GB2332578A - Demodulators for television receivers - Google Patents

Abstract

A TV demodulator having an I/Q isochronous detector and characterised by the output from the detector being input to a complex filter 16 whereby a vestigial sideband (VSB) modulated signal is converted to baseband. A complex filter 16 including four signal paths is described (Fig 6).

Description

2332578 DEMODUIATORS FOR TELEVISION RECEIVERS

Field of the Invention

The present invention relates to demodulators for television receivers and is more particularly concerned with providing such a demodulator in integrated form.

BackgIound of the Invention In situations where the demodulator must be designed to handle frequencies in the range 30-40 MHz there is the problem that it is difficult to realise the necessary circuit in integrated form because at this frequency, the manufacturing technology is at or even beyond its limit in terms of providing satisfactory performance.

The main constraint is the realisation, in integrated form, of the channel filter associated with such a demodulator. - The limit in accuracy, in terms of matching, of R and C elements would lead to a realisation having numerous parameters to be tuned, hence a very complicated and costly solution.

FIG. 1 illustrates a prior art known conventional demodulator for a television receiver. A radio frequency signal from an aerial I is input at A to an image filter 2, which is a relatively coarse filter designed to separate the image frequencies. The filtered signal with a picture carrier fp is fed to an IF conversion multiplier 3, controlled by an oscillator 4, which inputs a signal frequency fo.

The output from 3 is fed to a channel filter 5 which is a very sharp, precise filter, including a Nyquist flank, designed to separate the desired channel from the adjacent channels. (See FIG. 2). With regard to FIG. 1, the demodulation of the VSB spectrum is illustrated in FIG. 2 in terms of the signal at the points A, B, C & D in FIG. 1.

The output from 5 is fed to another multiplier 6 (the video detector) for video demodulation, 6 having an input from an oscillator 7 which inputs the signal having a frequency fo-fp. The resulting composite video signal is output at 8.

This prior art circuit has many discrete components, especially for the filtering functions. - For example, the image filter 2 includes capacitors, inductors, varicaps and dual-gate Field Effect Transistors (FETs) and the channel filter 5 would typically be a surface acoustic wave (SAW) filter. The circuit as a whole, therefore, takes up a large surface area and is of relatively high cost.

The main objective of the present invention is to provide a circuit which integrates the function of the channel filter because this filter is one of the critical items in reducing the cost and size of the circuit shown in FIG. 1.

In attempting to integrate the function of the circuit of FIG. 1, one of the key problems is the channel filter. This is because this filter has to be "sharp" and precise, because of the abrupt band transistions necessary to separate the desired channel from the adjacent channels, while keeping a flat response in the passband. Typically, a video SAW filter has 40 dB attenuation from pass band to stopbands, within a 2 MHz frequency interval (f centre =- 37 MHz); passband ripples of ldB and 50ns for magnitude and group delay, respectively; and a 1.5MHz width Nyquist flank on the high edge of the passband. This is very difficult if not impossible to achieve with the current integrated circuit manufacturing technology because of the frequency range involved of 30 to 40 MHz.

Summajy of the Invention

According to the present invention there is provided a television demodulator having an l/Q isochronous detector and characterised by the output from the detector being input to a complex filter whereby a VSB modulated signal is converted to baseband.

In this way the filtering operation is transposed into the base band, where the frequency is only 0 - 5 MHz. To achieve filtering in this area a complex filter is employed. Although doing this creates a roblem because of the IQ system, it is solvable within the constraints of the manufacturing technology compared with the problem it replaces.

The IQ problem is that the cancellation of the unwanted signal of the lower adjacent channel which occurs between the I and Q paths is limited by the accuracy of the quadrature operations in both I/Q detector and complex filter.

The invention is applicable to IF and to Zero IF (or direct) conversion systems.

In the case of both an IF and a Zero IF system, the present invention obviates the need for a channel filter (typically a surface acoustic wave (SAW) filter) because channel filtering takes place in the base-band and in the case of Zero IF the need for an image filter is also obviated, since only the band filter is required for limiting the RF energy.

