Abstract:
A method for digital preemphasizer high fidelity reconstruction of an original pulse code modulation (PCM) serial stream binary data signal which has suffered degradation of fidelity, and consequent increase in bit-error-rate, during transmission on a single channel from a transmitter to a receiver. Prior to transmission, amplitude encoded digital pre-emphasis of each bit of the original binary data signal to be transmitted mitigates or remedies the signal degrading frequency dependent losses concomitant with the signal transfer network characteristics of the fixed transmission line. Subsequently, the amplitude encoded signal is transmitted to the receiver connected to the other end of the fixed transmission line. Compared to a conventional transmitter, transmision line, and receiver system, the end result of amplitude encoded pre-emphasis is superior reconstructed fidelity and quality of the PCM waveform, i.e. lower bit-error-rate, at the output of the receiver for the same length of transmission line.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention, disclosed hereinafter, concerns generally an apparatus and method for reducing signal errors attendant to pulse or digital communications transmitted along lossy, band-limited, time-invariant, fixed length, single channel transmission path. In particular, the present invention provides apparatus and method for reducing the error-rate of digital PCM (pulse code modulation) data by means of preemphasis precoding or encoding of the binary PCM data signal prior to transmission on a single channel of a cable system. 
   2. Description of the Related Art 
   The electronic transmission of data and information by means of pulse and digital systems typically involves either a variable path, such as a radio link between a cellular or cordless is telephone and a repeater node, or alternatively, a fixed, time variant path, such as a copper fiber optic cable connecting a cable television subscriber&#39;s receiver to a provider&#39;s distribution transmitter. 
   The present invention concerns the latter fixed path. For fixed path cable transmissions, the fidelity, i.e. veracity, of the transmitted data or information is limited by the bandwidth characteristics (also called transfer characteristics, impulse response) of the equivalent, passive network representing each channel of the transmission line. In many cases, a transmission line for pulse and digital communications can feature multiple channels, permitting separate clocking and error correction channels for each data channel; however, in some cases, exigent economic and physical demands prescribe a minimal size, single channel cable or path for each data or information stream. These constraints foreclose the option of a separate clocking or timing channel. Also, there may be constraints to the low frequency response of the designed transmission system. To remedy or mitigate these channel availability and frequency response constraints bi-phase level, self clocking PCM techniques are commonly employed. Unfortunately, such codes effectively reduce the available data and information bandwidth by half. These issues and associated remedies are discussed in “Digital Recording Methods”, Magnetic Tape Recording Technical Fundamentals, Datatape Incorporated, 1976, pages 9599, incorporated by reference herein for purposes of indicating the background of the invention and illustrating the mature state of the art. The rationale and utilization of bi-phase PCM technology is well known to those practicing the art. 
     FIG. 1  illustrates a typical prior art, single channel, digital PCM data transmission system. 
   Biphase PCM Waveforn- 21  is the digital PCM data input to Buffer AmplifieI 22 , representing a simple transmitter, comprising either digital or analog circuitry. A Fixed Transmission Line  23 , featuring a single conductor per channel, each channel exhibiting both resistive and frequency dependent attenuation, conducts the output signal from Buffer Amplifier  22  to the input of simplified, typical Receiver  24 , comprising Wide-band Amplifier 25  and Multipole Equalization Network  26 . Wide-band Amplifier 25 , employing either analog or digital circuitry, is used to compensate for the attenuation of Waveforri 21  upon transmission through the lossy Fixed Transmission Line  23 . Output from Wide-Band Amplifier  25  thereupon goes to the input of Multipole Equalization Network  26 . Although typically an active network Multipole Equalization Network  26  can also be a passive network. Intended to reconstruct the degraded fidelity of the signal present at the input to Receiver  24  to a replication of Waveform  21  by compensating for the frequency dependent losses attendant to the Fixed Transmission Line  23 , Network  26  tends to be complex and difficult to implement. Although Network  26  may also include some techniques to correct the slope or rise time of the attenuated received signal, such techniques are often primitive and inadequate for reliable, high-quality digital PCM data transmission. This inadequacy typically is remedied by Bit Synchronizer  27 , which processes the output from Multipole Equalization Network  26 , correcting for slope and rise