DIGITAL RADIO FREQUENCY (RF) MODULATOR
Field of the Invention
This invention relates to a field of digital signal processing, and more specifically to a digital radio frequency (RF) modulator, which is used for modulation of an RF sinusoidal carrier with base band TV signal.
Background of the Invention
Modulation can be defined as an alteration of some characteristic of a known signal or
waveform, i.e., a carrier, as a Function of some unknown signal or waveform that conveys
information. In radio-frequency (RF) communication systems, the earner is typically a
sinusoid, and there are several methods of modulating the carrier. These include linear
modulation, angle modulation, and various types of pulse modulation. As defined in the
ITU-R BT.470 standard, the standard definition (SD) video signal AM-VSB (Amplitude
Modulation - Vestigial Side Band) modulates a vision carrier and the audio signal FM
(frequency modulation) modulates an audio carrier. .
In conventional systems the pulse code modulation (PCM) audio signals frequency modulates a earner whose frequency is the difference of frequency between vision carrier and audio carrier. The frequency modulated carrier is added to a base band video signal. The resulting signal is then shifted to an intermediate frequency, called an IF frequency. At the IF frequency the signal is vestigial side band (VSB) filtered and then translated to a desired channel frequency for broadcasting.
In earlier systems, the entire transmission pipeline is implemented in an analog domain using analog signal processing concepts. With the advancement of digital VLSI design and evolution of System-On-Chip concepts, it was desired to reduce the system cost by converging the functionality of discrete on-board components into cost effective integrated circuits (ICs). As a first step in this direction, today the pipeline till the IF stage has been widely replaced by digital components, followed by D/A converter and an
analog RF converter circuit to the desired TV channel frequency. To implement the RF converter block using digital logic, very high sampling rate is required. Digital logic operated at a higher sampling rates have the problems of higher power consumption, difficulty in meeting timing and higher area. Thus, it is difficult to replace the RF converter circuit by digital components.
Thus, there is a need for a novel digital RF modulator, which operates in a digital domain for modulating the base band TV signals to a desired channel frequency without requiring an analog up conversion. The invented RF modulator provides generic channel frequency and generates sinusoid carrier signal at a lower sampling rate for enhancing the efficiency. In the invented RF modulator, the band pass filter for selecting the required channel are implemented in polyphase structure which is quite area efficient.
Summary of the invention
It is an object of the present invention to provide a digital RF modulator, which utilizes an efficient digital architecture for modulating a desired channel carrier with the baseband TV signal without any analogue up conversion for frequencies up to higher VHF band.
It is another object of the present invention to provide a digital RF modulator, which provides a generic channel frequency and generates the sinusoidal carrier at a lower sampling rate for enhancing the efficiency.
It is yet another object of the present invention to provide a digital RF modulator, which utilizes low sampled sine waves and a poly-phase implementation of band pass filters (BPFs), which are area efficient.
To achieve said objectives, the present invention provides a digital radio frequency (RF) modulator for modulating a desired RF channel carrier with a baseband TV signal, the digital RF modulator comprising:
an audio modulator for receiving input audio signals to generate frequency
modulated (FM) audio signals;
a video modulator for receiving a composite video base-band signal (CVBS) to
generate a filtered output video signal; and
a RF converter connected to said audio modulator and said video modulator for
constructing the base band TV signal and for shifting said base band TV signal in
a frequency domain to the desired RF channel carrier.
The audio modulator as described above comprising:
a pre-emphasis filter for amplifying high frequency input audio signals;
a multi-stage audio interpolator connected to said pre-emphasis filter for
increasing audio sampling rate of pre-emphasized audio signals; and
a complex frequency modulator connected to said multi stage audio interpolator in which an exponential sinusoid carrier is frequency modulated with the interpolated audio samples.
The video modulator as described above comprising:
an optional interpolator for up-sampling the incoming CVBS signal by a factor of
2 if the incoming CVBS signal sample rate is 13.5 MHz. This block is not
required if the incoming CVBS signal sample rate is already at 27 MHz;
a group delay compensation filter connected to said digital circuit for pre-
correcting non linear phase characteristics of a receiver;
an IRE level adjuster connected to said group delay compensation filter for
scaling the incoming CVBS signals and for adding a DC value that represents the
channel carrier addition at the baseband and the addition required for appropriate
IRE level adjustment; and
a complex VSB filter connected to said IRE level adjuster for generating the
vestige of one of the sidebands and limiting the bandwidth of the other sideband
in order to avoid video interference into the audio spectrum.
