DE4102834A1 - Balancing arrangement for min. cross=talk in stereo decoder - synchronises phase of local oscillator using pilot tone filtered from auxiliary carrier of stereo multiplex signal - Google Patents

Abstract

From the input stereo multiplex signal (MPX) a 19 kHz pilot tone is extracted (2) and phase-shifted (8) for comparison (3) with outptut of the local oscillator (4) and two-stage frequency divider (5, 6). In one possible phase comparison control scheme a calibration signal is switched (13, 14) simultaneously to inputs of the pilot filter (2) and local oscillator (4). Alternatively a stereo multiplex calibration signal may be injected into the input, and crosstalk attentuation balanced to max., or mid-band filter frequency balanced to local min. USE/ADVANTAGE - Esp. for high precision stero measuring decoder. Channels are balanced automatically by phase setting of pilot tone filter branch.

Description

The invention relates to an arrangement for adjusting a Stereo decoders according to the preamble of the main claim.

With precision stereo measuring decoders it is important that the crosstalk attenuation between the left and right Output channel is maximum. Through aging and through Temperature response of the bandpass filter to filter out the Pilot tone signal of 19 kHz, for phase synchronization with the local oscillator on the decoder side is the The phase error of this bandpass filter is not always exact 0 °, then disturbing crosstalk occurs. It is also known, to adjust the phase error in the pilot ton filter branch to exactly 0 ° after the filter one of Hand adjustable phase shifters to arrange by on feed a multiplex calibration signal with optimal Crosstalk attenuation (it is only the right one or left channel of the multiplex signal modulated with 1 kHz) can be done by manually adjusting this phase shifter for minimal crosstalk between left and  right, right and left channel can be set (Stereo measuring decoder MSDC2 from Rohde & Schwarz). This manual setting of crosstalk attenuation a stereo decoder is relatively complex.

It is therefore an object of the invention to provide an arrangement create with which this alignment of a stereo decoder is simplified to minimal crosstalk.

This task is based on an order Preamble of the main claim characterized by its characteristics solved. Advantageous further training he give themselves from the subclaims.

The adjustment takes place in the arrangement according to the invention for minimal crosstalk fully automatically using a Control circuit by adjusting the phase accordingly in the pilot tone filter branch, whereby as a control variable for the Control loop one of the measured variables according to characteristics a), b) or c) of the main claim is suitable. With the Alternative according to feature a), the phase error of the Pilot tone filter branch automatically corrected to exactly 0 ° be, with possible phase errors in the phase syn chronization control loop are neglected. At the alternative according to feature b) is the sum of all Corrected phase errors, since the over speech attenuation itself is used. With that, too small phase errors in the synchronization control loop of the decoder can be corrected with. The alternative after Feature c) takes advantage of the knowledge that with sufficient supercritically coupled bandpass filters in the middle frequency the pass curve has a local minimum and at the same time the phase at this center frequency  is either exactly 0 ° like this for active ones, for example Bandpass filter applies or has a multiple of 90 °. By shifting the center frequency and adjusting the Filter pass curve to the local minimum can thus Phase in the pilot tone filter circuit can be set exactly.

In the alignment arrangement according to feature b), this can Stereo multiplex calibration signal either from a ge suitable stereo encoder from the outside in the calibrated Stereo decoder can be fed or there may be if there is a corresponding potash in the decoder itself Briersignalquelle be arranged for calibration purposes can be switched on at the input of the decoder. In the latter The adjustment according to feature b) can even be fully automatic run mathematically, because then via the internal tax pro processor also alternately either the calibration signal with modulation only in the right or only in the left channel can be fed.

The invention will now be described more schematically Drawings explained in more detail using exemplary embodiments.

