Modulator/demodulator using baseband filtering

A modulation and demodulation scheme for video signals may be used for HDTV signals using VSB-PAM, analog NTSC signals using VSB-AM and digital video signals using QAM. VSB-PAM modulation and demodulation may be performed using in-phase and quadrature baseband filters. By adjusting the filter taps, a single modulator structure may be used for QAM and VSB-PAM modulation. Similarly, a single demodulator structure may be used for QAM and VSB-PAM demodulation. This demodulator may also be used for VSB-AM modulation.

FIELD OF THE INVENTION 
The present invention relates to a modulator and demodulator used for the 
transmission and reception of information signals such as digital and 
analog video signals. Specifically, the present invention relates to a 
vestigial sideband (VSB) modulator and demodulator which utilizes in-phase 
and quadrature baseband filters. The present invention also relates to a 
modulator and demodulator which can be used for vestigial sideband-pulse 
amplitude modulation (VSB-PAM), as well as for Quadrature Amplitude 
Modulation (QAM). In addition, for video applications, the present 
invention provides a single demodulator structure which can demodulate QAM 
signals, VSB-PAM signals and VSB-AM signals, such as conventional NTSC 
video. 
BACKGROUND OF THE INVENTION 
A conventional QAM modulator 10 for a video signal is illustrated in FIG. 
1. The QAM modulator 10 has a first input 12 and a second input 14. The 
input 12 receives an in-phase baseband signal m.sub.I (k), where k is a 
discrete time variable. The baseband signal m.sub.I (k), where k is a 
discrete time variable. The input 14 receives a quadrature baseband input 
signal m.sub.Q (k). For example, both m.sub.I (k) and m.sub.Q (k) can take 
on one of four discrete symbol values in each symbol period T (e.g., -3, 
-1, +1, +3). The baseband information signals m.sub.I (k) and m.sub.Q (k) 
are filtered by the baseband digital filters 16 and 18 which operate at 
twice the symbol period by inserting zero values in between the symbol 
values. The outputs of the filters 16 and 18 are designated m.sub.I '(k) 
and m.sub.Q '(k). The signals m.sub.I '(k) and m.sub.Q '(k) are converted 
to analog form by the D/A (digital-to-analog) converters 20 and 22 clocked 
at twice the symbol rate to generate m.sub.I '(t) and m.sub.Q '(t) after 
low pass filtering by the low pass filters 21 and 23. The local oscillator 
24 outputs a carrier signal cos.omega..sub.o t, where .omega..sub.o is the 
frequency of an intermediate frequency (IF) band carrier. A phase shifter 
26 shifts the output of the local oscillator 24 by 90.degree. to generate 
the carrier sin.omega..sub.o t. The multiplier 28 multiplies m.sub.I '(t) 
and cos.omega..sub.o t. The multiplier 30 multiplies m.sub.Q '(t) and 
sin.omega..sub.o t. The two products are summed by the summer 32 to obtain 
the IF band signal r(t). A further frequency upshifting takes place 
through use of the local oscillator 34 and multiplier 36. The local 
oscillator 34 generates a radio frequency (RF) band carrier 
cos.omega..sub.c t. The radio frequency band carrier is multiplied with 
r(t) using the multiplier 36 to produce r'(t). The signal r'(t) is then 
processed by a conventional image rejection filter 35 and transmitted via 
a channel to a demodulator. 
A conventional QAM demodulator 40 is illustrated in FIG. 2. The QAM 
demodulator 40 receives the RF signal r'(t). The signal r'(t) is 
downshifted into the IF frequency band to reproduce r(t) using the local 
oscillator 42 which generates cos.omega..sub.c t, the multiplier 44, and 
the low pass filter 46. The baseband signals m.sub.I '(t) and m.sub.Q '(t) 
are then regenerated using the local oscillator 48 which generates the IF 
carrier cos.omega..sub.o t. The IF signal r(t) is multiplied by 
cos.omega..sub.o t in the multiplier 50 and filtered by the low pass 
filter 52 in the I channel 51 to regenerate m.sub.I '(t). Similarly, to 
regenerate m.sub.Q '(t), the IF carrier is phase shifted by 90.degree. in 
the phase shifter 54. Then sin.omega..sub.o t is multiplied with r(t) 
using the multiplier 56 in the Q channel 53. The result is filtered by the 
low pass filter 58 to reproduce m.sub.Q '(t). The baseband analog signals 
m.sub.I '(t) and m.sub.Q '(t) are then converted to digital signals 
m.sub.I '(k) and m.sub.Q '(k) using the A/D (analog-to-digital) converters 
60 and 62. The signals m.sub.I '(k) and m.sub.Q '(k) are then filtered 
using the baseband filters 64 and 66 and sampled at the symbol period T 
using the samplers 65 and 67 to reproduce m.sub.I (k) and m.sub.Q (k). 