time degradation, and producing as an output, Reconstructed PCM Waveform 28 , which is desired to be a replication of Bi-phase PCIVI Waveform  21  of sufficient veracity and reliability to fulfill the intended purpose of the data transmission system. Unfortunately, as illustrated in  FIG. 1 , Bit Synchronizer  27  is normally a separate, often necessary, add-on component to the typical receivef 24 . Containing expensive, complex circuitry, this synchronizer component can cost thousands to tens of thousands of dollars; proper utilization often entails lengthy, difficult adjustments, 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a means for high fidelity reconstruction, i.e. low, acceptable bit error-rate, of an original pulse code modulation (PCM) serial stream binary data signal which has suffered degradation of fidelity, and consequent increase in bit-error-rate, during transmission on a single channel from a transmitter to a receiver, connected to each other by a transmission line exhibiting both resistive and frequency dependent loss. This object is accomplished prior to transmission by amplitude encoded digital pre-emphasis of each bit of the original binary data signal to be transmitted in such manner as to mitigate or remedy the signal degrading frequency dependent losses concomitant with the signal transfer network characteristics of the fixed transmission line. Subsequent to this pre-emphasis process, the amplitude encoded signal is transmitted to the receiver connected to the other end of the fixed transmission line. Compared to a conventional transmitter, transmission line, and receiver system, the end result of amplitude encoded pre-emphasis is superior reconstructed fidelity and quality of the PCM waveform, i.e. lower bit-error-rate, at the output of the receiver for the same length of transmission lie or, alternatively, a longer transmission line for the same bit error-rate. 
   A further object of the present invention is to reduce the complexity and cost of signal processing equipment at the receiver while maintaining a low, acceptable bit error-rate. As a consequence of employing the present invention, the excellent high fidelity reconstruction of the received, amplitude encoded signal obviates many requirements for complex, expensive signal processing equipment at or following the receiver, such as bit synchronizers, filters, equalizers, and compensators. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified block diagram illustrating a typical prior art, single channel, digital PCM transmission system utilizing a bi-phase PCM waveform. 
       FIG. 2  is a simplified block diagram illustrating one embodiment of bi-phase PCM waveform pre-emphasis circuitry in accordance with the principles of the present invention. 
       FIG. 3  is a timing diagram illustrating various waveforms attendant to the present invention. 
       FIG. 4  is a schematic diagram of one embodiment of the PCM state detector and Multiplexer controller. 
       FIG. 5  is a schematic diagram of one embodiment of the PCM level adjustment. 
       FIG. 6  is a schematic diagram of one embodiment of the wide-band analog multiplexer and the buffer amplifier. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention amplitude encodes the serial stream binary PCM data signal containing digital PCM data which is to be transmitted to the receiver via a fixed transmission line. This encoding modifies the signal waveform by altering the individual amplitudes of each pulse-width interval prior to transmission. The present invention is particularly suited for performing digital pre-emphasis on the bi-phase level type binary serial PCM codes or the Delayed Modulation family of PCM codes. Individual amplitudes of these binary, pulse width intervals, commonly called Mark and Space, are manually or automatically mapped into series of discrete, constant amplitude steps, mitigating or remedying the undesired frequency dependent attenuation characteristics of the transmission line. The resultant encoded signal, which now varies in amplitude steps, is then transmitted through the transmission line. Upon transmission, frequency dependent characteristics of the transmission line degrade the transmitted data signal to an extent which is opposite to the mapped characteristics of the pre-emphasized signal prior to transmission. By compensating for the transmission line frequency dependent loss affecting digital PCM data, the present invention substantially reduced the increased bandwidth penalty imposed thereon and the received, amplitude encoded, signal is more easily de-commutated and reconstructed with greater fidelity, i.e. reduced bit-error-rate, than a similar signal lacking this pre-emphasis. The resulting effect at the receiver is a constant amplitude signal, essentially the original, albeit attenuated, digital PCM data signal prior to amplitude encoding. This received attenuated signal then requires only simple amplification to provide an amplitude level sufficient commutation (e.g., threshold detection) and subsequent reconstruction of the original digital PCM data. The present invention can be easily incorporated and operated with any transmission line network characteristic and length. 
