Separation of L+R from L-R in BTSC system

Abstract: A stereo signal separation system for separating the L+R and L-R components of a BTSC stereo signal mixes the BTSC signal with a multiple of the pilot frequency. To recover the L+R component the mixer output is combined with the input BTSC signal, and to recover the L-R component the mixer output is combined with a second mixer output shifted ninety degrees. The combined output is low pass filtered to recover the desired component.

Description: Background of the Invention The present invention relates to signal separation circuits, and more particularly to a circuit for separating the L+R and L-R components of stereo audio in the BTSC system. The BTSC system defines a main audio L+R channel up to 15 kHz, a pilot signal at the horizontal sweep frequency of 15.734 kHz, and a double side band, suppressed carrier stereo subchannel symmetrically about a frequence which is twice the pilot signal frequency. To design lowpass filters for this system requires a filter performance several orders of magnitude beyond performance adequate for an FM stereo system. The guardbands between the main channel, the pilot signal and the subchannel are much narrower, and a dbx noise reduction compressor is placed in the subchannel, increasing the potential for subchannel to main channel crosstalk. The guardbands between the main channel and the pilot signal and between the pilot signal and the subchannel are only 734 Hz each, with the separation between the main channel and the subchannel being 1.468 kHZ. Thus, a significant factor in channel separation is the subchannel to main channel crosstalk. Subchannel to main channel crosstalk occurs when the lower sideband of the subchannel leaks into the main channel due to inadequate lowpass filtering of the subchannel audio. This crosstalk is nonlinear, i.e., it is highly offensive to the ear because it is not harmonically related to the main channel signals. Additionally when the signal levels are low but the program material contains substantial L-R content, the dbx noise reduction compressor can cause subchannel levels to be 20-30 dB higher than main channel levels. When this occurs the main channel has negligible ability to psychoacoustically mask the crosstalk. Further the greatest gain is likely to be produced at high frequencies -- the very frequencies that appear at the edge of the lower sideband and which are most likely to cause audible crosstalk. High performance filters which are adequate for the BTSC system are quite complex, one such having as many as 29 poles of filtering overall, are expensive and require great stability. If the filter is too abrupt at cutoff, the result is ringing coming out of the filter. US-A-4,502,148 discloses an FM stereo demodulator in which an in-phase stereo FM pulse signal is input to a mixer, to whose other input is applied an inverted subcarrier signal at twice the pilot frequency. An inverted version of the FM signal is applied to a second mixer, to whose other input an in-phase version of the subcarrier frequency is applied. The outputs of the mixers are summed to provide a stereo channel signal. The demodulator of this prior art document is not suitable for BTSC signals. What is desired is a stable, simple circuit for separating the L+R and L-R components of the BTSC signal while still eliminating crosstalk between the main channel and the subchannel. Summary of the Invention According to a first aspect of the present invention there is provided a stereo signal separation system comprising: a mixer for mixing an input stereo signal, the input stereo signal having a lower frequency first component, an upper frequency double sideband component and a pilot signal at a frequency between the two components, with a reference frequency that is a multiple of the pilot signal such that the output of the mixer comprises the first component shifted in frequency above the second component; summing means for summing the output of the mixer with a signal derived from the input stereo signal to eliminate the second component; and filter means for filtering the output of the summing means to recover the first component. According to a second aspect of the present invention there is provided a stereo signal separation system comprising: mixing means having a pair of mixers for mixing an input stereo signal, the input stereo signal having a lower frequency first component, an upper frequency double sideband second component and a pilot signal at a frequency between the two components, with a reference frequency applied in quadrature to the pair of mixers, the reference frequency being a multiple of the pilot signal frequency such that the output of one mixer comprises the first component shifted in frequency above the pilot frequency and the second component shifted in frequency below the pilot frequency, and the output of the other mixer has the first component shifted above the pilot frequency; summing means for summing the outputs from the mixing means to eliminate the first component; and filter means for filtering the output of the summing means to recover the second component. Thus L+R or L-R components of a BTSC signal may be separated by mixing the components with a given multiple of the pilot signal frequency and then adding or subtracting the resultant signals. To recover the L+R component the BTSC signal is low pass filtered to obtain both components (L+R and L-R), the components are mixed with a signal four times the pilot signal frequency, and then subtracted from the unmixed, low pass filtered BTSC signal. To obtain the L-R component the BTSC signal is mixed with an unshifted subcarrier frequency and with a 90 degree phase shifted subcarrier frequency, the output of the shifted mixer being shifted an additional 90 degrees, and the outputs of the mixers then being either added or subtracted depending upon the direction of the additional phase shift. The objects, advantages and other novel features of the present invention will be apparent from the following detailed description when read in conjunction with the appended claims and attached drawing. Brief Description of the Drawing Fig. 1 is a frequency domain view of a portion of the BTSC stereo signal. Fig. 2 is a block diagram view of a circuit for obtaining the L+R component of the BTSC stereo signal according to the present invention. Fig. 3 is a frequency domain view illustrating the operation of the circuit of Fig. 2. Fig. 4 is a block diagram view of a circuit for obtaining the L-R component of the BTSC stereo signal according to the present invention. Fig. 5 is a frequency domain view illustrating the operation of the circuit of Fig. 4. Description of the Preferred Embodiment Fig. 1 illustrates a portion of the BTSC stereo signal having an L+R component; a double sideband, suppressed carrier L-R component; and a pilot signal. The respective signal deviations are 25, 50 and 5 kHz. The second program channel (SAP) and the professional channel are omitted as not involved in the present invention. As is apparent from this frequency domain view any filtering to separate L+R from L-R is extremely difficult and would require a very precise, and hence very complex and expensive, filter. Referring now to Fig. 2 the BTSC stereo signal is input to a first low pass filter (LPF) 10 which has a bandwidth sufficient to encompass both the L+R and L-R components. The filtered output from the LPF 10 is input to a mixer 12 and a summer 14. The second input to the mixer 12 is a frequency 4f H which is four times the pilot frequency f H . Since the L-R component is symmetrical about the suppressed carrier frequency 2f H , i.e., the lower band is the mirror image of the upper band, as illustrated in Fig. 3 the difference frequencies shift the L+R component above the L-R component while leaving the L-R component essentially unchanged. The sum and difference outputs from the mixer 12 are input to the summer 14, the difference outputs being the only significant outputs. The summer 14 subtracts the output 3(a) from the mixer 12 from the input 3(b) to the mixer to produce an output 3(c) which contains only the original L+R component and a frequency shifted L+R component. The output from the summer 14 is input to a second low pass filter 16 to recover the original L+R component. Alternatively the L-R component may be recovered as shown in Fig. 4. The BTSC stereo signal is input to two mixers 20 and 22. The second input to the mixers 20 and 22 is the subcarrier frequency 2f H in phase quadrature, i.e., identical frequencies shifted 90 degrees from each other. As illustrated in Fig. 5 the output 5(a) of the inphase mixer 20 within the portion of the frequency domain of interest results in the L-R component being shifted down to below the pilot frequency and the L+R component being shifted above the pilot frequency. The output 5(b) of the phase shifted mixer 22 has only the L+R component shifted above the pilot frequency, the L-R component having been cancelled due to the phase shift and the symmetry of the upper and lower bands. The output of the phase shifted mixer 22 is passed through a 90 degree phase shifter 24 so that the L+R component is either in phase or 180 degrees out of phase with the L+R component from the inphase mixer 20. The resulting outputs from the mixers 20 and 22 are input to a summer 26, which either adds or subtracts the two signals depending upon the direction of the 90 degree phase shift by the phase shifter 24. The resulting output 5(c) contains only the L-R component which can easily be obtained at this point by a low pass filter 28. Thus the present invention provides a circuit for separating the L+R and L-R components of a BTSC stereo signal by mixing the BTSC signal with a multiple of the pilot signal and then summing the results, requiring only simple low pass filters in the circuit.

