Advantages of Digital Advantages of Digital BroadcastingBroadcasting

• Better signal to noise ratioBetter signal to noise ratio

• Reduced interferenceReduced interference

• Possible single frequency networksPossible single frequency networks

• Less multipath distortionLess multipath distortion

• Automatic tuningAutomatic tuning

• Auxiliary data servicesAuxiliary data services

DRM - Digital Radio DRM - Digital Radio MondialeMondiale

• Developed by a group including broadcasters, Developed by a group including broadcasters, transmitter and receiver manufacturers, research transmitter and receiver manufacturers, research labs, universities, and government agencies.labs, universities, and government agencies.

• Designed to eventually replace AM in the present Designed to eventually replace AM in the present AM bands (0.15 to 30 MHz).AM bands (0.15 to 30 MHz).

• Transmitted bandwidth typically 9 kHz. Standard Transmitted bandwidth typically 9 kHz. Standard covers 4.5 to 20 kHz.covers 4.5 to 20 kHz.

• Complete specifications documented with ETSI and Complete specifications documented with ETSI and ISO. Consumer receivers just starting production.ISO. Consumer receivers just starting production.

• Different configurations for local and shortwave Different configurations for local and shortwave conditions.conditions.

Processing of SignalsProcessing of Signals• A/D conversionA/D conversion

• Audio processingAudio processing

• Source coding - MPEG Source coding - MPEG AACAAC

• Forward error correct-Forward error correct-ion (FEC) codingion (FEC) coding

• OFDM (Orthogonal OFDM (Orthogonal frequency division frequency division multiplex)multiplex)

• Channel coding - time Channel coding - time +frequency interleave+frequency interleave

• Digital modulation of Digital modulation of subcarriers subcarriers

• Inverse Fast Fourier Inverse Fast Fourier Transform (IFFT) Transform (IFFT)

• InterpolationInterpolation

• RF frequency RF frequency generation and generation and modulation in DDS modulation in DDS (direct digital (direct digital synthesizer) synthesizer)

• UpconversionUpconversion

• Power amplificationPower amplification

Audio ProcessingAudio Processing• The minimum amount of control is automatic The minimum amount of control is automatic peak limiting, that is, reducing the amplification peak limiting, that is, reducing the amplification whenever the input level is so high that overload whenever the input level is so high that overload and clipping will be produced in later steps.and clipping will be produced in later steps.

• Audio levels in program material vary with the Audio levels in program material vary with the source (live microphone, taped, satellite, CD’s etc source (live microphone, taped, satellite, CD’s etc and with the skill and attention level of the and with the skill and attention level of the operator. It is desirable to have some automatic operator. It is desirable to have some automatic control of audio levels.control of audio levels.

• When most listeners are located in noisy When most listeners are located in noisy environments such as automobiles, it is good to environments such as automobiles, it is good to reduce the dynamic range (ratio of soft to loud reduce the dynamic range (ratio of soft to loud sounds) so that soft music or speech will not be sounds) so that soft music or speech will not be lost in the ambient noise.lost in the ambient noise.

• Fortunately it’s possible to reduce these bit rates drastically Fortunately it’s possible to reduce these bit rates drastically with little or no perceptible loss of audio quality. with little or no perceptible loss of audio quality.

The coding technique for high quality audio signals The coding technique for high quality audio signals uses the properties of human sound perception by uses the properties of human sound perception by exploiting the spectral and temporal masking exploiting the spectral and temporal masking effects of the ear. effects of the ear.

Source CodingSource Coding• The output bit rates of the A/D converter are much The output bit rates of the A/D converter are much greater than can be transmitted in reasonable greater than can be transmitted in reasonable broadcast bandwidths. (1536 kbits/second from the broadcast bandwidths. (1536 kbits/second from the A/D converter for one channel.A/D converter for one channel.