   The present invention simplifies receiver electronics and permits increasing the length of cable linking the transmitter and the receiver without sacrificing increased degradation of the received PCM signal. Typically, PCM signals are transmitted through a cable with only single-pole pre-emphasis for improving transmission spectral characteristics. Multi-pole pre-emphasis can sometimes be utilized, however a change in the type of cable or cable length and its associated transmission spectral characteristics typically requires modification or redesign of the pre-emphasis applied to the signal. The same condition, and generally more severe and sophisticated, is the equalization or de-emphasis network at the receiver. Such networks, which can become quite complex, would require modification and redesign to restore the signal to a usable level if cable characteristics change or if the system is utilized on a different cable. 
   EXAMPLE 1 
   An example of the utility of the present invention entails the employment of oceanographic data systems which transmit data through very long cables from a towed, submerged electronics package. Different ships utilize different types and lengths of cables and such cables have changing characteristics over time or under alternating operating conditions. The elegant simplicity and utility of this exemplary embodiment of the present invention is illustrated in a signal degradation problem that occurred when high speed serial digital PCM data were transmitted through a 7,500 meter coaxial cable connecting a submerged oceanographic data acquisition system to a recording subsystem located on board the towing ship. 
   Self-clocking PCM signals (synchronous or asynchronous) commonly utilize coding schemes like the family codes for Bi-phase and Delayed Modulation when transmitting serial data through single-ended cable. At the shipboard receiver, the degraded signal suffered from, (1) base-band attenuation (in this example, more than −50 dB at I MHz); (2) additional attenuation of the high frequency band component for Bi-phase; and (3) considerable DC baseline shifting. 
   Adequately receiving such degraded signals required expensive Synchronizers (one such unit cost in excess of $10,000) and De-commutators for extracting useable data from the signal stream. Also included at the receiver were amplifiers and equalization circuits to compensate for signal attenuation and degraded fidelity. 
   To minimize or eliminate the need for such complex, expensive equalization and signal is reconstruction at the receiver, the present invention was incorporated into the oceanographic data gathering system to improve signal restoration and consequently, decrease the bit-error-rate. Although the PCM signal therefrom became adversely modified by the cable due to cable characteristics, altering the DC signal baseline and the amplitudes of both high and low frequency band components, the present invention compensated for the cable response characteristics by applying a manually calibrated, multiple stepped, amplitude level adjustment to the PCM signal prior to transmission. As a result of the employment of the present invention, less varying DC baseline and nearly flat amplitude for all frequency and components was received. Laboratory testing demonstrated no data loss at transmission rates of 1-2 MHz through 7,500 m of cable for more than 60 hours of continuous transmission. 
   In summary, the present invention eliminated the need for conventionally employed Bit-Synchronizer and De-commutator electronics and improved the bit-error-rate by several orders of magnitude. 
   To facilitate comprehending this particular and preferred embodiment of the present invention,  FIG. 2  illustrates, in block diagram form, a digital PCM data transmission system incorporating the present invention.  FIG. 2  shows the transmitter section connected to the Fixed Transmission Line  120 , which, in turn, is connected to the receiver section  130 . Bi-phase PCM waveform  60  represents the digital PCM data input to transmitted  50  and Reconstructed Digital PCM Waveform  60  represents the output of received  30 , the desired replication of the digital PCM data input. Also included in  FIG. 2  are signals C, D, E, E′, E″, and C′.  FIG. 3  portrays some of these various concomitant binary and analog waveforms associated with the present invention. The input Bi-phase PCM Waveform  60  is exemplified in  FIG. 3  as Signal C, Bi-phase Level. As illustrated in  FIG. 3 , Signal C is the resultant product of combining Signal A, an NRZ (non-return to zero) signal containing data or information, and Signal B, a clock signal for synchronizing data flow. Techniques for combining Signal A and Signal B into Signal C are common and well known to those practicing the art. Signals A, B, C, and D are binary signals, whereas signal E is an analog signal appearing as a series of discrete, stepped amplitude levels. Signal D is a binary signal operating at twice the frequency of Signal B; and Signal E illustrates the pre-emphasized, multiple step level, analog bi-phase signal in accordance with the present invention. Signal E′ is an amplified version of Signal E; Signal E″ is an attenuated version of Signal E. 