The RF converter as described above comprising:
a complex adder for adding the frequency modulated (FM) complex audio carrier
and the VSB filtered video signal to form the base band TV signal;
a complex frequency shifter connected to said complex adder for multiplying said
base band TV signal with an exponential carrier to shift said base band TV signal
by the frequency of the exponential carrier; and
a RF interpolator connected to said complex frequency shifter for generating
modular output at a higher sample rate.
Further, the present invention provides a RF module for converting digital base band audio signals and digital video TV signals to a desired RF channel carrier, said RF module comprising:
a RF modulator for modulating base-band TV signals at the desired RF channel
carrier;
a PCM/SPDIF interface connected to the RF modulator through an audio interface
for providing audio samples to said RF modulator;
a digital encoder connected to the RF modulator through a video interface for
converting input video signals into composite video base-band signals (CVBS);
a digital circuit connected to the RF modulator through a processing interface for
configuring the said RF modulator;
a digital to analog converter block (DAC) connected to said RF modulator
through an output interface for converting the modulated RF carrier from digital
to analog domain; and
an analog low pass filter block connected to said DAC block for retaining only
the fundamental spectrum and rejecting the images of fundamental spectrum.
Further, the present invention provides a method of modulating base band TV signals to a desired RF channel carrier, said method comprising the steps of:
processing input audio signals to generate frequency modulated (FM) audio
signals through an audio modulator;
processing input composite video base-band (CVBS) signals to generate a filtered
output video signal through a video modulator;
adding the frequency modulated (FM) audio signals and the filtered output video
signals to form the base band TV signals through a RF converter; and
shifting the base band TV signals in a frequency domain to the desired RF
channel carrier through the RF converter.
Brief Description of the drawings
The present invention is described with the help of accompanying drawings.
FIGURE 1 illustrates a block diagram of a digital RF modulator in accordance with the present invention.
FIGURE 2 illustrates a block diagram of the system in which the RF modulator can fit into, in accordance with the present invention.
FIGURE 3 shows a block diagram of an audio modulator in accordance with the present invention.
FIGURE 4 shows a block diagram of a video modulator in accordance with the present invention.
FIGURE 5 shows a block diagram of a RF converter in accordance with the present invention.
FIGURE 6 illustrates the output spectrum of FM modulated audio signals. FIGURE 7 illustrates the output spectrum of an output of a VSB filter. FIGURE 8 illustrates the spectrum of the base band TV signals.
FIGURE 9 illustrates the unidirectional shift of the spectrum by +10 MHz frequency in an embodiment of the present invention.
FIGURE 10 shows a resultant spectrum after passing through upsampler and a quadrature band pass filter in an embodiment of the present invention.
FIGURE 11 shows a flow diagram of a method for modulating the base band TV signals to a desired RF channel carrier in accordance with the present invention.
Detailed Description of the Invention
The present invention provides a digital radio frequency (RF) modulator for modulating a sinusoid carrier of desired frequency with the baseband TV signal. The novel idea in this invention is that the frequency shifting on base band TV signal is done at 27 MHz sample rate. This enables to generate the effective channel carrier frequency at much lower frequency (27 MHz).
The modulated carrier at base band is up-converted from 27 MHz to the desired sample rate and the desired image around the RF carrier is captured using a quadrature band pass filter (BPF).
For applications, which require the channel to be in a higher VHF band, the interpolation factor for the base band TV signal becomes high and the process of interpolation followed by band pass filter (BPF) could be split in multistage. The BPF and the up-sampling have been combined to implement the BPF in a polyphase structure, which is quite area efficient.
FIGURE 1 illustrates a block diagram of a digital RF modulator (100) in accordance with the present invention. The RF modulator (100) provides modulation of a base band TV signal at a desired RF channel carrier. The RF modulator (100) includes an audio modulator (102), a video modulator (104), a RF converter (106). The RF modulator (100)
further includes an audio interface (108), a video interface (110), a processing interface (112), and an output interface (114) for providing interfacing among its components. The audio modulator (102) receives input audio signals through the audio interface (108) to generate frequency modulate (FM) audio signals. The video modulator (104) receives composite video base band signals (CVBS) through the video interface (110) to generate filtered output video signals. The RF converter (106) is connected to the audio modulator (102) and the video modulator (104) to construct the base band TV signal. The RF converter (106) further shifts the base band TV signal in a frequency domain to the desired RF channel carrier. The output interface (114) provides the modulated carrier samples to DAC followed by analog LPF. The processing interface (112) configures the RF modulator (100) with an external general purpose processor and an external memory module.
