IE45167B1 - Improvements in or relating to television signal transmission systems - Google Patents

Abstract

The transmission is intended to be carried out on transmission paths with a baseband input and output, in particular on coaxial cable paths, microwave radio paths and laser transmission paths. An RF-baseband converter with a mixing stage (3) is in this case connected upstream of the baseband input (6) of the transmission path (7). A conversion of the TV signal to the frequency range of the baseband is performed in said mixing stage, the TV signal in the baseband having a non-zero minimum frequency spacing from the lower baseband limit. An inverse-operating baseband-RF converter (9) is connected downstream of the baseband output (8) of the transmission path. Modulation/demodulation and the associated disadvantages can be avoided by means of this circuit arrangement.

Description

^^^.s^nvention relates to a television signal transmission system comprising a transmission link, e.g. a coaxial cable, radio relay, or laser transmission link, having a base-band input and a base-band output.

Conventionally, the directional transmission of television signals (video and audio signals) is carried out on post office transmission links complying with international CCITT standards. The television signal Ϊ3 normally available in a high-frequency (HF) modulated form and must be converted to a base band. This is conventionally achieved by modulation which involves distortion and intermodulition phenomena between the video and audio signals. As in modulation processes it is necessary to represent the image content down to fhe lowest occurring frequencies, virtually to the frequency aero, it is necessary to employ bulky components. A further disadvantage of modulation processes is that tha black-level value must be constantly corrected. Furthermore, if it is necessary to employ modulation several times during the course of a transmission link, it is necessary to regenerate the residual side band in order to obtain the ilyquist flank.

In accordance with CCIR standards, the audio signal must possess a predetermined frequency spacing from the video signal. If this spacing does not 4510? exist in the presented signal, for purposes of demodulation the audio signal must he separated from the video signal by means of a branching circuit being subsequently brought into the correct frequency relationship This invention seeks to provide an improved television signal transmission system comprising a transmission link having a base-band input and a baseband output in which modulation and demodulation and their diaadvantavea are avoided.

According to this invention there is provided a television signal transmitter for use in a carrier transmission link for receiving an HF modulated television signal and transmitting it to a desired location said transmitter comprising a carrier transmission transmitter having a base-band input, an HF to baseband converter connected to.· so as to supply a baseband signal to, the base-banc input ar.d including a iu.xer arranged for mixing < received HF modulated television signal with an auxiliary frequency so as to displace the tel«vi«.i ··;. signal band such that fche video carrier of the television signal has a predetermined non-zero frequency spacing from zero and the television signal band so positioned in frequency provides a base-band for the carrier transmission transmitter whereby the desired television signal band can be transmitted.

According to a second aspect of this invention there is provided a television signal receiver for use with a transmitter according to said first aspect, said receiver comprising a carrier transmission receiver having a base-band output connected to a base-band to HF converter which includes a mixer arranged for mixing the base-band of a received television signal modulated carrier v/ith an auxiliary frequency so as to produce an HF modulated television signal.

Such a system enables the modulated HP signal to bs converted to as low a frequency as possible (to form the base-band for the carrier transmission link) while avoiding any demodulation, and in the case of the restoration of the HF-signal, avoiding any modulation. Furthermore a greater degree of flexibility of the system is achieved as it is also possible to employ carrier transmission base-band inputs (lower cut-off frequency of SO IcHs). Furthermore it is possible to carry out transmission on existing video-cables with improved quality. The positioning of the television signal band wit': the minimum frequency spacing from sere can avoid LF-interference (mains hum, audio-frequency power line carrier control signals, etc.) affecting the video content.

The invention will be further understood from the following description by way of example of an embodiment thereof v/ith reference to the accompanying drawings v/hich schematically illustrates a television transmission system comprising a transmission link having a base-band input and a base-band output, via v/hich a television sxgnal which is available in HF modulated form can be transmitted.

From a receiver 1, the HF modulated television signal is fed to an HF to base-band converter 2, v/hich contains a mixer stage 3 by means of whieh the HF modulated television signal is mixed with an auxiliary frequency obtained from an oscillator 4. 451β? - 5 The mixed product is fed to a filter 5, whose output signal is forwarded to the base-band input 6 of a carrier transmission link, e.g. a radio relay link 7.

At the end of the radio relay link 7 there is a base-band output 8 whose output signal is fed to a base-band to HF converter 9 which is constructed similarly to the HF to base-band converter 2. The converter 9 contains a mixer stage 10 which·, with a frequency supplied by an oscillator 11 regain^ the HF modulated signal, which is output-coupled by a filter 12 tc a following device 13.

The auxiliary frequency produced in the oscillator 4 is selected to be such that, together with the carrier frequency of the HF modulated television signal, it forms a video-carrier frequency which, for example in the normal situation {video-carrier frequency^audiocarrier frequency) lies between 1.25 MHz and 2.5 MHz when using a carrier tranes.-’ ssion system for 960 speech channels.

By means of the converters 2 and 9 a transmission link having a base-band input and a basa-band output can be used for the high-quality direct transmission of HF modulated television signals, without the need to convert the broadcasting standard. This is particularly advantageous in che case cf long distance transmission between large party antenna installations, in magnetic re-recording, and in connections between television studios and transmitters.

Claims (5)

1. c l a ι a s 8 1. A television signal transmitter for use in a carrier transmission link for receiving an HF modulated television signal and transmitting it to a desired 5 location, said transmitter comprising a carrier transmission transmitter having a base-band input, an HF to base-band converter connected to, so as to supply a base-band signal to, the base-band input and including a mixer arranged for mixi.ng a received HF modulated 10 television signal with sn auxiliary frequency so as to displace the television signal band such that the video carrier of the television signal has a predetermined non-zero frequency spacing from zero and the television signal band so positioned in frequency 15 provides a base-band for the carrier transmission transmitter whereby the desired television signal band can be transmitted.

2. A television signal transmitter for use in a carrier transmission link for receiving an HF modul20 ated television signal and transmitting it to a desired location, said transmitter being substantially as described herein with reference to the accompanying drawing.

3. A television signal receiver for use with a 25 transmitter according to claim 1 or claim 2, eaid receiver comprising a carrier transmission receiver having a base-band output connected to base-band to HF converter which includes a mixer arranged for mixing the base-band of a received television signal modulated 30 carrier with an auxiliary frequency so as to produce an HF modulated television signal. - 7

4. A television signal receiver for use with a transmitter according to claim 1 or claim 2, said receiver being substantially as described herein with reference to the accompanying drawing.

5. 5. A television signal transmission system including a transmitter according to claim 1 or claim 2 and a receiver according to claim 3 or claim 4 arranged at respective opposite ends of a carrier transmission link.