• MPEG Advanced Audio Coding (AAC) perceptual coding MPEG Advanced Audio Coding (AAC) perceptual coding allows great bit rate reduction while maintaining allows great bit rate reduction while maintaining excellent audio quality. CD quality stereo requires only excellent audio quality. CD quality stereo requires only 96 to 128 k bits/s. and good monaural AM 16 to 24 kb/s. 96 to 128 k bits/s. and good monaural AM 16 to 24 kb/s. It is specified as the source coding method for DRM. It is specified as the source coding method for DRM.

Error Correction CodingError Correction Coding• Bit errors due to noise, interference from other Bit errors due to noise, interference from other trans-mitters, and multipath propagation produce trans-mitters, and multipath propagation produce audible clicks, squawks, and other noises at the audible clicks, squawks, and other noises at the receiver. Forward Error Correction (FEC) coding receiver. Forward Error Correction (FEC) coding adds calculated bytes or bits to the signal to make adds calculated bytes or bits to the signal to make it possible to detect and in many cases correct it possible to detect and in many cases correct erroneous bits at the receiver.erroneous bits at the receiver.

• Similar codes are used to make compact disks Similar codes are used to make compact disks (CD’s) and hard disk drives nearly error free in (CD’s) and hard disk drives nearly error free in spite of imperfections in the recording medium.spite of imperfections in the recording medium.

• Good error correction is a major consideration in Good error correction is a major consideration in the design of these systems. FEC is most effective the design of these systems. FEC is most effective when error bits are isolated or come in short when error bits are isolated or come in short bursts. bursts.

Error Correction Coding (2)Error Correction Coding (2)

• Multipath propagation tends to make Multipath propagation tends to make the delayed signal interfere with later the delayed signal interfere with later bits of the desired signal. It can also bits of the desired signal. It can also result in a delayed signal being 180 result in a delayed signal being 180 deg. out of phase and canceling a deg. out of phase and canceling a desired signal. With a multicarrier desired signal. With a multicarrier signal this normally affects one or a signal this normally affects one or a few adjacent carriers at a given time. few adjacent carriers at a given time. The bits of the data stream are The bits of the data stream are scattered among the carriers so that scattered among the carriers so that errors on several adjacent carriers errors on several adjacent carriers become isolated errors in the become isolated errors in the reconstituted bit stream.reconstituted bit stream.

Orthogonal Frequency Orthogonal Frequency Division MultiplexDivision Multiplex

•Uses a relatively large number of subcarriers Uses a relatively large number of subcarriers that fill the allocated bandwidth with a low data that fill the allocated bandwidth with a low data rate per carrier.rate per carrier.

•Used to minimize multipath problems.Used to minimize multipath problems.

• DRM Mode A DRM Mode A - 24 carriers/kHz = 204 in - 24 carriers/kHz = 204 in 9 kHz9 kHz

• Mode B (SW) - 21 carriers/kHz = 182 in 9 Mode B (SW) - 21 carriers/kHz = 182 in 9 kHzkHz

•Modulation of each carrier is QAM 4, 16, or 64 Modulation of each carrier is QAM 4, 16, or 64 states.states.

•Synchronization signals are distributed thru each Synchronization signals are distributed thru each frame to give the receiver a phase and amplitude frame to give the receiver a phase and amplitude reference for each carrier.reference for each carrier.

History of OFDMHistory of OFDM• OFDM has been used in radio OFDM has been used in radio

communication for 50 years. The Collins communication for 50 years. The Collins Kineplex system in the 1950’s put 24 300-Kineplex system in the 1950’s put 24 300-baud teletype channels on a single SSB baud teletype channels on a single SSB transmitter. The mathematical theory of transmitter. The mathematical theory of the system probably dates from the 1930’s.the system probably dates from the 1930’s.

• Implementation of OFDM with analog Implementation of OFDM with analog circuits was very complex. The 24 channel circuits was very complex. The 24 channel receiver took up nearly all of a 6 foot rack.receiver took up nearly all of a 6 foot rack.