FIGURE 2 illustrates a block diagram of the system (200) in accordance with the present invention. The system (200) converts the digital base band audio signals and digital video component signals to a desired RF channel carrier after proper encoding of the component video signals. The RF module (200) includes the RF modulator (100), a PCM/SPDIF interface (202), a digital encoder (204). a digital circuit (206), a digital to analog converter (DAC) block (208), and an analog filter block (210). The RF modulator (100) internally generates the base band TV signal and modulates the desired channel carrier frequency. The PCM/SPDIF interface (202) is connected to the RF modulator (100) through the audio interface (108) to provide processed PCM audio signals to the audio modulator (102). The digital encoder (204) is connected to the RF modulator (100) through the video interface (110) to convert input video signals into composite video base band signals (CVBS) signal. The CVBS signal is defined by various standards like National Television Standards Committee (NTSC) and Phase Alternation Line (PAL) defined in the ITU-R 470.6 standard. If the video source generates RGB signals, then the digital encoder (204) converts the samples from RGB domain to Y Cr Cb domain before generating the CVBS signals. The CVBS signals consist of the luma samples (Y samples), quadrature modulated chroma samples (Cr and Cb samples), blanking and sync signal. The CVBS signal is then applied to the RF modulator (100) through the video
interface (110). The digital circuit (206) includes a memory & lO's block (212), and a general purpose processor (214). The digital circuit (206) is connected to the RF modulator (100) through the processing interface (112) to configure the digital encoder (204) and the PCM/SPDIF receiver (202) and the RF modulator (100). The DAC block (208) is connected to the RF modulator (100) through the output interface (114) for converting the modulated RF carrier from digital to analog domain. The analog filter block (210) is connected to the DAC block (208) for filtering the output of the DAC to reject the images that occur at every samples frequency and retaining only the fundamental spectrum around 0 Hz. The final RF output signals are then transmitted or broadcasted to desired destinations.
FIGURE 3 shows a block diagram of the audio modulator (102) in accordance with the present invention. The audio modulator (102) includes a pre-emphasis filter (302), a multi stage audio interpolator (304), and a complex frequency modulator (306). The cutoff frequency of a pre-emphasis filter is specified in ITU-R BT 470. The ideal magnitude response of the filter is such that it remains constant till the cutoff frequency and increases with a rate of 20db/decade from there on. The pre-emphasis filter has a high-pass filter like response. The pre-emphasis filter (302) results in increased signal-to-noise ratio (SNR).
The multi stage audio interpolator (304) is connected to the pre-emphasis filter (302) to increase the sampling rate of the pre-emphasized audio signals. Typical audio sampling rates are 32 KHz/48 KHz /64 KHz/96 KHz /128 KHz/144 KHz /192 KHz. The output sampling rate of the multistage audio interpolator is twice the video pixel rate (2 x 13.5 MHz) which is 27 MHz.
The complex frequency modulator (306) is connected to the multi-stage audio interpolator (304) to modulate the exponential carrier with the interpolated audio signal. The frequency of the exponential carrier is programmable. The value to be programmed depends on the TV system targeted. Typical values of the audio carrier frequency are 4.5, 5.5, 6.5 MHz as defined by ITU-R 470.6 standard for different TV systems.
Examples of TV standards include a National Television Standards Committee (NTSC) standard, a phase alternation line (PAL) standard, and a SECAM. The output frequency spectrum of the complex frequency modulator (306) is shown in FIGURE 6.
FIGURE 4 shows a block diagram of the video modulator (104) in accordance with the present invention. The video modulator (104) includes a digital circuit (402), a group delay compensation filter (404), an IRE level adjuster (406), and a complex VSB filter (408). Generally, the digital encoders take incoming video data at 13.5 MHz pixel rate, and up-sample it to 27 MHz frequency to construct the base band signal. The digital circuit (402) is used for up-sampling the incoming CVBS signal by a factor of 2, if the sampling rate of the incoming signal is 13.5 MHz. This block is not required if the sampling rate of the incoming CVBS signal is already 27 MHz. The upsampling can be achieved by using a simple interpolation filter.
The group delay compensation filter (404) is connected with the digital circuit (402) for pre-correcting the non-linear phase characteristics of a receiver. The group delay compensation filter (404) is an all-pass filter with the group delay characteristics as defined in the ITU-R BT.470.