Fig. 1 shows the schematic diagram of a conventional precision stereo measuring decoder, as used for decoding stereo multiplex signals for measurement purposes. The multiplex signal is fed in at the input E of the actual decoder 1 , the decoded output signals are available on the left channel L or right channel R for further evaluation. The pilot tone of 19 kHz is filtered out of the stereo multiplex signal MPX via a bandpass filter 2, the pilot tone signal of 19 kHz is compared in phase in a phase detector 3 with a 19 kHz signal which is obtained from a local oscillator 4 of frequency 2N 19 kHz is initially generated locally by division by N in a frequency divider 5 and further division by 2 in a frequency divider 6 . By phase comparison of the divided output frequency of the oscillator 4 with the phase of the filtered pilot tone signal in the phase detector 3 , the 38 kHz subcarrier signal for the decoder 1 is recovered in the correct phase, and the oscillator 4 is regulated by the integrator 7 .

According to the invention, a device for adjusting the phase is provided in the pilot tone filter branch, namely either a phase shifter 8 is arranged at the output of the filter, which can be electronically adjusted electronically via a D / A converter 9 and a control processor 10 , or the band filter 2 is itself electronically adjustable in its phase profile in a manner known per se, as is indicated schematically by the dashed-line circuit 11 .

For the automatic and electronically controlled Pha There is the following adjustment of the pilot tone filter circuit three options:

• a) At the input 12 of the bandpass filter 2 , in the "kal" position, the switches 13 and 14 from the local oscillator 4 , which is then no longer phase-controlled, are fed a calibration signal of 19 kHz via the additional frequency divider 15 . At the output of the phase detector 3 , an additional calibration detector 16 is switched on, this regulates via the control processor 10 and the D / A converter 9 either the phase shifter 8 or the phase error of the filter 2 to the phase difference of 0 ° at the output of the phase detector 3 , whereby possible phase errors by frequency dividers 5 , 6 and 15 can be assumed to be negligible. In this way, the phase error between filter input 12 and output 26 of the pilot tone filter branch is set to zero and thus minimal talk over between L and R is achieved, because in the subsequent operating switch position (phase locked loop closed) the 38 kHz subcarrier for the Decoder is recovered exactly in phase.
• b) Another way to set the phase error in the pilot tone filter branch to exactly 0 ° is to feed a stereo multiplex calibration signal with optimal crosstalk attenuation at input E of the decoder and by measuring the output voltage in channels L and R in each case to regulate minimal talk, as indicated by the control circuit 17 . For example, a calibration signal is fed in at input E, which is fully modulated at 1 kHz from the right and left channels. When feeding in the right-hand modulated calibration signal at input E, only the voltage detector 18 in the left channel L is evaluated via the control processor 10 and by changing the phase in the pilot tone filter branch (via phase shifter 8 or in filter 2 ) it is regulated until the Voltage in left channel L is a minimum. The corresponding phase setting is stored in the processor. Then only the calibration signal modulated in the left channel is fed in and then only measured via the voltage detector 19 in the right channel R and this channel is also adjusted to the minimum voltage. The control processor selects an average value from these measured values determined in successive measurement cycles, which results in a favorable compromise for crosstalk between L and R and R and L.
• The calibration signal can either be fed in from an external stereo encoder at input E, but it is also possible to arrange an internal calibration signal source 20 directly in the decoder itself, which supplies this calibration signal and which is preferably controlled directly via the control processor 10 in such a way that the automatically one after the other Only left or right modulated calibration signals mentioned can be fed.
• c) Another control possibility arises from the relationship between the transmission curve 21 of a customary via critically coupled bandpass filter with the associated phase profile 22 , as is shown schematically in FIG. 2. With active filters, for example, the phase between the input and output of the filter at the center frequency f _{m is} exactly 0 °, with conventional passive filters there is an exact phase shift of M · 90 ° at the center frequency, where M = 1, 2, 3. . . is. This relationship can be used for another type of control of the phase curve in the pilot tone filter branch exploited to exactly 0 ° phase shift who to by an electronically adjustable in its center frequency and thus also in its passband pilot-tone filter 2 is used, which via the D / A- Converter 9 and the control processor 10 is precisely tuned to its center frequency as a function of an attenuation measurement on the filter 2 . For this purpose, the knowledge that the filter transmission curve 21 has a local minimum 23 at the center frequency f _m is used as a control criterion. By a damping measurement by means of a level detector 24 at the output of the filter 2 of this band-pass filter can 2 via the control loop 25, 10, 9, 11 precisely regulated at the local minimum 23, the bandpass filter 2 is then exactly to its center frequency f _m tuned and an active The filter has exactly the phase 0 ° or a passive filter exactly the phase 90 ° or a multiple M thereof. In the latter case, only a corresponding 90 ° phase shift in the pilot tone branch is then required, which can either be achieved by an additional phase shifter in the pilot tone branch or simply by swapping channels R and L at the output of decoder 1 , since a 90 ° phase shift in the pilot tone branch corresponds to a phase rotation of 180 ° of the 38 kHz subcarrier.