Phase and Amplitude Phase and Amplitude Modulation of SubcarriersModulation of Subcarriers

• Various possibilitiesVarious possibilities

• Simplest form is Simplest form is BPSK (binary phase BPSK (binary phase shift keying) shift keying)

• Reference phase = Reference phase = 0 degrees (0 0 degrees (0 Logical)Logical)

• Other state = Other state = 180 degrees (1 180 degrees (1 Logical)Logical)

• 1 bit / Hz1 bit / Hz

Quadrature Amplitude ModulationQuadrature Amplitude Modulation

Quadrature Amplitude Modulation Quadrature Amplitude Modulation (2)(2)

Inverse Fast Fourier Inverse Fast Fourier TransformTransform

• Converts desired frequency spectrum to Converts desired frequency spectrum to in-phase and quadrature waveformsin-phase and quadrature waveforms

• Efficiently executed in DSP (digital signal Efficiently executed in DSP (digital signal processor)processor)

Interpolation (smoothing)Interpolation (smoothing)• Output of IFFT is a stepped waveformOutput of IFFT is a stepped waveform

• Must be smoothed to avoid producing Must be smoothed to avoid producing spurious sidebandsspurious sidebands

Power AmplificationPower Amplification•Typical peak-to-average power ratio for multi-Typical peak-to-average power ratio for multi-carrier systems is 10:1 (10 dB) with infrequent carrier systems is 10:1 (10 dB) with infrequent higher peaks.higher peaks.

•DRM requires high power for international DRM requires high power for international broadcasting (25 to 100 kW average power)broadcasting (25 to 100 kW average power)

•A linear amplifier will have about 20 % efficiency A linear amplifier will have about 20 % efficiency for this type of signal. For example, 125 kW dc in for this type of signal. For example, 125 kW dc in for 25 kW average power out.for 25 kW average power out.

• A multicarrier signal can be amplified with 75 to A multicarrier signal can be amplified with 75 to 80% efficiency by splitting it into two signals.80% efficiency by splitting it into two signals. 1. A constant amplitude RF signal with the phase 1. A constant amplitude RF signal with the phase variations of the desired output signal (including variations of the desired output signal (including phase reversals). 2. An amplitude (envelope) phase reversals). 2. An amplitude (envelope) signal defining the instantaneous amplitude of the signal defining the instantaneous amplitude of the desired output.desired output.

Power Amplification (2)Power Amplification (2)• The phase modulated signal can be amplified by The phase modulated signal can be amplified by

the class C intermediate and driver amplifiers of the class C intermediate and driver amplifiers of a typical local AM or short wave transmitter.a typical local AM or short wave transmitter.

• A class C anode modulated amplifier effectively A class C anode modulated amplifier effectively multiplies the phase modulated signal by its multiplies the phase modulated signal by its anode voltage (the envelope signal), producing anode voltage (the envelope signal), producing the desired output.the desired output.

• The time delay (phase shift) of the phase and The time delay (phase shift) of the phase and amplitude signals must be matched to a fraction amplitude signals must be matched to a fraction of a microsecond . The OFDM signal contains of a microsecond . The OFDM signal contains phase reversals (points at which the amplitude phase reversals (points at which the amplitude goes rapidly to 0 and then increases with goes rapidly to 0 and then increases with reversed phase).reversed phase).

• Mathematical Mathematical analysis shows that analysis shows that though the OFDM though the OFDM I/Q signal is limited I/Q signal is limited to its designated to its designated bandwidth with bandwidth with only a small only a small amount of out-of-amount of out-of-band energy, the band energy, the phase and phase and amplitude signals amplitude signals both are wideband. both are wideband. If they are If they are properly matched properly matched their out-of-band their out-of-band sidebands cancel.sidebands cancel.

•Spectra of signals Spectra of signals for amplitude-for amplitude-phase amplificationphase amplification

Transmitter RequirementsTransmitter Requirements• The fundamental requirement is that the The fundamental requirement is that the

transmitter function as a linear amplifier and this transmitter function as a linear amplifier and this section shows how linear amplification can be section shows how linear amplification can be achieved with existing AM broadcast transmitter achieved with existing AM broadcast transmitter designs.designs.

