GB2234652A - Apparatus for generating digital wideband frequency modulated signals - Google Patents
Apparatus for generating digital wideband frequency modulated signals Download PDFInfo
- Publication number
- GB2234652A GB2234652A GB8917531A GB8917531A GB2234652A GB 2234652 A GB2234652 A GB 2234652A GB 8917531 A GB8917531 A GB 8917531A GB 8917531 A GB8917531 A GB 8917531A GB 2234652 A GB2234652 A GB 2234652A
- Authority
- GB
- United Kingdom
- Prior art keywords
- signal
- digital
- adder
- synthesiser
- frequency
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/10—Frequency-modulated carrier systems, i.e. using frequency-shift keying
- H04L27/12—Modulator circuits; Transmitter circuits
- H04L27/122—Modulator circuits; Transmitter circuits using digital generation of carrier signals
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03C—MODULATION
- H03C3/00—Angle modulation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/02—Amplitude-modulated carrier systems, e.g. using on-off keying; Single sideband or vestigial sideband modulation
Landscapes
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Transmitters (AREA)
Abstract
The apparatus comprising a converter arranged to convert the analog signal into a digital signal. Deviation means is arranged to subject the digital signals to a scaling factor, and an adder/subtractor means is arranged to modify the digital signal. A frequency synthesiser is arranged to generate a digital wideband frequency modulated signal in accordance with the modified digital signal under control of frequency setting signal applied to the synthesiser by way of the adder/subtractor circuit. <IMAGE>
Description
APPARATUS FOR GENERATING DIGITAL VVIDEBAND FREQUENCY MODULATED SIGNALS.
The present invention relates to apparatus for generating digital wideband frequency modulated signals.
New methods of broadband signal distribution will require very wide bandwidth frequency modulation of carrier signals. Examples are in cable or other means of television signal distribution. ,%rile fully digital distribution is possible, it is very demanding in use of bandwidth. Conventional amplitude modulation suffers too many defects and distortions for any further improvement over present broadcast systems; and is barely adequate even in these circumstances, and will not be the ideal choice for new networks.
Frequency modulation (FM), as used in almost all satellite systems, is capable of excellent picture quality even under marginal signal conditions, and has the potential for further improvement using high definition TV (KDTV! techniques. The receiving equipmen: in use for satellite receivers is capable of being adapted to such systems so the user investments could be relatively small.
However, if full benefit is to be gained from wideband F.M. 1W', there is a need for very linear modulators at the transmission end, preferably at low cost so that such systems can be readily integrated into the new networks.
Present means of achieving wideband FM have the disadvantages of being relatively large and costly pieces of precision analogue equipment. The present invention overcomes these disadvantages by providing a very simple means of achieving very linear Fhl by digital means. It has the advantage that the output consists essentially of only one sideband ie. it is inherently of vestigial sideband (VSB) format. The apparatus needs no adjustments and has no operational drift. Linearity of operation is arbitrarily set by the choice of analog to digital converter used.
According to the present invention, there is provided apparatus for generating digital wideband frequency modulation signals from an analog signal, the apparatus comprising a converter arranged to convert the analog signal into a digital signal, deviation means arranged to subject the digital signal to a scaling factor, an adderisubtractor means arranged to modify the digital signal, and a frequency synthesiser arranged to generate a digital wideband frequency modulated signal in accordance with the modified digital signal under control of a frequency setting signal applied to the synthesiser by way of the adder/substractor circuit.
An embodiment of the present invention will now be described with reference to the accompanying drawings, wherein: Figure 1 shows a block diagram of apparatus for generating digital wideband frequency modulated signals. and,
Figure 2 shows an output spectrum of a synthesiser used in the block diagram of Figure 1.
Referring to Figure 1, the video signal, which could be standard composite video or a signal such as MAC-coded video, is fed into an analogue to digital converter 1, for example of 8 bits accuracy. The
AID converter 1, would need to sample at a rate of, say, three times the colour subcarrier, i.e. 13.29MHz, although other sample rates could be used. There would be an optimum means of sampling the data for the various video coding techniques, probably involving higher sample rates, but the method of modulation would remain the same. In any of these cases, a digital representation of the required modulation is obtained. The transmission requirement is for vestigial sideband FM (VSB-FM). This is achieved by filtering and scaling the digitised video signal.The filters 2 are connected to the AID converter 1 and apply a suitable pre-emphasis, and the multiplier 3 applies a scaling factor corresponding to the required deviation. A digital word is applied through an adder/subtractor 4 to the frequency setting inputs of a direct frequency synthesiser 5 such as the Plessey SP 2002. The output of the multiplier is also applied to the adder/subtractor 4, the output of which is passed to the synthesiser 5. The synthesiser 5 generates an output frequenW X directly related to the input setting word so by changing this word in a way directly related to the video signal, a very linear form of FM is generated.Taking the specific device (SP 2002j as an example. an update interval of 20ns is possible, so deviation of centre frequency up to 50N1Hz is possible. The clock applied to the AID convene 1 would conveniently also synchronise the updating of the frequency setting word of the synthesiser 5. A further advantage of this technique over analogue systems is that the signal produced would be directly in the required VSB-FM form, without additional filtering and would be as linear as the AID converter, except of course for any rounding errors in the scaling multiplication.