   As illustrated in  FIG. 3 , and common to those practicing the art, the pulse width interval of the bi-phase level, Signal C, may be categorized in one of four possible states,
         Long Mark, (designated: LM)—The longer duration of two possible binary signal levels designated as a Mark, the Mark level typically more positive than the alternative level designated as a Space   Long Space (designated—LS): The longer duration of two possible binary signal levels designated as a Space, the Space level typically less positive than the alternative level designated as a Mark   Short Mark (designated, SM): The shorter duration of two possible binary signal levels designated as a Mark   Short Space (designated: SS)—The shorter duration of two possible binary signal levels designated as a Space       

   Grouping together these four possible states as adjacent pairs of Space Mark or Mark Space states produces a new universe of 8 possible paired states:
         Long Mark followed by Long Space, designated: LM LS   Long Mark followed by Short Space, designated: LIVISS   Short Mark followed by Long Space, designated—SIVILS   Short Mark followed by Short Space, designated: SMSS   Long Space followed by Long Mark, designated: LSLM   Long Space followed by Short Mark, designated: LSSM   Short Space followed by Long Mark, designated—SSLM   Short Space followed by Short Mark, designated: SSSM       

   These 8 possible conditions of level or states constitute the universe of mapped phase levels with digital preemphasis in accordance with the preferred embodiment of the present invention, illustrated in  FIG. 3  as Signal E. 
   In  FIG. 2 , transmitter  50  of the present invention comprises 2X Clock  70 , PCM State Level Adjustment  80  (shown in more detail in FIG.  5 ), PCM State Detector and Multiplexer Controller  90  (shown in more detail in FIG.  4 ), Wide-bandwidth Analog Multiplexer  100  (shown in more detail in FIG.  6 ), and Buffer Amplifierl  10  (shown in more detail in FIG.  6 ). As mentioned earlier, input to transmitter  50  is Biphase PCM Waveform  60  (containing digital PCM data). At this point, biterror-rate is negligible and Signal C and Waveform  60  are essentially identical. 
   The PCM State Detector and MultiplexerController  90  performs the dual function of detecting the 8 possible MarkSpace and SpaceMark states of input Signal C and providing a corresponding multiplexer control signal to the Wide-bandwidth Analog Multiplexer  100 . Responding to the multiplexer control signal, the Analog Multiplexer  100  is a wide-bandwidth multiplexer which selects a specific MarkSpace or SpaceMark mapping level. Adjustable DC voltages from the PCM State Level AdjustmerSO are selected by the Wide-bandwidth Analog Multiplexer  100  to map the detected, binary PCM Mark-Space or Space-Mark states of Signal C into new, variable step amplitude MarkSpace or SpaceMark levels to form the amplitude modulated PCM, Signal E. The mapped output, Signal E, from Multiplexer  100  is the input signal to Buffer Amplifierl  10 , which provides buffering, impedance matching, and amplitude control for thepreemphasized signal to be subsequently transmitted through a single channel of Fixed Transmission Line  120 . In  FIG. 2 , Signal E′ is the amplified output signal from Buffer Amplifier  110 ; Signal E″ is the attenuated output signal from the Fixed Transmission Line  120 . 
   A preferred embodiment of PCM state Detector and Multiplexer Controller  120  is illustrated in  FIG. 4  in circuit diagram schematic format. Signal C, i.e. the digital PCM data, Biphase PCM Waveform  60 , is the input to Programmable Read-Only-Memory (PROM) U 1 , universal integrated circuit nomenclature  2764 , or the like. This input is at address terminal A 8  of PROM U 1 . 
   PROM U 1  terminals, A 10 , A 11 , A 12 , CE, and OE are connected to the system ground bus. PROM U 1  terminals PGM and VPP are connected to +5 VDC source of power. Appropriate connection of the integrated circuitry of the present invention for purposes of system grounding and sources of power are common and well known to those skilled in the art and will not be discussed hereinafter. Upon detection of a particular Mad-Space and Space-Mark state, a corresponding state detector signal is generated as output from PROM U 1 , on detector output terminals, D 0 , D 1 , D 2 , D 3 , D 4 , D 5 , D 6 , and D 7 , connected, respectively, to controller input terminals,  1  D,  2 D,  3 D,  4 D,  5 D,  6 D,  7 D, and  8 D of Digital Latch U 2 , universal integrated circuit nomenclature  74 HC 574 , or the like. These eight data lines represent the eight possible Mark-Space and Space-Mark states of the PCM signal, Signal C. PROM U 1  and Digital LatchU 2  are together configured as a serial shift register providing detection of the eight different Mark-Space and Space-Mark state conditions, listed hereinbefore, that can exist for the Signal C. 