• As has been described in the previous section, the As has been described in the previous section, the DRM signal from the OFDM modulator takes the DRM signal from the OFDM modulator takes the form of a group of equally spaced carriers, with the form of a group of equally spaced carriers, with the digital information being modulated onto the digital information being modulated onto the carriers in terms of phase and amplitude.carriers in terms of phase and amplitude.

• This signal is termed an I/Q signal, as it is complex, This signal is termed an I/Q signal, as it is complex, and contains In-phase and Quadrature and contains In-phase and Quadrature components.components.

• In raising the level of this signal to the power In raising the level of this signal to the power required for broadcast transmission it is imperative required for broadcast transmission it is imperative that the correct phase and amplitude relationship that the correct phase and amplitude relationship of the "I" and "Q" components is maintained. In of the "I" and "Q" components is maintained. In other words, the DRM signal must not be distorted other words, the DRM signal must not be distorted in the power amplification process. in the power amplification process.

• If the signal is distorted, errors will be introduced If the signal is distorted, errors will be introduced and the Bit Error Rate (BER) may fall to and the Bit Error Rate (BER) may fall to unacceptable levels and the DRM signal be unacceptable levels and the DRM signal be unusable and/or out-of-band radiation become unusable and/or out-of-band radiation become excessive. To avoid distortion, the power amplifier excessive. To avoid distortion, the power amplifier must therefore have a linear transfer function such must therefore have a linear transfer function such that the output signal is an exact replica of the that the output signal is an exact replica of the input but at a higher power level.input but at a higher power level.

• A factor that has to be borne in mind for a A factor that has to be borne in mind for a DRM amplifier, is the peak to mean ratio of DRM amplifier, is the peak to mean ratio of the DRM signal. The power level of a DRM the DRM signal. The power level of a DRM signal is generally stated in terms of its mean signal is generally stated in terms of its mean or average value; however the instantaneous or average value; however the instantaneous peak amplitude of the combined carriers peak amplitude of the combined carriers exceeds this value by a large amount. A exceeds this value by a large amount. A typical DRM signal has a peak to mean ratio typical DRM signal has a peak to mean ratio of 10dB, thus an amplifier producing a signal of 10dB, thus an amplifier producing a signal having an average power of 10kW needs a having an average power of 10kW needs a peak power capability of 100kW.peak power capability of 100kW.

• In a practical amplifier, a point is reached In a practical amplifier, a point is reached where gain compression occurs and the where gain compression occurs and the transfer characteristic flattens out. The transfer characteristic flattens out. The amplifier must be operated on the linear amplifier must be operated on the linear portion of its characteristic.portion of its characteristic.

• It is possible to construct a linear amplifier to It is possible to construct a linear amplifier to provide the power level required for broadcast provide the power level required for broadcast transmission, but its energy conversion efficiency transmission, but its energy conversion efficiency is very poor, typically 20 – 30%; significant cooling is very poor, typically 20 – 30%; significant cooling is required and operating costs will be high.is required and operating costs will be high.

• Although some earlier low power transmitters used Although some earlier low power transmitters used linear amplifiers, high power AM transmitters linear amplifiers, high power AM transmitters invariably use non-linear class C operation to invariably use non-linear class C operation to achieve high conversion efficiency. In a valve achieve high conversion efficiency. In a valve (electronic tube) transmitter, the grid bias voltage (electronic tube) transmitter, the grid bias voltage is chosen such that the valve conducts over a is chosen such that the valve conducts over a limited range of the RF cycle and effectively limited range of the RF cycle and effectively delivers energy to the resonant anode circuit as a delivers energy to the resonant anode circuit as a series of pulses. The resulting RF power is coupled series of pulses. The resulting RF power is coupled from the anode circuit to the antenna. from the anode circuit to the antenna.

• With the use of modern high power valve With the use of modern high power valve technology and efficient cooling systems, very technology and efficient cooling systems, very high output power can be achieved for relatively high output power can be achieved for relatively low drive power with high conversion efficiency. low drive power with high conversion efficiency.