A schematic of the spectral output is shown in Figure 2. The choice of addition or subtraction of the digitised video signal determines whether the upper of lower sideband is produced.
Simpler systems are possible, where some of the functions are carried out prior to digitisation, but this would result in reduced precision and linearity. In most practical systems, the variables of filter coefficients, scaling factors and sideband selection would remain fixed for the lifetime of the apparatus, and only in the most general case would these need to be programmed.
The basic SP 2002 device is capable of operation up to 500,MHz output. This could be mixed or multiplied up in frequency to the required output frequency, or could be translated.
The invention as described, using readily available components gives a very linear and relatively low cost means of achieving vestigial sideband FM at VHF, UHF or microwave frequencies without the complications of the current analogue systems, and with the potential for accuracy and linearity sufficient for the most demanding high definition TV systems.
The above description has been of one embodiment and is not limited thereto. It will readily be appreciated that the invention could be used for narrowband modulation of signals for voice or data transmission as VSB-FM as a means of bandwidth saving compared with existing FM systems. Also, the invention could also be used, under softy are control, to provide an extremely versatile signal source for VSB-FM.
Claims (7)
1. Apparatus for generating digital wideband frequency modulation signals from an analog signal, the apparatus comprising a converter arranged to convert the analog signal into a digital signal, deviation means arranged to subject the digital signal to a scaling factor, an adder/subtractor means arranged to modify the digital signal, and a frequency synthesiser arranged to generate a digital wideband frequency modulated signal in accordance with the modified digital signal under control of a frequency setting signal applied to the synthesiser by way of the adder/substractor circuit.
2. Apparatus as claimed in Claim 1, wherein the deviation means is a digital multiplier to which a scaling factor is applied.
3. Apparatus as claimed in Claim 1 or 2 wherein the output of the converter is applied to a digital filter controlled by coefficient values to subject the digital signal to pre-emphasis before applying the signal to the deviation means.
4. Apparatus as claimed in Claim 1, wherein the adder/subtrac tor has a control signal applied to it so that its output is in vesti gia sideband form.
5. Apparatus as claimed in Claim 1, wherein the input signal is a video signal.
6. Apparatus substantially as hereinbefore described.
7. Apparatus substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8917531A GB2234652A (en) | 1989-08-01 | 1989-08-01 | Apparatus for generating digital wideband frequency modulated signals |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8917531A GB2234652A (en) | 1989-08-01 | 1989-08-01 | Apparatus for generating digital wideband frequency modulated signals |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB8917531D0 GB8917531D0 (en) | 1989-09-13 |
| GB2234652A true GB2234652A (en) | 1991-02-06 |
Family
ID=10660951
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8917531A Withdrawn GB2234652A (en) | 1989-08-01 | 1989-08-01 | Apparatus for generating digital wideband frequency modulated signals |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2234652A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0527660A1 (en) * | 1991-08-14 | 1993-02-17 | Nec Corporation | Digital FM modulator |
| US5497399A (en) * | 1992-09-10 | 1996-03-05 | Nec Corporation | Digital FM modulating apparatus |
| DE102005024169A1 (en) * | 2005-05-23 | 2006-11-30 | Astro Strobel Kommunikationssysteme Gmbh | A method of modulating a first baseband signal and a second baseband signal, modulator for performing the method, and using the modulator in a cable head location |
-
1989
- 1989-08-01 GB GB8917531A patent/GB2234652A/en not_active Withdrawn
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0527660A1 (en) * | 1991-08-14 | 1993-02-17 | Nec Corporation | Digital FM modulator |
| US5272454A (en) * | 1991-08-14 | 1993-12-21 | Nec Corporation | Digital FM modulator using direct digital synthesizer |
| US5497399A (en) * | 1992-09-10 | 1996-03-05 | Nec Corporation | Digital FM modulating apparatus |
| DE102005024169A1 (en) * | 2005-05-23 | 2006-11-30 | Astro Strobel Kommunikationssysteme Gmbh | A method of modulating a first baseband signal and a second baseband signal, modulator for performing the method, and using the modulator in a cable head location |
Also Published As
| Publication number | Publication date |
|---|---|
| GB8917531D0 (en) | 1989-09-13 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 730A | Proceeding under section 30 patents act 1977 | ||
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |