during data recording or signal reproduction.

Operating Principle –

When a magnetic field is applied to the iron oxide particles in a tape and then removed, a residual flux is left behind. An earlier state of magnetization and the magnetization curve of recording medium govern the relationship between the residual flux and recording field.

Figure 4. A typical magnetization curve

Typical magnetization curve for a single magnetic particle is shown in the figure 4. Here one can observe relationship between magnetization force H and flux density, B. A completely degaussed material is at point O. As the current in the head increase, flux density in the direction of the increasing magnetization force gets saturated at point C. This can be observed along the path OAC. In case the operating point is brought to point A and magnetization force H is brought to zero, the flux density follows a hysteresis path to point K. Therefore when large value of current passes through the coil it will leave a higher magnetic flux and lower current will leave lower residual flux but linearity between the flux and the current is very poor due to hysteresis. To secure a linearity of the recording data various techniques like direct method, Frequency Modulation and Pulse Duration modulation are used.

Now we will discuss various data recording methods –

Direct Method- This is the most economical and simple method for analog recording of the signal. A signal of a certain amplitude and frequency are linearly recorded as is the variation of magnetization amplitude and wavelength of the tape. By adding high frequency ac bias to the recording signal one can get amplitude linearity when recoding data.

Figure 5. Recording and Reproducing Head

Through the coils input voltage gets converted into proportional current. A magnetic flux created at the recoding gap is given by

Ø  = KØi —– (1)

With sinusoidal input signal we get

i  = io sin 2  ft ——- (2)

and with tape speed of v m/s, the intensity of magnetization along the tape varies sinusoidally with distance x as

Where m = Km. Ø. The wavelength of the magnetization variation is then v/fm. Thus the resultant magnetic field in the recording gap enables magnetic recording as the tape passes under the gap.

Direct Recording is mainly employed for recording the speech and music as in sound recording and the ear averages the amplitude variation errors.

Frequency Modulation (FM) Recording- Input signal is modulated by a frequency modulator and is recorded linearly using ac bias or non-linearly without bias on a magnetic tape. For reproduction, the recorded information is reconstructed by frequency demodulation and filtering. In this system variation is frequency is used, instead of amplitude for carrying desired information. This system is preferred when a more accurate recording and response to dc voltages are required.

Pulse Duration Modulation (PDM) Recording- Here input signal is converted into pulse, duration of which is proportional to the amplitude of the signal at that instant. For example if sine wave is to be recorded, it is sampled and recorded at uniformly spaced discrete intervals. For playback, original sine is reconstructed by passing it through appropriate filter. It has high signal to noise ratio, high accuracy and information can be recorded from large number channels. Its main drawback is use of highly complex circuitry, low reliability and limited frequency response.

Applications

There are number of application of recording devices. We have discussed one such application where one can use magnetic heads to store data within the audible range by using magnetic tape recorders. Other applications of recorders for Biomedical application are Electroencephalograph (EEG) and Electrocardiograph (ECG).

Electrocardiograph (ECG)–It is an instrument that records electrical activity of the heart. Electrical signal generated in the heart, precede the normal mechanical beat that distributes the blood through out the body. These signals are of great clinical significance and are used for diagnosing wide range of cardiac disorders. The block diagram of EEG machine is illustrated in figure 6.

Figure 6. Block diagram for ECG machine

The signals (potential) picked from the patient electrodes are taken to the lead selector switch. Lead program in the selector switch selects two by two electrodes. Via capacitive coupling, the signal is connected symmetrically to the long tail pair differential preamplifier. The Single ended output from the preamplifier is fed into power amplifier, which is usually of the push-pull differential type. The base of the one transistor in the power amplifier is driven by unsymmetrical signal from preamplifier whereas base of the other is driven by feedback signal resulting from the pen position that is connected via frequency selective network. The output from the power amplifier is fed to the pen motor, which deflects the writing arm of the paper. Cardiograms are recorded on graph paper with horizontal and vertical lines at 1 mm interval and thicker lines at 5 mm interval. For routine work paper recording speed is 25 mm/s. Amplitude readings are recorded vertically in millivolts. The sensitivity is typically set at 10 mm/mV.

Electroencephalograph (EEG)–It is an instrument that records electrical activity of the brain. Electrodes for the measuring electrical activity are placed on the surface of the scalp. The electrodes are small in size and are applied separately or mounted over special bands placed on the patient’s head. Signal picked by surface electrodes are smaller than the signal picked up by ECG. There range is several hundred microvolts, but peak-to-peak voltage of 50 microvolt is most often observed.

Brain waves unlike electrical measurements of heart, lack periodicity, i.e. the same pattern is not repeated over and over again. Recordings are conducted over longer interval of time to detect any abnormalities. The block diagram of EEG machine is shown in figure 7.

Figure 7. Block diagram of EEG Machine

EEG potentials are small in amplitude therefore preamplifiers used here must have high gains and low noise characteristics. Plus, amplifier must have very high common mode rejection to minimize any stray signal interference from power line or any other electrical equipment. EEG simultaneously records signal from an array of electrodes. Electrodes are separately connected to amplifiers and writing systems. A commercial set-up can have upto 32 channels, though 8 or 16 channels are more easily available.

Summary

In this module we studied about modern digital systems.

1. First we studied about digital display devices
2. In detail we looked into magnetic recording systems
3. In the end we looked into few applications of the modern recording systems with biomedical applications like EEG and ECG.

     References :-

1. Electrical and Electronic Measurements and Instrumentation, Sawhney A. K., Dhanpat Rai & Sons, Reprint 1985
2. Measurements and Instrumentation, Bakshi U.A., Bakshi A.V., Technical Publications, 2009
3. Principles of instrumental analysis, Skoog, Douglas A., F. James Holler, and Stanley R. Crouc,. Cengage learning, Edition 2017
4. Instrumentation, measurement and analysis. Nakra, B.C. and Chaudhry, K.K., Tata McGraw-Hill Education, 2003.
5. Measurement and instrumentation: theory and application, Morris, A. S., & Langari, R. , Academic Press, 2012