The IRE level adjuster (406) is connected with the group delay compensation filter (404) for scaling and adding the output of the group delay compensation filter with pre-calculated values which are programmable. This operation is required to adjust the amplitude levels of the CVBS signal. This adjustment is required so that the signal meets the percentage modulation requirements (for example the percentage modulation of the sync level is 100%) of the ITU-R BT 470. The addition factor also contains the value that is required to be added to represent carrier addition at baseband.
The complex VSB filter (408) is connected to the IRE level adjuster (406) for generating a vestige of one of the sidebands and limiting the bandwidth of the other sideband in order to avoid video interference into the audio spectrum. The complex VSB filter (408) is a symmetric coefficients FIR filter so that the phase response is linear. The complex
VSB filter (408) operates at a base band sampling frequency of 27 MHz. The bandwidth of the complex VSB filter (408) depends on a targeted TV system. Since the filter is complex in nature (complex valued coefficients), the magnitude response is asymmetric across 0 Hz. The output spectrum at output of the complex VSB filter (408) is shown in FIGURE 7.
FIGURE 5 shows a block diagram of the RF converter (106) in accordance with the present invention. The RF converter (106) includes a complex adder (502), a complex frequency shifter (504), and a RF interpolator (506). The complex adder (502) includes two simple adders, which adds the frequency modulated audio signals (from the audio modulator 102) with the filtered output video signal (from the video modulator 104). The complex adder (502) works at 27 MHz to form the base band TV signal. The spectrum of the base band TV signal is shown in the FIGURE 8.
The complex frequency shifter (504) is connected to the complex adder (502) for multiplying the base band TV signal with an exponential carrier value to shift the base band TV signal uni-directionally by the frequency of the exponential carrier. The exponential carrier value lies within a range of +/- 13.5 MHz. The complex frequency shifter (504) works at 27 MHz sampling rate.
The frequency of the exponential carrier determines the required channel frequency.
Example 1: Assuming that a required channel carrier frequency is 62 MHz.
In order to bring the image at 54 MHz to 62 MHz, there is a need to shift the image by 8
MHz towards right, so in this case exponential carrier frequency of 8 MHz should be
chosen.
Example 2: Now, assuming that a required channel carrier frequency is 70 MHz. So, an exponential carrier frequency of 16 MHz (70-54) will be chosen to shift the image at 54 MHz to 70 MHz, but this carrier can not be generated at a sampling rate of 27 MHz.
However, we can bring the image at 81 MHz to 70 MHz by using an exponential carrier of-11 MHz frequency, which can be generated with a 27 MHz sampling rate.
To generalize, the frequency of an exponential carrier is either (Fch - n * 27MHz) or (Fch - (n+1) * 27MHz) whichever value lies in the range of+/- 13.5 MHz, where Fch is the desired channel carrier frequency and n is an integer.
After a complex multiplication, the video carrier also gets inserted automatically due to an initial DC addition in a video processor block. The spectrum after the uni-directional frequency shift of 10 MHz (required channel carrier frequency = 64 MHz) is shown in FIGURE 9.
The RF interpolator (506) is connected to the complex frequency shifter (504) for generating a modulator output at a higher sampling rate. The output samples after complex multiplication are passed through a zero-padder block (508). The zero padder block (508) inserts sufficient number of Os between 2 input samples. Zero-padding by N (N is an integer) implies we need to insert N-l zeros between two adjacent samples.
Zero padding not only increases the sampling rate by N times (27*N MHz), but also causes the N replicas of the initial spectrum to appear across the new sampling rate.
These samples are then provided as input to a quadrature band pass filter to select the spectrum replica of interest (containing the desired channel frequency). After the quadrature band pass filter, the resultant spectrum for a channel frequency of 64 MHz is shown in the FIGURE 10.
The zero-padding block (508) and the BPF (510) can be merged together as a poly-phase structure in which each poly-phase structure effectively operates at 27 MHz only. The output data rate of the BPF is the desired sample rate.
The following description explains the process of generating an output at various bands (VHF/UHF).
VHF band up to 85 MHz
The lower VHF band implies a channel carrier whose frequency is less than 80-85 MHz. The lower VHF also covers VCR outputs and up to TV channel 6. We have to increase the sample rate from 27 MHz to a higher value for generating an RF modulated output. The increased sample rate is a multiple of 27 MHz for simplicity in interpolation (interpolation factor becomes an integer). For generating a VHF band output whose channel carrier frequencies are less than 85 MHz, the possible choices of the output sample rates are 189/216/ 243/270 MHz (multiples of 27 MHz) and so on. If we choose 243 as an output sample rate, then we need to interpolate it by 9 times (N=9).
VHF band up to 250 MHz
The upper VHF band implies channel frequencies till 250 MHz. For generating a VHF band output with channel carrier frequency less than 250 MHz,1he desired sample rate should be greater than 600 MHz (after allowing some guard band in the specrum), which results in a large interpolation factor. It would be expensive (requires high order BPF) to perform the RF interpolation (zero-pad + quadrature BPF) in one step. In this case, we can have two stages of interpolation , in which the first stage interpolates the samples to 243 MHz using a 9 times zero-padder followed by quadrature BPF and the second stage interpolates the samples by 3 times (M=3) (243*3 = 729 MHz) using a 3 times zero-padder followed by real band pass filter (BPF). Both the band pass filters can be optimized to implement in a polyphase structure because of the zero-padder logic preceding them.
For supporting UHF band (up to 1.1 GHz), the final analogue VHF-II channel output may be treated as an IF (intermediate frequency) input to an analogue converter, which moves it to desired UHF channel frequency.
FIGURE 11 shows a flow diagram of a method for modulating the base band TV signals to a desired RF channel carrier in accordance with the present invention. At step 1102, input audio signals are processed to generate frequency modulated (FM) audio signals through an audio modulator. At step 1104, input composite video base-band (CVBS) signals are processed to generate a filtered output video signal through a video modulator. At step 1106, the frequency modulated (FM) audio signals and the filtered output video signals are mixed to form the base band TV signals through a RF converter. At step 1108, the base band TV signals are shifted in a frequency domain, upsampled, passed through quradrature band pass filter and an optional upsampler followed by a real bandpass filter, if the desired band is in the higher VHF range, to capture the required RF channel.
The proposed digital RF modulator offers various advantages. The RF modulator provides direct conversion of digital base-band audio and video TV signals to a desired RF channel frequency, without any analogue up conversion at higher frequencies. The proposed RF modulator provides generic channel frequency and generates sinusoidal carrier at a lower sampling rate for enhancing the efficiency. The band pass filter that selects the desired channel is implemented in a polyphase structure. The proposed approach provides digital conversion of base band TV signals directly to any frequency up to higher VHF band, which is much simpler, flexible, robust as well as cost effective as compared to full analog up conversion from baseband TV signal.

We claim:
1) A digital radio frequency (RF) modulator for modulating a base band TV signal
with a desired RF channel carrier, said digital RF modulator comprising:
an audio modulator for receiving input audio signals to generate frequency
modulated (FM) audio carrier;
a video modulator for receiving a composite video base-band signal
(CVBS) to generate a filtered output video signal; and
a RF converter connected to said audio modulator and said video
modulator for constructing the base band TV signal and for shifting said
base band TV signal in a frequency domain to the desired RF channel
carrier.
2) The digital modulator as claimed in claim 1 further comprising an audio interface
for providing the input audio signals to said audio modulator.
3) The digital modulator as claimed in claim 1 further comprising a video interface
for providing the composite video base-band signal (CVB&) to said video
modulator.
4) The digital modulator as claimed in claim 1 further comprising an output interface
for providing the modulated TV signal to a DAC.
5) The digital modulator as claimed in claim 1 further comprising a processing
interface for configuring the digital RF modulator with one or more of a general
purpose processor and a memory module.
6) The digital modulator as claimed in claim 1, wherein said audio modulator
comprising:
a pre-emphasis filter for amplifying high frequency input audio signals;
a mufti stage audio interpolator connected to said pre-emphasis filter for increasing audio sampling rate of pre-emphasized audio signals; and a complex frequency modulator connected to said multi stage audio interpolator in which the interpolated audio samples frequency modulate an exponential carrier.
7) The digital modulator as claimed in claim 1, wherein said video modulator
comprising:
a digital circuit for upsampling the incoming CVBS signals to twice the
pixel rate. This module is not required if the incoming CVBS signal is
already upsampled to 27 MHz in the previous block;
a group delay compensation filter connected to said digital circuit for pre-
correcting non linear phase characteristics of a receiver;
ar) IRE level adjuster connected to said group delay compensation filter
for scaling and adding the incoming CVBS signal with pre-calculated
constant values; and
a complex VSB filter connected to said IRE level adjuster that passes one
complete sideband and a vestige of the other sideband.