All three measures have in common that the crosstalk attenuation of the decoded stereo signals in the output channels L and R by regulating the phase in the pilot tone filter branch to exactly 0 ° phase error a maximum is set, the crosstalk attenuation So it becomes a minimum, this being through a control loop takes place fully automatically.

Claims (6)

1. Arrangement for adjusting a stereo decoder for minimal crosstalk between the left and right output channels, in which the pilot tone is filtered out from the subcarrier of the stereo multiplex signal by means of a bandpass filter and thus a local oscillator is phase-synchronized by adjusting the phase in the pilot tone filter branch by means of a phase adjustment device , characterized in that, using an electronically adjustable phase setting device in the pilot tone filter branch by means of a control loop, the phase can be set automatically, specifically

• a) by feeding a calibration signal with the frequency of the pilot tone (19 kHz) directly at the input ( 12 ) of the pilot tone bandpass filter ( 2 ) and adjusting the phase error between input ( 12 ) and output ( 16 ) of the pilot tone filter branch to zero or
• b) by feeding a stereo multiplex calibration signal at the input of the decoder ( 1 ) and adjusting the crosstalk attenuation measured to a maximum, or
• c) when using a pilot tone bandpass filter ( 2 ), whose pass curve ( 21 ) has a local minimum ( 23 ) at the center frequency (f _m ) and a phase error of M · 90 ° (M = 0, 1, 2, 3. .) between input and output, by feeding any stereo multiplex signal at the input (E) of the decoder ( 1 ) and adjusting the center frequency (f _m ) of the bandpass filter ( 2 ) to the local minimum ( 23 ).

2. Arrangement according to claim 1, characterized in that the electronically adjustable phase adjustment device is an electronically adjustable phase shifter ( 8 ) arranged at the output of the pilot tone bandpass filter ( 2 ). 3. Arrangement according to claim 1 or 2, characterized in that the pilot tone bandpass filter ( 2 ) in its phase (feature a) or b) according to claim 1) or in its center frequency (feature c) according to claim 1) electronically is adjustable. 4. Arrangement according to claim 1, feature a), or one of claims 2 and 3, characterized in that the phase-locked loop ( 5 , 6 , 3 , 7 ) of the local oscillator ( 4 ) can be separated (switch 14 ) and the output signal of Local oscillator ( 4 ) is used to generate the calibration signal according to feature a) (frequency divider 15 , switch 13 ). 5. Arrangement according to claim 1, feature b) or one of the preceding claims, characterized in that as a stereo multiplex Kali briersignal a only in one channel (left or right) modulated multiplex signal at the input of the decoder ( 1 ) is fed and the control variable of the control circuit is the voltage of the other channel (right or left) of the decoder, the phase in the pilot tone filter branch being adjusted to a minimum of the measured variable in the other channel. 6. Arrangement according to one of the preceding claims, characterized in that the calibration signal according to feature b) is generated locally (internal calibration source 20 ).