Claims: A stereo signal separation system comprising: a mixer (12) for mixing an input stereo signal, the input stereo signal having a lower frequency first component, an upper frequency double sideband component and a pilot signal at a frequency between the two components, with a reference frequency that is a multiple of the pilot signal such that the output of the mixer comprises the first component shifted in frequency above the second component; summing means (14) for summing the output of the mixer (12) with a signal derived from the input stereo signal to eliminate the second component; and filter means (16) for filtering the output of the summing means (14) to recover the first component. The system as recited in claim 1 wherein the reference frequency is four times the pilot signal frequency. The system as recited in claim 2 wherein the summing means comprises a summing circuit (14) having as inputs the output of the mixer and the input stereo signal such that the second component is eliminated and the first component is recovered by the filter means. A stereo signal separation system comprising: mixing means having a pair of mixers (20, 22) for mixing an input stereo signal, the input stereo signal having a lower frequency first component, an upper frequency double sideband second component and a pilot signal at a frequency between the two components, with a reference frequency applied in quadrature to the pair of mixers (20, 22), the reference frequency being a multiple of the pilot signal frequency such that the output of one mixer comprises the first component shifted in frequency above the pilot frequency and the second component shifted in frequency below the pilot frequency, and the output of the other mixer has the first component shifted above the pilot frequency; summing means for summing the outputs from the mixing means to eliminate the first component; and filter means for filtering the output of the summing means to recover the second component. The system as recited in claim 4 wherein the reference frequency is twice the pilot signal frequency. The system as recited in claim 5 wherein the mixing means further comprises a phase shifter to shift the output of one of the pair of mixers ninety degrees prior to input to the summing means. The system as recited in claim 6 wherein the summing means comprises a summing circuit having as inputs the output of the phase shifter and the output of the other one of the pair of mixers, the summing circuit combining these two inputs to eliminate the first component so that the second component is recovered by the filter means.

Inventors: ZINK SCOTT EUGENE, ZINK, SCOTT EUGENE
Applicants: TEKTRONIX INC, TEKTRONIX, INC.
Application year: 1987
Main classification: H04N5
IPC codes: H04H5, H04S7, H04N5