   Signal D from the 2X Clock  70  is applied to Digital Latch U 2  to synchronize identification of each of the eight possible Mark-Space and Space-Mark conditions of state that can exist in Signal C. As shown in  FIG. 4 , Signal D, a clock signal from 2X Clock  70 , operating at twice the clock frequency of SignaB, is applied to the input of FirstInvertor U 7 A, universal integrated circuit nomenclature  74 HC 14 , or the like. The output of the Fist Inverter U 7 A is applied both to the clocking terminal, CLK, of Digital Latch U 2  and also to Second Inverter U 713 , universal integrated circuit nomenclature  74 HC 1   4 , or the like. Output of Second Inverter U 7 B appears on Signal Line AA, reconstructed as Signal D (i.e. a double inversion of Signal D). From PROM U 1 , input address terminals, A 0 , A 1 , A 2 , A 3 , A 4 , A 5 , A 6 , and AT are connected respectively to controller output terminals,  1  Q,  2 Q,  3 Q,  4 Q,  5 Q,  6 Q,  7 Q, and  8 Q, of Digital Latch U 2 . These connections enable the state detector control signal generated by Digital Latch U 2  to feed back PROM U 1  in order to detect the next condition of state of the serial data. This state detector control signal is presented as an address to PROW. The portion of the state detector control signal generated at controller output terminal  1 Q appears on Signal Line BB; the portion of the state detector control signal generated at controller output terminal  2 Q appears on Signal Line CC; and the portion of the state detector control signal generated at controller output terminal  3 Q appears on Signal Line DD. These three output signals from  1  Q,  2 Q, and  3 Q of Digital Latch U 2 , appearing respectively on Signal Line BB, Signal Line CC, and Signal Line DD, plus Signal D (the 2X clock signal) appearing on Signal Line AA, comprise the multiplexed control signal which is applied to the multiplexer input terminals of the Widebandwidth Analog Multiplexer  100  (see FIG.  2  and  FIG. 6 ) to control selection of a specific DC adjusted mapping voltage representing the amplitude of the one of eight possible Mark-Space and Space-Mark states of Signal IC. In this particular embodiment, Multiplexer  100  is represented in  FIG. 6  as integrated circuit Multiplexer U 3 , universal integrated circuit nomenclature MAX 440 , or the like. The purpose of the output control signals utilized by MultiplexerU 3  from outputs  1 Q,  2 Q, and  3 Q of U 2  is to select one of eight individually variable DC voltages representing an adjusted mappling amplitude of a specific Mark-Space or Space-Mark state condition. 
   These eight discrete DC voltage levels are determined by PCM State Level Adjustment  60 , shown schematically for this embodiment as FIG.  5 . Circuitry constituting Resistor R 8 , Resistor R 4 , Capacitor C 1 , Capacitor C 2 , Capacitor C 3 , and Capacitor C 4  serves to provide voltage dividing from a source of DC power and also for suppression of transient noise. Such circuitry is well known by those practicing the art. Eight variable resistance devices, VR 1 , VR 2 , VR 3 , VR 4 , VR 5 , VR 6 , VR 7 , VR 8 , connected between a source of DC power and system ground, are manually adjusted to provide preemphasizer mapping voltages, respectively, to Mapping Voltage Line HH, Mapping Voltage Line GG, Mapping Voltage Line FF, Mapping Voltage Line EE, Mapping Voltage Line JJ, Mapping Voltage Line II, Mapping Voltage Line KK, and Mapping Voltage Line LL. These mapping voltage lines EE, FF, GG, HH, II, JJ, KK, LL serve as input, respectively, to IN 0 , IN 1 , IN 2 , IN 3 , IN 4 , IN 5 , IN 6 , and IN 7 , level adjustment input terminals of Multiplexer U 3 , shown in FIG.  6 . These DC mapping voltages are individually and manually adjusted to compensate for frequency dependent attenuation of Signal E′, upon transmission through Fixed Transmission Line  20 , as represented herein by Signal E″. Such frequency dependent transmission line loses occur because a Short Mark or Short Space features a higher frequency component than a Long Mark or Long Space, and consequently, yielding differing amplitudes at the output of the transmission line. Thus, the amplitude mapping of the higher frequency signal component must be of higher magnitude than that of the lower frequency signal component to compensate for the greater attenuation suffered in the transmission line. 