8) The digital modulator as claimed in claim 7, wherein said VSB filter has a
bandwidth that is programmable (coefficients are stored in registers) and the
required bandwidth depends on the targeted television system.
9) The digital modulator as claimed in claim 8. wherein said TV system could be
National Television Standards Committee (NTSC) system, a phase alternation
line (PAL) system, and a SECAM system.
10) The digital modulator as claimed in claim 1, wherein said RF converter
comprising:
a complex adder for adding the frequency modulated (FM) audio signals and the filtered output video signal to form the base band TV signal;
a complex frequency shifter connected to said complex adder for multiplying said base band TV signal with an exponential carrier to shift the spectrum of the said base band TV signal uni-directionally by the frequency of the exponential carrier; and
a RF interpolator connected to said complex frequency shifter for generating modular output at a desired sample rate.
11) The digital modulator as claimed in claim 10, wherein said frequency of the
exponential carrier lies in a range of+/- 13.5 MHz.
12) A RF module for converting digital base band audio signals and digital video TV
signals to a desired RF channel, said RF module comprising:
a RF modulator for modulating base-band TV signals on to the desired RF . channel carrier;
a PCM/SPDIF interface connected to the RF modulator through an audio interface for providing PCM samples to said RF modulator; a digital encoder connected to the RF modulator through a video interface for converting input video pixels (luma and chroma values) into composite video base-band signals (CVBS);
a digital circuit connected to the RF modulator through a processing interface for configuring the said RF modulator;
a digital to analog converter block (DAC) connected to said RF modulator through an output interface for converting the modulated RF carrier from digital to analog domain; and
an analog low pass filter block connected to said DAC block for capturing the actual spectrum till half of the sampling frequency.
13) The RF module as claimed in claim 12, wherein said RF modulator comprising:
an audio modulator for receiving input audio signals to generate frequency modulated (FM) audio signals;
a video modulator for receiving a composite video base-band signal
(CVBS) to generate a filtered output video signal; and
a RF converter for constructing the base band TV signal and for shifting
said base band TV signal in a frequency domain to the desired RF channel
carrier.
14) The RF module as claimed in claim 12, wherein said digital circuit comprising:
a memory block for storing data;
a general purpose processor connected to said memory block for
programming the registers of the RF modulator; and
an Input Output (IO) block for interfacing with said processor.
15) A method of modulating base band TV signals to a desired RF channel carrier,
said method comprising the steps of:
processing input audio signals to generate frequency modulated (FM)
audio signals through an audio modulator;
processing input composite video base-band (CVBS) signals to generate a
filtered output video signal' through a video modulator;
adding the frequency modulated (FM) audio signals and the filtered output
video signals to form the base band TV signals through the RF converter;
and
shifting the base band TV signals in a frequency domain to the desired RF
channel carrier through the RF converter.
16) The method as claimed in claim 15, wherein said processing the input audio
signals comprising:
amplifying high frequency input audio signals through a pre-emphasis
filter;
increasing audio sampling rate of pre-emphasized audio signals through a
multi stage audio interpolator; and
modulating the frequency of the exponential carrier with the interpolated audio samples through a complex frequency modulator.
17) The method as claimed in claim 15, wherein said processing the input composite
video base-band (CVBS) signals comprising:
sampling the input CVBS signals to twice the pixel rate through a simple
interpolator if the input CVBS samples are at the pixel rate;
pre-correcting non linear phase characteristics of a receiver through a
group delay compensation filter;
adjusting the amplitude of the CVBS signal through an IRE level adjuster;
and
generating a VSB spectrum through a complex VSB filter.
18) The method as claimed in claim 15, wherein said mixing comprising:
adding the frequency modulated exponential audio carrier and VSB
filtered CVBS samples through a complex adder;
multiplying the base band TV signals with an exponential carrier to shift
the base band TV signals by the frequency of the exponential earner
through a complex multiplier; and
generating modulator output at a desired sample rate through a zero-
paddcr logic and quadrature band pass filter.
the output of the quadrature band pass filter is passed through another
zero-padder logic and real band pass filter if the required RF channel is in
the higher VHF band.
19. A digital radio frequency (RF) modulator substantially as herein described with
reference to and as illustrated in the accompanying drawings
20. A RF module substantially as herein described with reference to and as illustrated
in the accompanying drawings
21. A method of modulating base band TV signals to a desired RF channel carrier substantially as herein described with reference to and as illustrated in the accompanying drawings.