• Solid-state modular MF/LF transmitters use a Solid-state modular MF/LF transmitters use a switching technique to achieve high conversion switching technique to achieve high conversion efficiency, typically between 70 and 80%. The efficiency, typically between 70 and 80%. The output stage of each power amplifier module uses output stage of each power amplifier module uses MOSFET transistors as switches arranged in an "H MOSFET transistors as switches arranged in an "H Bridge" arrangement. RF power is taken from a Bridge" arrangement. RF power is taken from a transformer connected between the mid-points of transformer connected between the mid-points of each arm. In operation, diagonally opposite each arm. In operation, diagonally opposite transistors are sequentially switched at carrier transistors are sequentially switched at carrier frequency rate to produce alternate current frequency rate to produce alternate current reversals in the output transformer primary. In reversals in the output transformer primary. In this way high RF power levels are generated at this way high RF power levels are generated at high conversion efficiency.high conversion efficiency.

• These non-linear amplifiers cannot be used These non-linear amplifiers cannot be used directly for DRM signals. However a modulated directly for DRM signals. However a modulated non-linear amplifier can be driven with suitable non-linear amplifier can be driven with suitable RF and base-band signals derived from the RF and base-band signals derived from the original low level complex I/Q signal, such that original low level complex I/Q signal, such that the component signals combine in the modulated the component signals combine in the modulated final amplifier to form a high level replica of the final amplifier to form a high level replica of the original signal. original signal.

• The overall effect is that the modulated amplifier The overall effect is that the modulated amplifier functions as a linear amplifier even though the functions as a linear amplifier even though the amplifier itself continues to work in a non-linear amplifier itself continues to work in a non-linear manner. manner.

• Although a modular solid-state MF/LF transmitter Although a modular solid-state MF/LF transmitter does not have a separate modulated amplifier as does not have a separate modulated amplifier as such, the functionality is identical.such, the functionality is identical.

• The two signals derived from the basic DRM The two signals derived from the basic DRM I/Q signal are termed the RF Phase (RFP) I/Q signal are termed the RF Phase (RFP) and Amplitude (A). The processing to derive and Amplitude (A). The processing to derive these signals is contained within the DRM these signals is contained within the DRM Modulator and generally the modulator will Modulator and generally the modulator will provide both I/Q and A/RFP outputs.provide both I/Q and A/RFP outputs.

• For this technique to work correctly there For this technique to work correctly there are a number of requirements that must be are a number of requirements that must be satisfied by the transmitter. Firstly, there satisfied by the transmitter. Firstly, there must be a direct (DC) connection between must be a direct (DC) connection between the modulator and the final amplifier. the modulator and the final amplifier. Unfortunately this means that the A/RFP Unfortunately this means that the A/RFP technique cannot be used with transmitters technique cannot be used with transmitters having Class B transformer coupled having Class B transformer coupled modulators.modulators.

• For this reason the widely used Continental For this reason the widely used Continental 418 A-F short wave transmitters cannot be 418 A-F short wave transmitters cannot be used for DRM without modification, but used for DRM without modification, but Continental can supply a G upgrade using a Continental can supply a G upgrade using a pulse step modulator which makes the pulse step modulator which makes the transmitter DRM capable.transmitter DRM capable.

• Operation in a DRM simulcast mode does Operation in a DRM simulcast mode does not require DC coupling, but it still requires not require DC coupling, but it still requires a greater audio bandwidth than is possible a greater audio bandwidth than is possible with typical modulation transformers.with typical modulation transformers.

• The Harris HF100 PDM modulator can be DC The Harris HF100 PDM modulator can be DC coupled but it is necessary to raise the PDM coupled but it is necessary to raise the PDM switching frequency to get the needed audio switching frequency to get the needed audio bandwidthbandwidth..