Brief Description of the Drawings

An exemplary embodiment of the invention will now be described with reference to the drawing in which: FIGs. 1 & 2 show a prior art circuit and its characteristics as previously 30 described; FIG. 3 shows a first embodiment of the present invention; FIG. 4 shows a second embodiment of the present invention; FIG. 5 is a very simplified illustration of the I/Q detector and complex filter arrangement of FIG.s 3 & 4;

FIG. 6 is a detailed illustration of a complex filter including the four signal paths; FIG. 7 illustrates the RF TV channel translated to base band; FIG. 8 illustrates the magnitude responses of H(f) and H'(f); FIG. 9 illustrates the magnitude responses of Hr(f) and Hi(f); FIG. 10 is derived from FIG. 5, where the Hr bloclj has been moved to the output of the adder; FIG. 11 illustrates the phase response of the Hi/Hr block in FIG. 10; FIG. 12 illustrates the magnitude response of the Hiffir block of FIG. 10; FIGs. 13a, 13b and 13c are vector diagrams of the IlQ signal components and in particular the DSB components, the VSB components and the lower channel components respectively.

Detailed Description of a Preferred Embodiment

Efforts have been made in the past few years to realise demodulator circuits with a high degree of integration to reduce- the number of external components such as front end tuned circuits and surface acoustic wave filters (SAW). Zero IF and low IF circuits using l/Q quadrature demodulation present attractive options in this context. These have already been applied to broadcast receivers and cellular telephones and they allow the RF pre-filtering requirements to be simplified and lead to the realisation of fully integrated base band or "quasi base band" (low IF) post filters.

This integration is possible because the channel band width is generally several times smaller than the channel spacing and consequently the desired channel can be extracted using reasonably low Q filters.

However, a different situation applies in the case of analogue TV transmissions, particularly those employing cable. In th!se situations, the bandwidth of the video channel is large compared with its channel spacing, typically in the proportion of about 75%. Furthermore, the asymmetrical spectrum shape of the channel around the picture carrier, due to the vestigial side band (VSB) modulation, requires the use of a Nyquist flank for the channel separation filter as discussed earlier.

Hence, a low IF method has already been proposed in which a judicious choice of the IF frequency (about equal to half of the channel bandwidth) enables the desired channel to be extracted using two low pass filters placed in the I and Q signal paths. In addition, the Nyquist flank on each filter response allows the correct spectrum re-construction in the IF to baseband conversion.

However, when it comes to TV receivers the introduction of a Zero IF system is not straightforward since the separation of the desired channel from the adjacent channel cannot be achieved by using standard lowpass filters.

The present invention overcomes this problem by using a complex filter with a Nyquist flank centred at the origin of the frequency axis, the passband of the filter fitting onto the upper side band of the desired channel while its stopband rejects the unwanted lower adjacent channel.

The present invention involves a TV demodulator system providing a direct RF to baseband or an IF to baseband conversion without the need of any additional SAW filter.

Referring to FIG. 3, there is shown a Zero IF (direct conversion) I/Q demodulator embodying the present invention.

The incoming RF signal is indicated at 1 which then passes through a bandpass filter 9 roughly tuned to a given frequency range including the desired TV channel. Such a filter is intended to be adjustable to different bands of frequency in order to cover the whole spectrum of TV channels to be received.

The output signal from the filter 9 contains the desired channel component with a picture carrier fp. This signal is fed to either two multipliers or two mixers, 10 and 11, which are driven in quadrature by pilot signals Si and Sq, of frequency fp, issued from an oscillator 12.

In the case where multipliers are used, Si and Sq are sinusoidal and Si is isochronous (i.e. exactly in phase) to the picture carrier fp. In this case, the role of the front end filter 9 is just to limit the energy of unwanted signals around the desired channel, which then relaxes the dynamic range requirements of the following stages.

The choice of mixers rather than multipliers leads to an easier implementation with respect to the dynamic range just mentioned above.

However, in this case, the front end filter 9 must be sufficiently selective to reject high frequency signal components laying around Up, 5fp... etc., in order to prevent those components beating the odd harmonies of the square waves which would produce undesired signals in the baseband.

The function of the complex filter 16 is to perform a good rejection of the lower adjacent channel components whilst preserving correct phase and magnitude characteristics within the demodulated channel. The way in which it operates will be described later with reference to FIG.s 5 - 13.

The output from the complex filter 16 is a composite video signal which may or may not include the sound channel, depending on the conversion system planned for the sound.

If the output signal does include the sound, the latter can be demodulated after passing through a narrow bandpass filter which removes the video part of the signal, whilst the video signal has to pass through a notch filter removing the sound before feeding the video decoder.