The combined transfer function of the I-channel filters 16 (see FIG. 1) and 
64 (see FIG. 2) is designated H(.omega.). The combined transfer function 
of the Q-channel filters 18 (see FIG. 1) and 64 (see FIG. 2) is also 
H(.omega.). The I channel transfer function may be partitioned so that it 
is entirely at the modulator (in which case filter 64 may be omitted) or 
entirely at the demodulator (in which case filter 16 may be omitted) or 
may be partitioned between the modulator and demodulator. Similarly, the Q 
channel transfer function may be partitioned so that it is entirely at the 
modulator, entirely at the demodulator, or partitioned between the 
modulator and demodulator. 
Another form of modulation which may be used is single sideband (SSB) 
modulation. Before SSB is discussed the following should be noted. 
Consider the message signal m(t). The frequency domain representation of 
this signal M(.omega.) is illustrated in FIG. 3. As can be seen in FIG. 3, 
the signal M(.omega.) has a bandwidth W. When the signal m(t) is upshifted 
by modulation of m(t) onto an IF band carrier with frequency 
.omega..sub.o, the bandwidth is 2W centered around .omega..sub.o as shown 
in FIG. 4. In single sideband modulation, either the lower sideband 70 or 
the upper sideband 72 of the double sideband (DSB) signal of FIG. 4 is 
suppressed. FIG. 5A shows the spectrum after suppression of the lower 
sideband. 
One form of SSB modulator 70 is illustrated in FIG. 5. In FIG. 5, the 
baseband digital video message signal m(k) is converted to analog form by 
the D/A converter 72 and low pass filtered using the low pass filter 73 to 
produce the analog message signal m(t). The message signal m(t) is then 
frequency upshifted into the IF band using the local oscillator 74 and 
multiplier 76. The result is a signal with a frequency spectrum such as 
that shown in FIG. 4 with a bandwidth of 2W. To reduce the bandwidth to W, 
the single sideband filter 78 is utilized. The filter 78 has a passband 
from .omega..sub.o to .omega..sub.o +W if the upper sideband is to be 
transmitted and a passband from .omega..sub.o -W to .omega..sub.o if the 
lower sideband is to be transmitted. The output of the filter 78 is the 
single sideband signal r(t) which is an IF band signal. The IF band signal 
r(t) is then upshifted to the RF band using the local oscillator 80 which 
generates the RF band carrier cos.omega..sub.c t and the multiplier 82 to 
form the RF band signal r'(t). A conventional image rejection filter (not 
shown in FIG. 5) is used to filter r'(t). 
Another form of SSB modulator 90 is illustrated in FIG. 6. An input video 
data signal m(k) is directed into an I (In-phase) channel 92 and a Q 
(quadrature) channel 94. The signal in the Q-channel 94 is subjected to a 
Hilbert transform using conventional Hilbert Transform circuit 98. The 
signals in both channels are then converted to analog form using the D/A 
converters 100 and 102 and low pass filters 101 and 103. The local 
oscillator 104 generates an IF band carrier signal cos.omega..sub.o t 
which is shifted 90.degree. by the phase shifter 106 to form 
sin.omega..sub.o t. The I-channel baseband signal is upshifted into the IF 
band using the multiplier 108 which multiplies the I-channel baseband 
signal by cos.omega..sub.o t. The Q-channel baseband signal is upshifted 
into the IF band using the multiplier 110 which multiplies the Q-channel 
baseband signal by sin.omega..sub.o t. The outputs of the multipliers 108 
and 108 are summed by the summer 112 to form the IF band signal r(t). The 
signal r(t) is then upshifted into the RF band through use of the local 
oscillator 114 which generates the RF carrier cos.omega..sub.c t and the 
multiplier 116 which outputs the RF band signal r'(t). The signal r'(t) is 
then filtered by a conventional image rejection filter (not shown) and 
transmitted to a remote location. 
