Pulse or digital communications – Transmitters – Amplitude modulation
Reexamination Certificate
1999-01-29
2004-04-20
Liu, Shuwang (Department: 2734)
Pulse or digital communications
Transmitters
Amplitude modulation
C375S240000, C375S295000, C348S723000, C348S724000
Reexamination Certificate
active
06724832
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to vestigial sideband generators or modulators and more particularly to a new and improved vestigial sideband generator or modulator employing modified Weaver modulator techniques and arrangements. Another aspect of the invention relates generally to digital television transmitters and more particularly to a digital television transmitter including a digital modulator including a modified Weaver modulator arrangement and technique.
BACKGROUND ART
A typical prior art digital television transmitter adapted to transmit signals containing information indicative of digitally encoded video and aural signals for deriving an ATSC A/53 standard signal is illustrated in
FIG. 1
as including multi-bit digital baseband television signal source
10
which drives the cascaded combination of data randomizer
11
, Reed-Solomon encoder
12
, data interleaver
14
, trellis encoder
16
and multiplexer
18
. The signals derived from source
10
, randomizer
11
, encoders
12
and
16
, as well as interleaver
14
and multiplexer
18
are typically three or four parallel bit signals having a symbol rate (i.e., sampling frequency) of
1.539
×
10
9
143
,
i.e., the encoded television signal is sampled 10,762,237.76 times per second. Because each symbol includes two, three or four bits, the bit rate is substantially higher than the symbol rate. The three or four parallel bits represent 8 or 16 amplitude levels of the encoded television signal.
Multiplexer
18
, in addition to being responsive to the output of trellis encoder
18
, responds to segment synchronizing source
20
and field synchronizing source
22
to derive an output having the same number of bits as applied to the multiplexer by encoder
16
. Multiplexer
14
supplies a multi-bit output signal to pilot inserter
24
which inserts a 309.44056 KHz pilot carrier on the signal applied to it. Pilot inserter
24
derives a multi-bit output signal which it applies to pre-equalizer filter
26
. Pre-equalizer filter
26
supplies a multi-bit intermediate frequency (I.F.) signal to vestigial sideband modulator or generator
28
. Generator
28
feeds a multi-bit digital I.F. output signal to digital to analog converter
30
, which supplies an analog I.F. signal to frequency up converter
32
, a frequency synthesizer for heterodyning the I.F. output frequency of converter
30
to a radio frequency (R.F.) carrier frequency. Up converter
32
also inverts the I.F. spectrum derived from digital to analog converter
30
so the lowest frequencies in the I.F. spectrum are converted into the highest frequencies in the R.F. spectrum derived by converter
32
and the highest frequencies in the I.F. spectrum are converted to the lowest frequencies in the R.F. spectrum. The modulated carrier frequency signal derived by R.F. up converter
32
is applied to antenna
34
via power amplifier
36
.
The output signal of digital to analog converter
30
includes orthogonal I and Q channels or components. At predetermined time intervals the I channel has one of multiple levels, corresponding to the number of amplitude levels in the 3 or 4 bit signal derived by signal source
10
. The Q channel contains no independent information, but causes part of the unwanted lower sideband appearing at the output of up converter
32
to be reduced substantially to zero amplitude. The unwanted lower sideband is removed by circuitry included in vestigial sideband generator
28
and up converter
32
does not reintroduce it. Because up converter
32
“flips” (i.e., inverts) the I.F. spectrum derived by digital to analog converter
30
, the upper sideband R.F. output of converter
30
is reduced substantially to zero.
To enable digital to analog converter
30
to produce the desired vestigial sideband signal, vestigial sideband modulator or generator
28
derives the spectrum illustrated in
FIG. 2
having a 6 Mhz bandwidth and including the 309.44056 kHz pilot carrier provided by pilot inserter
24
, as well as a vestigial sideband of 309.4405594 kHz, to the left of the pilot carrier frequency.
The prior art vestigial sideband modulators or generators for deriving the ATSC A/53 standard have generally used a filter or phasing method. In the filter method the vestigial sideband modulator generates a double sideband signal that is filtered to produce a vestigial sideband signal at an I.F. of about 10 MHz. Sidebands extend equally around the 10 MHz I.F. in accordance with:
0.5
F
sym
+F
pilot
=6 MHz−
F
pilot
where F
sym
is the symbol clock frequency of 10.76223776 . . . MHz of the bits derived from source
10
in accordance with the ATSC A/53 standard, and
F
pilot
=
59
3
·
FH
NTSC
=
59
3
·
4.5
MHz
286
where FH
NTSC
is the NTSC horizontal line frequency
Based on the foregoing, the sidebands of the double sideband modulator extend ±5.690559441 . . . MHz on either side, of the 10 MHz carrier. A convenient sampling frequency is four times the 10.76223776 . . . MHz symbol clock rate, i.e., 43.04895105 . . . MHz.
The ATSC A/53 standard requires the vestigial sideband generator to have a root-raised cosine (RRC) response. Obtaining a proper root-raised cosine response for vestigial sideband shaping at the 43.04895105 . . . MHz sampling rate requires a finite impulse response (FIR) filter having about 2048 filter coefficients. Implementation of such a filter is difficult.
The phasing method uses a Hilbert transform to partially cancel the unwanted sideband of a double sideband signal. The Hilbert transform can easily generate a vestigial sideband signal such that DC is 6 dB down with respect to the sidebands. This is because the response of any Hilbert transform approximation is always zero at DC. With only one of the I and Q modulators included in such a vestigial Hilbert transform sideband generator contributing at DC, the vector sum of the outputs of the two modulators drops in half at DC relative to the vector sum at a frequency where both the I and Q channels contribute to the generator output. However, in the ATSC A/53 standard, the requirement for the root-raised cosine response places the DC output at −3 dB instead of −6 dB. Therefore, the Hilbert transform method of vestigial sideband digital television modulation requires a low frequency equalizer to produce a +3 dB “shelf” at the DC and low frequency portions of the response.
To achieve the ATSC A/53 standard the vestigial sideband generator has a linear phase requirement. Consequently, equalizer filter
26
is generally implemented as a finite impulse response filter having a large number of coefficients. Further, the −3 dB requirement exists at the Nyquist frequency of the symbols, i.e., half the symbol frequency, with certain modifications. Hence, equalizer
26
must include a high frequency portion operating at a sampling frequency higher than twice the symbol rate to avoid aliasing, i.e. insertion of information at frequencies that do not exist in the sample frequency due to sampling at a frequency less than twice the highest frequency component being filtered. In this case, the highest frequency being filtered is 5.690559441 . . . MHz, which is more than half the symbol rate. Hence, the Hilbert transform method of producing a vestigial sideband signal with root-raised cosine sideband shaping is also quite difficult to implement.
Because power amplifier
34
has a non-linear amplitude response, a nonlinear equalizer must apply a substantial non-linear correction to the signal applied to it. Because of the possibility of aliasing and spectral folding through zero frequency, the amount of nonlinear correction which may be applied at 10 MHz is limited, resulting in distortion in the transmitted signal.
I have realized that the non-linear correction can be more effectively implemented to substantially reduce distortion in the transmitted signal by employing an I.F. digital signal having a frequency approximately twice the approximately 10 MHz frequency of the prior art digital I.F. T
ADC Broadband Wireless Group, Inc.
Fogg & Associates LLC
Liu Shuwang
Lundberg Scott V.
LandOfFree
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