If the output signal does not include the sound, the lowpass filter function embedded in the complex filter directly removes the sound signal from the video, then no additional notch filter is required. However, in this case, a separate sound conversion system must be used. The description of such a sound system, for which different solutions exist, is beyond the scope of invention.

Referring now also to FIGA, there is shown an IF demodulator, those parts of the circuit which are the same as shown in FIG. 3 being indicated by the 10 dotted boxes A and B in both FIGs.

The only difference is that the front end where, instead of the band selection filter 9, there is an image filter 19 followed by an IF conversion stage 20 consisting of an oscillator 21 and a multiplier 22.

In the circuits of both FIG.s 3 and 4 the SAW filter 5 of FIG. 1 has been dispensed with and in addition the image filter to FIG. 1 has been dispensed with in the circuit of FIG. 3.

In FIG.s 5 and 6, those items which correspond to items already illustrated in FIG.s 1 and 2 have been given the same reference numerals.

Referring to FIG. 5, the RF signal u(t) is applied to the inputs of the two multipliers 10 and 11 which are driven in quadrature by two pilot signals 25 cos (wpt) and sin (wpt).

The first signal, cos (wpt) is phase locked to the picture carrier fp of the desired TV channel and thus the pair of multipliers 10 and 11 effects an I/Q isochronous detection of the modulated desired signal.

The outputs of the two multipliers are two signals vl(t) and v2(t) which are fed to the complex filter 16 and in particular form inputs to a twopath transfer function, Hr and Hi, the outputs of which are fed to an adder 23.

The structure indicated at 16 in broken lines is in fact only a half complex filter (for ease of illustration) in a real arithmetic form.

The whole complex filter is shown in FIG. 6.

The complex filter 16 acts as a bandpass transfer function whose magnitude response 1 H(f) 1 is represented in FIG. 7. This response exhibits a Nyquist flank centred on the origin of the frequency axis and extending at +A.75MHz from this origin. It should be noted that usually, (but not always) the term "Nyquist flank" applies to a linear phase filter and consequently in this case the half value of the magnitude occurs exactly in the middle of the flank.

The high frequency cut off of the filter is situated at about 5.5 MHz which corresponds to the frequency offset of the sound carrier relative to the picture carrier. This corresponds to the case mentioned above, where the sound channel is included in the output signal.

The desired RF TV channel, translated to baseband, is superimposed in dotted lines in FIG. 7.

The way in which the RF to baseband conversion is effected will now be described with reference to FIGs. 8 - 13.

By using a complex signal approach it is possible to explain the reconstruction of the entire base band of the desired channel by the filter structure. A real signal interpretation is carried out below by observation of magnitude and phase of signal components in the system of FIG. 5.

Firstly, the transfer function of the complex filter mentioned above can be written, using the real filter responses HrGw) and Hiw) Hw) = Hrow) + jHiow) Then, if we consider the complex image filter defined as EGw)= conjugatel H(-jw) = HrClw)-jHiGw) (1) (2) we can express the reciprocal relationships of Hr(jw) and HiGw) in terms of the function of H(Jw) and RGw); from (1) and (2) we get:

Hrow) 1How) + Sow) 1/ 2 and: Hiow) 1How) - Row)) / 2j (3) (4) The magnitudes of H(f), H'(f) and Hr(f), Hi(f) are represented in FIGs 8 and 9, respectively. Due to the Nyquist flanks on both fimctions H(f) and H'(f), it turns out that Hr is a flat lowpass filter response. In addition, I Hr(f) I and I Hi(f) I have the same behaviour within the frequency ranges I -5.5MHz to 0.75MHz I and I 0.75MHz to 5.5MHz 1. Only the positive part of the frequency axis will be considered subsequently.

The system of FIG. 5 can be then transformed to that of FIG. 10, by extraction of the filter block Hr from both I and Q signal paths, moving that block into the output path, after the adder. The base band reconstruction of the desired channel and the rejection of the adjacent channels can be thus understood as follows:

a) First of all, as show-n above, Hr(f) is a flat lowpass response with a cutoff at 5.5MHz. Therefore, after down conversion by the I/Q detector, any RF component from the upper unwanted channels is rejected by that filter. In the same way, the lower channel components situated below fp-5.5MHz are also rejected by the Hr filter.

b) Secondly, by observation of relations (3) and (4), it turns out that the remaining block Hi/Hr in the Q path acts as a wide band -90deg phase shifter, with a unity gain, valid from 0.75MHz to beyond 5.5MHz (FIGs 11 and 12). Then, the whole demodulator system behaves differently for the two frequency ranges, 0 to 0.75MHz and 0.75MHz to 5.5MHz, of the base band: - From zero to 0.75MHz: the DSB part of the input spectrum contains symmetrical frequency components, (m/2) cos [ (wp+wl) t + a] and (m/2)cos [ (wp-wl) t - a], in the upper and lower side bands, respectively.