It should be noted that in FIG. 6, if the summer 112 performs addition, the 
lower sideband is suppressed, and if the summer 112 performs subtraction, 
the upper sideband is suppressed. 
The SSB techniques described in connection with FIG. 5 and FIG. 6 both have 
significant shortcomings. The SSB filter 78 of FIG. 5 is very hard to 
implement practically because the sharp cutoff at .omega.=.omega..sub.o 
cannot be synthesized exactly. Thus, there is a problem at the low 
frequency portion of the baseband signal. Similarly, in the modulator of 
FIG. 6, the Hilbert transform circuit 98 cannot be implemented exactly and 
there is significant distortion of the low frequency modulating 
components. 
The vestigial sideband pulse amplitude modulation (VSB-PAM) technique may 
be used to overcome the shortcomings of the SSB technique. VSB-PAM is 
derived by filtering DSB in such a fashion that one sideband is passed 
completely while just a trace or vestige of the other sideband remains. A 
conventional VSB-PAM modulator is illustrated in FIG. 7. The VSB-PAM 
modulator 120 of FIG. 7 is identical to the SSB modulator 70 of FIG. 5 
except that the VSB filter 99 replaces the SSB filter 78. 
The transfer function of the SSB and VSB filters 78, 99 are illustrated in 
FIG. 8. As shown in FIG. 8, the SSB filter has a sharp cut-off at 
.omega.=.omega..sub.o. This is very hard to implement in practice. The 
exact shape of the transfer function of the VSB filter is not critical, 
but the VSB filter transfer function has a response such that 
EQU H(.omega..sub.o -.omega..sub.o ')+H(.omega..sub.o 
+.omega.')=2H(.omega..sub.o) 
However, the VSB modulation technique described above also has certain 
shortcomings. Specifically, the VSB filter is an IF or RF band filter 
which is implemented using filter devices such as inductor-capacitor (L-C) 
filters, surface-acoustic-wave (SAW) filters, helical filters, or 
stripline filters. 
In view of the foregoing, it is an object of the present invention to 
provide a VSB-PAM modulator and demodulator which utilizes baseband 
filtering rather than an IF or RF VSB filter. 
Another form of modulation which is used for video signals is VSB-AM. The 
VSB-AM modulation technique is used for conventional analog NTSC video. 
An NTSC VSB-AM modulator 150 is shown in FIG. 8A. The analog video baseband 
signal m(t) is upshifted into the IF band using the local oscillator 152 
which generates the IF band carrier cos.omega..sub.o t, and the AM 
modulator 154. The resulting IF band signal is then filtered by the VSB-AM 
filter 156 to produce the IF band signal r(t). The transfer function of 
the VSB-AM filter 156 is shown by the solid curve A of FIG. 8B. The 
frequency domain representation of the signal outputted by the AM 
modulator 154 is indicated by curve B in FIG. 8B. Note that the curve A is 
not symmetric with respect to the IF band carrier frequency .omega..sub.o. 
Returning now to FIG. 8A, the IF band signal r(t) is upshifted to the RF 
band using the local oscillator 159 which generates an RF band carrier 
cos.omega..sub.c t, and the multiplier 158 which multiplies r(t) by 
cos.omega..sub.c t to form the IF band signal r'(t). The signal r'(t) is 
filtered by a conventional image rejection filter 160 and then broadcast 
to a plurality of receivers. 