• Secondly, the bandwidth of the audio path in the Secondly, the bandwidth of the audio path in the transmitter needs to be significantly greater than transmitter needs to be significantly greater than that required for normal AM working. Typically, it that required for normal AM working. Typically, it should be three to four times the bandwidth of the should be three to four times the bandwidth of the wanted DRM signal. wanted DRM signal.

• The sampling frequency of solid-state Pulse Step or The sampling frequency of solid-state Pulse Step or Pulse Duration Modulators (PDM/PSM) must be more Pulse Duration Modulators (PDM/PSM) must be more than twice this frequency limit to meet Nyquist than twice this frequency limit to meet Nyquist criteria. criteria.

• Any bandwidth limiting filters in the audio path must Any bandwidth limiting filters in the audio path must be removed and the modulator output filter will need be removed and the modulator output filter will need to be modified to achieve the required bandwidth. In to be modified to achieve the required bandwidth. In modifying the filter response it is important to modifying the filter response it is important to ensure that a substantially flat group delay ensure that a substantially flat group delay characteristic is maintained over the pass-band. characteristic is maintained over the pass-band.

• In modifying filters there are two points to In modifying filters there are two points to bear in mind. Firstly, in some solid-state bear in mind. Firstly, in some solid-state modulator designs the modulator output modulator designs the modulator output is slightly inductive and this will need to is slightly inductive and this will need to be taken into account when designing the be taken into account when designing the filter. Secondly, the final amplifier valve filter. Secondly, the final amplifier valve provides the load for the filter. It is provides the load for the filter. It is important to remember that although this important to remember that although this is predominantly resistive, the transmitter is predominantly resistive, the transmitter RF output circuit will provide some shunt RF output circuit will provide some shunt capacitance. Depending on output circuit capacitance. Depending on output circuit configuration, the shunt capacitance configuration, the shunt capacitance existing at some tuning settings may be existing at some tuning settings may be significant and need to be taken into significant and need to be taken into account in the filter design.account in the filter design.

• Current HF transmitter designs use a tetrode Current HF transmitter designs use a tetrode valve as the modulated amplifier. In order to valve as the modulated amplifier. In order to achieve linear modulation to 100% with a achieve linear modulation to 100% with a tetrode, it is necessary to apply modulation to tetrode, it is necessary to apply modulation to the screen grid electrode. This is usually done the screen grid electrode. This is usually done by applying a fraction of the modulating signal by applying a fraction of the modulating signal to the screen grid, or by allowing the screen to the screen grid, or by allowing the screen grid to "self modulate", by including an grid to "self modulate", by including an inductance in the screen grid circuit. The audio inductance in the screen grid circuit. The audio component in the screen current develops an component in the screen current develops an AC voltage across the inductor and this AC AC voltage across the inductor and this AC voltage serves to modulate the screen.voltage serves to modulate the screen.

• It is also necessary to decouple the screen grid, It is also necessary to decouple the screen grid, so that it is at RF ground potential, for proper so that it is at RF ground potential, for proper operation of the amplifier.operation of the amplifier.

• In some transmitter designs the value of In some transmitter designs the value of decoupling capacitance used may be decoupling capacitance used may be sufficiently high to become significant at the sufficiently high to become significant at the higher frequencies now required to be handled higher frequencies now required to be handled in the amplitude path. This can degrade the in the amplitude path. This can degrade the response at the upper end of the pass band. It response at the upper end of the pass band. It may therefore be necessary to reduce the may therefore be necessary to reduce the value of the screen decoupling capacitor, but value of the screen decoupling capacitor, but this must be done with great care in order not this must be done with great care in order not to disturb the RF performance of the amplifier.to disturb the RF performance of the amplifier.

• The decoupling capacitor is often an integral The decoupling capacitor is often an integral part of the final amplifier valve socket made in part of the final amplifier valve socket made in the form of an annular ring of dielectric film, the form of an annular ring of dielectric film, with electrodes deposited on each side of the with electrodes deposited on each side of the ring. One possible technique is to add a second ring. One possible technique is to add a second ring – this effectively halves the decoupling ring – this effectively halves the decoupling capacitance.capacitance.