   In the preferred embodiment, calibration of PCM State Level Adjustment  20  to subsequently provide pre-emphasis amplitude mapping of Signal C into the digital pre-emphasized signal, Signal E, is accomplished by observing, with an appropriate measuring instrument, such as an oscilloscope, the received signal at the output of Wide-Band Amplifier  140  (see  FIG. 2 ) and manually and individually adjusting the eight variable resistance devices, V 13 , VR  2 , VR  3 , VR  4 , VR  5 , VR  6 , VR  7 , and VR  8  of PCIVI State Level Adjustment  60  until the received signal features a constant amplitude. This procedure is not difficult and normally can be completed within three to five minutes by those of average skill in the art. Differences between Signal C and Signal E occur only in respect to individual amplitudes of the pulses, the width of such pulses does not differ; consequently, the original digital PCM data, represented as bi-phase PCM waveform  60 , has not been altered. 
   In  FIG. 6 , the preemphasized mapped output signal of Multiplexer U 3 , represented as Signal E, is presented as an input signal to Buffer Amplified  10 , Integrated Circuit Amplifier U 4 , universal integrated circuit nomenclature AD 848 , or the like. Additional circuitry concomitant with U 4  includes, (1) a resistive network R 1 , R 2 , R 3 , and R  10 , connected between the output of U 3  and the input of U 4 , providing impedance matching and voltage dividing, and (2) a resistive network R 7  and R 9 , connected between the output of U 4  and input of Fixed Transmission Linel  20 , providing impedance matching. Design and implementation of both networks are well known in the art, and are not further discussed. Shown in  FIG. 2 , the output of Buffer Amplified  10 , an amplitude adjusted replication of Signal E, is Signal E′, which is the input signal to Fixed Transmission Line  20 . In like manner, the output signal from Fixed Transmission Line  20 , an attenuated, frequency degraded version of Signal E′, is Signal E″, which is the input signal to Receiver  130 . 
   As illustrated in  FIG. 2 , Signal ″ is applied as an input signal to Received  30 , comprising Wide-Band Amplifier  140  and Threshold Detector  150 . The design and implementation of circuitry for Wide-Band Amplifier  140  and Threshold Detector  150  are well known to those practicing the art. Both circuits are common and implementation is relatively inexpensive. The output of Receiver  130  is Reconstructed PCM Waveform  160 , represented as Signal C′. With proper calibration of PCM State Level Adjustment  80 , mentioned hereinbefore, the veracity of Signal C′, compared to Signal C, can be made sufficient to perform the intended purpose and fulfill requisite operational characteristics of the aforementioned PCM data transmission system. 
   EXAMPLE 2 
   The present invention described hereinbefore provides the basic or elementary level of implementation of transmitting pre-emphasized PCM codes through cable. In a second, more sophisticated (and expensive) embodiment of the present invention, calibration of PCM State Level Adjustment  80  can be automatic, vice manual, using a microcontroller and additional digital electronics to provide for remote calibration of the circuit through the cable. This more technically advanced version allows the calibration process to be performed dynamically and at a remote facility. 
   As applied to the example of the oceanographic data system, the digital reemphasizer was located within pressure vessels which were submerged to depths of up to 5,000 meters of water (about 7500 meters of cable). Since retrieval of the pressure vessels was both labor-intensive and time consuming, it was important to enable calibration of the reemphasizer while deeply submerged. A cost effective solution entailed the implementation of a microcontroller, firmware, and the addition of more electronic components to the basic circuit. It should be noted also that the basic description and application is applied for use in bi-phase level PCM code. The invention can use other codes which simply require a differently coded PROM (U 1 , FIG. 4 ) depending on the code used. The Delayed Modulation family of codes can be utilized by simply adding an additional PROM to detect the inherent additional states in the Delayed Modulation code. There are a host of other codes which this invention can be used for preemphasis. Because of the simple nature of its use, the invention could be implemented into a Very Large Scale Integrated (VLSI) circuit which could provide for a single chip solution. Note also that this invention has commercial application in digital recording and in systems using data telemetry. 
   Although only a single exemplary embodiment of the invention has been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-functions clauses are intended to cover the structures described herein as performing the recited functions and not only structural equivalents but also equivalent structures.