FIG. 8C illustrates a conventional NTSC demodulator 160. The RF band signal 
is downshifted to the IF band using a local oscillator 161 which generates 
cos.omega..sub.o t, the multiplier 162, and the low pass filter 163. The 
multiplier 162 multiplies r'(t) and cos.omega..sub.o t. The low pass 
filter 163 filters harmonics of the RF carrier .omega..sub.c to regenerate 
r(t). The signal r(t) is filtered by the VSB-AM filter 164. The resulting 
signal is downshifted to the baseband using the local oscillator which 
generates the IF band carrier, the AM demodulator 166, and the low pass 
filter 167, which eliminates harmonics of the IF carrier, to regenerate 
the baseband signal m(t). 
The transfer function of the filter 164 of FIG. 8C is indicated by the 
curve A of FIG. 8D. The frequency domain representation of the signal r(t) 
which is inputted to the filter 164 is indicated by the curve B in FIG. 
8D. 
A further object of the invention is as follows. It is now expected that 
HDTV (High Definition Television) signals will be transmitted using 
VSB-PAM. However, as indicated above, conventional analog NTSC television 
signals are transmitted using VSB-AM. In addition, it is expected that 
digital television signals will be transmitted using QAM. It is expected 
that a typical user will simultaneously have access to some HDTV channels 
using VSB-PAM and some conventional analog NTSC channels which are 
transmitted using VSB-AM. The user will also have access to some digital 
TV channels transmitted using QAM. It is therefore an object of the 
invention to provide a single integrated demodulator which can perform 
QAM, VSB-AM, and VSB-PAM demodulation. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the invention a unique VSB-PAM modulator 
is disclosed- This modulator eliminates the need for a VSB IF or RF band 
filter and instead uses baseband filtering. An input message signal which 
undergoes VSB modulation is divided into an I-(In phase) signal and a Q 
(quadrature) signal. The I and Q signals are filtered by baseband filters 
(whose transfer functions are discussed in detail below). The filtered I 
and Q signals are then multiplied by in phase and quadrature carriers 
(e.g., cos.omega..sub.o t and sin.omega..sub.o t) and the result is summed 
to produce a VSB signal. Illustratively, .omega..sub.o is a carrier in the 
IF band. The inventive VSB modulator may also include further circuity 
(i.e., local oscillator and multiplier) for translating the signal up to 
the RF band. 
In a preferred embodiment, the I and Q baseband filters are implemented as 
linear phase Finite Impulse Response (FIR) filters which means that the 
filtering takes place in the digital domain. 
The VSB signal may be demodulated as follows. The RF band signal is first 
stepped down to the IF band. The IF band signal is then divided into an I 
channel signal and a Q channel signal. The I-channel signal is multiplied 
by cos.omega..sub.o t and processed by a low pass analog filter to form an 
I-channel baseband signal. The Q-channel signal is multiplied by 
sin.omega..sub.o t and filtered by a low pass analog filter to form a 
Q-channel baseband signal. The I and Q channel baseband signals are then 
converted to digital form and filtered using digital baseband filters 
which preferably are linear phase FIR filters. 
The overall transfer function of the baseband filters in the in-phase 
channel of the inventive VSB modulator and demodulator is denoted G.sub.I 
(.omega.). The transfer function G.sub.I (.omega.) may be partitioned so 
that the entire transfer function G.sub.I (.omega.) is located in the 
modulator or the entire transfer function G.sub.I (.omega.) is located in 
the demodulator or the transfer function G.sub.I (.omega.) may be 
partitioned between the modulator and demodulator. Similarly, the baseband 
filters in the quadrature channel of the inventive modulator and 
demodulator may be partitioned so that the entire transfer function 
G.sub.Q (.omega.) is located in the modulator, the demodulator, or 
partitioned between the two. When expressed as a function of filter tap 
number n, the transfer function of the in-phase channel filters g.sub.I 
(n) is even symmetric about the center tap and the transfer function 
g.sub.Q (n) of the quadrature channel filters is odd symmetric about the 
center tap. 
It may now be noted that both a QAM modulator and demodulator and a VSB-PAM 
modulator and demodulator may be implemented through use of baseband FIR 
filters. In accordance with a second aspect of the invention, this permits 
a single modulator structure to be used as a-QAM modulator or a VSB-PAM 
modulator by varying the filter coefficients of the FIR baseband filters. 