• Even when all of the above points have been Even when all of the above points have been taken into account, it may not always be taken into account, it may not always be possible to directly achieve the required possible to directly achieve the required bandwidth and group delay response from the bandwidth and group delay response from the transmitter. In this situation some degree of transmitter. In this situation some degree of pre-correction may be needed. DRM pre-correction may be needed. DRM Modulators now include some form of pre-Modulators now include some form of pre-correction. In some implementations, the pre-correction. In some implementations, the pre-correction is user-adjustable and is set on test correction is user-adjustable and is set on test to give the best response, in others, the pre-to give the best response, in others, the pre-correction is set at the factory and detailed correction is set at the factory and detailed response measurements of the transmitter response measurements of the transmitter are taken and provided to the DRM Modulator are taken and provided to the DRM Modulator supplier, who sets the pre-correction. supplier, who sets the pre-correction.

• For frequency agile HF transmitters, it For frequency agile HF transmitters, it will generally be necessary to use some will generally be necessary to use some form of dynamic phase equalisation in form of dynamic phase equalisation in order to retain the correct phase order to retain the correct phase relationship between the A and RFP relationship between the A and RFP signals as the transmitter switches signals as the transmitter switches between the various broadcast bands. between the various broadcast bands. Dynamic phase equalisation is Dynamic phase equalisation is generally provided as an option with a generally provided as an option with a DRM Modulator.DRM Modulator.

Transmitter RequirementsTransmitter Requirements• The HC100 and other modern transmitters using pulse-The HC100 and other modern transmitters using pulse-

step modulation can be modified to transmit OFDM step modulation can be modified to transmit OFDM signal in conformance with the DRM specification. They signal in conformance with the DRM specification. They typically have 10 to 150 microseconds of delay in their typically have 10 to 150 microseconds of delay in their amplitude circuits but only a few microseconds in the RF amplitude circuits but only a few microseconds in the RF amplifiers.amplifiers.

• (1) An audio server includes A/D conversion, audio (1) An audio server includes A/D conversion, audio processing and MPEG source encoding to deliver a signal processing and MPEG source encoding to deliver a signal at the required bitrate (An industrial PC).at the required bitrate (An industrial PC).

• (2) An OFDM exciter implements Forward Error (2) An OFDM exciter implements Forward Error Correction, Channel Coding, the IFFT and interpolation, Correction, Channel Coding, the IFFT and interpolation, in-phase (I) and quadrature (Q) signal generation, in-phase (I) and quadrature (Q) signal generation, derives Amplitude and Phase signals from I and Q, derives Amplitude and Phase signals from I and Q, compensates for time delay of the amplitude signal, and compensates for time delay of the amplitude signal, and sends phase and operating frequency data to the Direct sends phase and operating frequency data to the Direct Digital Synthesizer (one PCB or DSP and many lines of Digital Synthesizer (one PCB or DSP and many lines of embedded software).embedded software).

HC100 ChangesHC100 Changes• (1) An Audio Controller with a higher switching rate to produce the (1) An Audio Controller with a higher switching rate to produce the

fast amplitude transitions of the OFDM signal and control the screen fast amplitude transitions of the OFDM signal and control the screen supply. (One PCB)supply. (One PCB)

• (2) A high voltage low pass filter with wider bandwidth and less (2) A high voltage low pass filter with wider bandwidth and less phase shiftphase shift

• (3) A modulated Power Amplifier Screen supply. Basically a (3) A modulated Power Amplifier Screen supply. Basically a simplified low power version of the pulse- step modulated anode simplified low power version of the pulse- step modulated anode (plate) supply.(plate) supply.

• (4) Redesign of the fiber optic links and switching modules in the (4) Redesign of the fiber optic links and switching modules in the modulator for more uniform turn-on and turn-off times. modulator for more uniform turn-on and turn-off times.

• This redesign is in process at the Engineering Center in Elkhart This redesign is in process at the Engineering Center in Elkhart