These side band signals, converted by the isochronous I/Q detector, are represented in a vector diagram in FIG. 13a: - two identical (superposed) base band components (m/2)cos(wlt + a) in the I signal path; - two opposite phase components +/-(m/2)sin(wlt + a) which cancel each other in the Q-path.

Thus, the base band conversion, in this frequency range, is entirely achieved through the I-path and the output Hr filter, resulting in frequency components of type:

mcos(wlt + a - d) (5) where "d" is the additional phase shift produced by the Hr filter.

Between 0.75MHz to 5.5MHz: the VSB part of the input spectrum contains asymmetrical frequency components, (m/2)cos[(wp+wl)t + a] and k(m/2)cos[(wp-wl)t -a + bl, in the upper and lower side bands, respectively. In addition, the upper region of the lower adjacent channel will produce undesired base band signals. Signal components of this channel are of type: Acos [(wp-wa)t + cl. Then, in base band, the conjugate operation of the isochronous l/Q detector and the -90deg phase shifter allows both the complete reconstruction of the wanted signal and the rejection of the adjacent channel part (see vector diagrams in FIGs. 13b and 13 c):

- the upper side band of the input spectrum yields base band components of types (m/2)cos(wlt + a) and -(m/2)sin(wlt + a), in I and Q signal paths, respectively. Those components have the same magnitude, but Q signal part is phase shifted by +90deg with respect to I signal part. Therefore, after a -90deg rotation through HiMr block, the Q -signal is added in phase to the I signal, which yields the whole base band part at the system output as:

mcos(wlt + a - d) (6) - the lower side band of the input spectrum yields base band components of types k(m/2)cos(wlt + a - b) and +k(m/2)sin(wlt + a - b), in I and Q signal paths, respectively. Those components have the same magnitude, but Q signal part is phase shifted by -90deg with respect to I signal part; then, after an additional -90deg phase shift through HiMr block for the Q part, I and Q signals cancel each other at the adder output; - similarly to the latter case, the upper region of the lower adjacent channel also produces base band components of types (A/2)cos(wat - c) and + (A/2)sin(wat - c), in I and Q signal paths, respectively. Those components have the same magnitude, but Q signal part is phase shifted by -90deg with respect to I signal part; again these contributions are cancelled by the conjugate effect of the -90deg phase shifter and the adder.

The Zero IF system described earlier is specially dedicated to the demodulation of VSB signals like standard RF TV transmission channels.

The conjugate actions of an isochronous I/Q detector and a half complex filter with an appropriate Nyquist flank allows the correct base band reconstitution of the desired channel and the rejection of all undesired components of the other channels.

The realisation of full integrated complex filters and the small number of external components make the arrangement according to the present invention preferable from a cost point of view to the prior art demodulators because in the zero IF system there are no image filter components or IF 5 SAW filter, and in the IF system there is no IF SAW filter.

It will be appreciated that alternative embodiments to the one described above are possible. For example, the precise configurations of the circuits and the precise values of the components may differ from those described 10 above.

Claims (6)

Claims

1. A television demodulator having an I/Q isochronous detector and characterised by the output from the detector being input to a complex filter whereby a VSB modulated signal is converted to baseband.

2. A demodulator as claimed in claim 1 which is a Zero IF demodulator, whereby a direct RF to baseband conversion is achieved.

3. A demodulator as claimed in claim 1 which is an IF demodulator, whereby the IF to baseband conversion is achieved.

4. A TV demodulator as herein described with reference to and shown in FIGs. 3 & 5 - 13 of the accompanying drawings.

5. A TV demodulator substantially as hereinbefore described with reference to and as shown in FIGs. 4 - 13 of the accompanying drawings.

6. A TV demodulator as claimed in any previous claim in which at least the complex filter is manufactured as an integrated circuit.