Similarly, a single demodulator structure may be used as a QAM demodulator 
or VSB-PAM demodulator by varying the filter coefficients. Preferably, the 
change in filter coefficients can be accomplished automatically under the 
control of a controller such as a microprocessor. This permits a 
significant advantage in that a user's television set may be provided with 
a single demodulator which can demodulate both QAM and VSB signals. 
Moreover, the inventive demodulator can also be used to demodulate VSB-AM 
signals. As is shown in detail below, this can be accomplished using the 
same filter coefficients in the demodulator as in the VSB-PAM case. 
The inventive demodulator is particularly useful as it enables a television 
receiver unit to receive analog NTSC channels modulated using VSB-AM, 
digital video channels modulated using QAM, and HDTV channels modulated 
using VSB-PAM.

DETAILED DESCRIPTION OF THE INVENTION 
A VSB modulator 200 in accordance with the invention is illustrated in FIG. 
9. An input symbol stream m(k) enters into an I-channel 204 and a 
Q-channel 206. The I-channel 204 includes a baseband filter 208 for 
processing the I-channel symbols. The Q-channel 206 includes a baseband 
filter 210 for processing the Q-channel symbols. The outputs of the 
filters 208, 210 are converted to analog form by the D/A converters 212, 
214 (and low pass filters 213, 215) to produce the analog signals m.sub.I 
(t) and m.sub.Q (t). The local oscillator 216 produces the IF band 
in-phase carrier cos.omega..sub.o t. The 90.degree. phase shifter 218 
phase shifts the output of the local oscillator 216 to produce the IF band 
quadrature carrier sin.omega..sub.o t. The multiplier 220 multiplies 
m.sub.I (t) and cos.omega..sub.o t. The multiplier 222 multiplies m.sub.Q 
(t) and sin.omega..sub.o t. The products are summed by the summer 224 to 
produce the IF band VSB signal r(t). The local oscillator 226, which 
produces an RF band carrier cos.omega..sub.c t, and the multiplier 228 are 
used to translate r(t) into the RF band, thereby producing the RF band 
signal r'(t). The signal r'(t) may be processed by a conventional image 
rejection filter (not shown). 
It should be noted that there is no IF or RF band VSB filter such as the 
filter 99 of FIG. 7. Instead, the baseband filters 208, 210 are used. 
A VSB demodulator 300 according to the invention is illustrated in FIG. 10. 
The demodulator 300 receives the RF band signal r'(t) at the input 302. 
This signal is downshifted to the IF band using the local oscillator 304 
which generates cos.omega..sub.c t, the multiplier 306, and the low pass 
filter 308. The low pass filter 308 suppresses harmonics of the RF carrier 
frequency .omega..sub.c and outputs the IF band signal r(t). The signal 
r(t) is distributed to the I-channel 310 and the Q-channel 312. A local 
oscillator 314 generates cos.omega..sub.o t. This is phase shifted by the 
phase shifter 316 which outputs sin.omega..sub.o t. The multiplier 320 
multiplies r(t) and cos.omega..sub.o t. The result is low pass filtered by 
the filter 322 to suppress harmonics of the IF band carrier .omega..sub.o 
and to reproduce the I-channel baseband signal m.sub.I (t). The multiplier 
324 multiplies r(t) by sin.omega..sub.o t. The result is low pass filtered 
by the filter 326 to suppress harmonics of the IF band carrier 
.omega..sub.o and to reproduce the Q-channel baseband signal m.sub.Q (t). 
The signals m.sub.I (t) and m.sub.Q (t) are reconverted to digital form by 
the A/D converters 328 and 330. 
The I-channel and Q-channel signals are filtered by the I-channel and 
Q-channel baseband filters 340 and 350. The outputs are then summed by the 
summer 352 to reconstruct the original symbol stream. 
The I-channel filters 208 (see FIG. 9) and 340 (see FIG. 10) have a 
combined transfer function G.sub.I (.omega.). This transfer function may 
be implemented totally by the modulator filter 208, in which case the 
demodulator filter 340 is omitted, or implemented totally by the 
demodulator filter 340, in which case the modulator filter 208 is omitted. 
Alternatively, the transfer function G.sub.I (.omega.) may be partitioned 
into a product G.sub.I (.omega.)=G.sub.Im (.omega.) G.sub.Id (.omega.) 
where G.sub.Im (.omega.) is implemented at the filter 208 and G.sub.Id 
(.omega.) is implemented at the filter 340. Preferably G.sub.Im 
(.omega.)=G.sub.Id (.omega.). Similarly, the Q channel filters 210 (see 
FIG. 9) and 350 (see FIG. 10) have a combined transfer function G.sub.Q 
(.omega.) . This transfer function may be implemented totally by the 
modulator filter 210, in which case the demodulator filter 350 is omitted, 
or implemented totally by the demodulator filter 350, in which case the 
modulator filter 210 may be omitted. Alternatively, the transfer function 
G.sub.Q (.omega.) may be partitioned into a product G.sub.Q 
(.omega.)=G.sub.Qm (.omega.).multidot.G.sub.Qd (.omega.) where G.sub.Qm 
(.omega.) is implemented at the filter 210 and G.sub.Qd (.omega.) is 
implemented at the filter 350. Preferably, G.sub.Qm (.omega.)=G.sub.Qd 
(.omega.). 
FIG. 11A is a plot of an exemplary transfer function G.sub.I (.omega.). 
FIG. 11B illustrates the transfer function G.sub.I (.omega.) in dB. FIG. 
11C illustrates the impulse response of a filter with a transfer function 
G.sub.I (.omega.). It should be noted that G.sub.I (.omega.) is purely 
real and has an even symmetry with respect to .omega.=o. FIG. 12A is a 
plot of an exemplary transfer function G.sub.Q (.omega.). FIG. 12B 
illustrates G.sub.Q (.omega.) in dB. FIG. 12C is the impulse response of a 
filter with a transfer function G.sub.Q (.omega.) . The function G.sub.Q 
(.omega.) is purely imaginary and has an odd symmetry about .omega.=o. 
The filters 208, 210, 340, 350 may be implemented as FIR filters. An FIR 
filter is schematically illustrated in FIG. 11. The FIR filter 500 of FIG. 
11 comprises a shift register 502 with positions, 0, 1, 2, . . . , N-2, 
N-1. The symbols to be filtered arrive at input 504 and in each succeeding 
cycle the inputted symbols are shifted one position to the right. 
In each cycle, each of the symbols stored in the shift register 502 is 
multiplied by a tap weight w.sub.o,w.sub.1, . . . , w.sub.N-2,w.sub.N-1 
using a multiplier 504. The products are summed using the summer to 
generate an output symbol at 508. 
FIG. 13 is a list of tap weights for an FIR filter with N=61 which 
implements G.sub.I (.omega.) of FIG. 11A. FIG. 14 is a list of tap weights 
for an FIR filter with N=61 which implements G.sub.Q (.omega.) of FIG. 
12A. 
In accordance with the invention, the baseband filters in a QAM modulator 
or demodulator and a VSB-PAM modulator or demodulator may both be 
implemented using FIR filters. It is thus possible to form a single 
modulator structure which can perform QAM modulation and VSB-PAM 
modulation. Such a single modulator structure is shown in FIG. 15. It is 
also possible to form a single demodulator structure which can demodulate 
both QAM and VSB-PAM. Such a demodulator structure is shown in FIG. 16. 
More specifically, in a QAM modulator and demodulator, the combined 
transfer function of the I-channel baseband filter in the modulator (e.g., 
filter 16 of FIG. 1) and the I-channel baseband filter in the demodulator 
(e.g., filter 64 of FIG. 2) is H(.omega.). Similarly, the combined 
transfer function of the Q-channel baseband filter in the modulator (e.g., 
filter 18 of FIG. 1) and the Q-channel baseband filter in the demodulator 
(e.g., filter 66 of FIG. 2) is also H(.omega.). FIG. 17 is a list of tap 
weights for an N=31 FIR filter which has an illustrative transfer function 
H(.omega.). The function H(.omega.) has even symmetry with respect to 
.omega.=0. From the function H(.omega.), it is possible to derive G.sub.I 
(.omega.) and G.sub.Q (.omega.) , the baseband filter transfer functions 
for the VSB-PAM modulator and demodulator. 
The following steps are used to derive G.sub.I (.omega.) and G.sub.Q 
(.omega.) from H(.omega.). The steps are explained by representing 
H(.omega.), G.sub.I (.omega.) and G.sub.Q (.omega.) as h(n), g.sub.I (n), 
and g.sub.Q (n), respectively, where n is a tap number in a FIR filter. In 
the FIR filter of FIG. 11, n=0, 1, 2, . . . , N-2, N-1. 
1) h.sub.2 (n)=.uparw.2h(n). This means that h(n) is up sampled by a factor 
of two by inserting zero tap weights between the tap weights of h(n). That 
is: 
h(n)=h(0), h(1) 2, . . . , h(N--2), h(N-1) 
h.sub.2 (n)=h(0), 0, h(1), 0, . . . , 0, h(N-2), 0, h(N-1) 
2) h.sub.3 (N)=h(N)* h.sub.2 (N) 
3) h.sub.4 (n)=h.sub.3 (n)e.sup.j.pi.n/4 where .pi. is the digital Nyquist 
frequency 
4) g.sub.I (n)=R.sub.e {h.sub.4 (n)} 
5) g.sub.Q (n)=I.sub.m {h.sub.4 (n)} 
When h(n) is implemented by an FIR filter having the tap weights shown in 
FIG. 17, steps (1)-(5) above result in the tap weights for g.sub.I (n) and 
g.sub.Q (n) shown in FIGS. 13 and 14. 
A modulator which can be used for VSB-PAM and QAM is illustrated in FIG. 
15. The modulator 700 of FIG. 15 has three inputs 702, 704, 706. When the 
modulator 700 is used as a QAM modulator a baseband I-channel signal 
arrives via input 702 and a baseband Q channel signal arrives via input 
706. The switches 708 and 710 are set in a manner to pass the I and 
Q-channel baseband signals to the I-channel and Q-channel baseband filters 
712 and 714. Preferably, the filters 712 and 714 are linear phase FIR 
filters. 
When the modulator 704 is used for a VSB signal, the baseband signal 
arrives on input 704. Note that the symbol rate of the signals in inputs 
702 and 706 is half the symbol rate of a signal on input 704. The VSB-PAM 
baseband signal at input 704 is passed to both the I-channel and the 
Q-channel. The states of the switches 708 and 710 are set to pass the 
symbols from the input 704 to the I-channel and Q channel baseband filters 
712 and 714. 
The states of the switches 708 and 710 are controlled by the controller 
718, which illustratively is a CPU, depending on whether VSB-PAM or QAM is 
selected. In addition, the memory 720 connected to the CPU stores the tap 
weights for the filters 712 and 714. Depending on whether VSB-PAM or QAM 
is selected, a particular set of tap weights is automatically applied to 
the filters 712 and 714 from the memory 720 by the CPU 718. 
After processing by the filters 712 and 714 the signals in the I and Q 
channels are converted to analog form by the D/A converters 730 and 732. 
The low pass filters 731 and 733 remove harmonics induced by the D/A 
conversion. A local oscillator 734 generates an in-phase IF band carrier 
cos.omega..sub.o t. The cos.omega..sub.o t signal is shifted 90.degree. by 
the phase shifter 736 to generate an IF band quadrature carrier 
sin.omega..sub.o t. The frequency .omega. of the local oscillator is 
controlled by the CPU so that different frequencies can be used for QAM 
and VSB. For example, for QAM the frequency may be 44 MHz and for VSB-PAM 
the frequency may be 46.69 MHz. The in-phase baseband signal is multiplied 
by cos.omega..sub.c t using multiplier 738. The quadrature baseband signal 
is multiplied by sin.omega..sub.o t using the multiplier 740. The results 
are summed using the summer 742 to generate an IF band modulated signal 
r(t). This IF band signal is then shifted to the RF band using the local 
oscillator 744 which generates an RF carrier and the multiplier 746. The 
output of the multiplier 746 r'(t) may be filtered by a conventional image 
rejection filter 748. 
A demodulator 800 which can demodulate VSB or QAM modulated signals is 
shown in FIG. 16. The modulator 800 receives a RF band QAM or VSB signal 
r'(t) on the input 802. The local oscillator 804 generates the RF carrier 
cos.omega..sub.c t. The multiplier 806 multiplies cos.omega..sub.c t with 
the RF input signal r'(t). The low pass filter removes harmonics of the RF 
carrier .omega..sub.c and outputs the IF signal r(t). 
The signal r(t) is distributed to the I-channel 810 and the Q-channel 812. 
A local oscillator 816 generates the in-phase IF carrier, e.g., 
cos.omega..sub.o t. The phase shifter 820 provides a 90.degree. phase 
shift to generate the IF band quadrature carrier sin.omega..sub.o t. The 
multiplier 822 multiplies r(t) and cos.omega..sub.o t. The product is then 
filtered by the low pass filter 824 which removes harmonics of 
.omega..sub.o and outputs an I-channel baseband signal. Similarly, the 
multiplier 826 multiplies r(t) and sin.omega..sub.o t. The product is then 
filtered by the low pass filter 828 to remove harmonics of .omega..sub.o 
and to output a Q-channel baseband signal. The I-channel and Q-channel 
baseband signals are then converted to digital form by the A/D converters 
830,832. The I-channel and Q-channel baseband signals are then filtered by 
the FIR filters 834 and 836. 
The demodulator 800 includes controller 838 (e.g., a CPU) and a memory 840. 
The memory stores tap weights that are applied by the CPU to the filters 
834 and 836 depending on whether the user selects a QAM or VSB channel. 
Optionally, the CPU 838 also controls the local oscillator 816 so that 
this oscillator outputs a carrier .omega..sub.o corresponding to the 
channel selected by the user. 
The demodulator 800 also includes the switches 842 and 844. The state of 
the switches 842 and 844 are controlled by the CPU 838 depending on 
whether the user has selected a VSB or QAM channel. In the case of a QAM 
channel, independent I and Q signals are outputted at outputs 846 and 848. 
In the case of a VSB signal, the switches 842 and 844 set so that the 
outputs of the filters 834 and 836 are summed by the summer 850 and the 
result outputted at the output 852. 
It is a significant feature of the invention that the modulator 800 can 
also demodulate a VSB-AM signal (as well as QAM and VSB-PAM signals). The 
filter coefficients of the filters 834 and 836 are the same for VSB-PAM 
and VSB-AM. However, the IF carrier frequency is different. For example, 
the IF carrier for VSB-PAM is 46.69 MHz and for VSB-AM it is 45.75 MHz. 
The IF carrier may be adjusted under the control of the CPU 838 depending 
on whether VSB-PAM or VSB-AM demodulation is performed. The demodulation 
of VSB-AM using the filters 834 and 836 may be understood in connection 
with FIGS. 18A and 18B. FIG. 18A shows the I-channel spectrum (dashed 
curve) and the I-channel filter transfer function (solid curve). FIG. 18B 
shows the Q-channel spectrum (dashed curve) and the Q-channel filter 
transfer function (solid curve). When the I and Q channel spectrums are 
summed the baseband signal is reconstructed. 
In the case of VSB-AM, the baseband signal is converted to analog form 
using D/A converter 854. A low pass filter 856 removes harmonics resulting 
from the Digital-to-Analog (D/A) conversion. The output is NTSC baseband 
video. 
In short, there has been disclosed a new modulation and demodulation scheme 
for video signals including HDTV signals using VSB-PAM, analog NTSC 
signals using VSB-AM, and digital video signals using QAM. In particular, 
according to the invention, VSB-PAM modulation and demodulation may be 
performed using in-phase and quadrature baseband filters. Preferably, the 
filters are linear phase FIR filters. By adjusting the filter taps, a 
single modulator structure may be used for QAM and VSB-PAM demodulation. 
Similarly, a single demodulator structure may be used for QAM and VSB-PAM 
demodulation. This demodulator may also be used for VSB-AM modulation. 
Finally, the above-described embodiments of the invention are intended to 
be illustrative only. Numerous alternative embodiments may be devised by 
those skilled in the art without departing from the spirit and scope of 
the following claims.