DE4128727C2 - Method for switching to mono, stereo or two-tone in a television set - Google Patents

Description

The invention relates to a method for mono, stereo or Two-tone switching in a television with one device device for stereo or two-tone playback.

Identification and switchover security can be timed long detection of the stereo sound or two-tone carrier he pass. But if an interfering signal appears for a long time occurs, even with a very long detection period Misrecognition and wrong switchover lead.

In DE-A-36 22 016 the difference of the direct voltage Portions of the sound signals modulated onto the two sound carriers used as a criterion for mono switching. DE 35 17 495 C1 discloses the derivation of a switching criterion from the resultant in the absence of the second sound carrier Offset of the average DC voltage in the demodulator for the second sound carrier.

DE-A-33 40 325 the presence of a certain Fre frequency in an input containing several frequencies Signal detected. This turns a reference frequency two switching signals with the specified frequency and one 90 ° phase difference generated. The input signal is then mixed with the switching signals and a DC voltage Proportion in the mixed product determined.

DE 32 44 983 C2 determines whether the identifier decoder Noise components with frequencies greater than 54.68 kHz occur. The operating modes are switched accordingly. In GB-A-2122458 the height of the Noise component determined in the pilot tone. If this is higher than is a predefined threshold value, the operating mode becomes mono maintained.

In GB-A-2097229 a mode is determined by a mod gated pilot tone signal is fed to a PLL and that  PLL phase detector output signal with two different ones Threshold values are compared.

It is an object of the invention, the identifier or Umschaltsi security between stereo, two-tone or mono broadcasts poor reception or interference disturbed by interference signals improve.

This object is achieved with the method according to claim 1. Advantageous developments of the invention are in the dependent claims described.

The invention opens up the possibility of switching to carry out, the identification or switching criterion un is dependent on interference signals and therefore after a short time a safe switchover can be made. The following are available for the detection of the transmission state Signals available: pilot carrier A, stereo carrier B and / or two-tone carrier C.

With good signals you get egg after a certain time ne certain number of identifiers. If the signal gets bad ter or you are on a channel with interference signals, you get a small number of identifiers. If a transmission is disturbed by interference signals, a over the pilot carrier as well as the stereo carrier or the two sound carrier disturbed in the same way. The ratio of the amp litude of the pilot carrier to the amplitude of the stereo carrier thus regardless of interference or fluctuations in field strength in the transfer.

Now the relationship between pilot carrier and identifier is ge forms. To do this, it makes sense if the individual parameters a specific one for the stereo carrier or two-tone carrier Exceed threshold (with hysteresis). A switchover is otherwise pointless, since it is accepted with great certainty that the audio signals are too small in amplitude are falsified.

Whether the circuit carries out stereo or two-tone detection  or makes such a switch depends on the invention depends on whether the ratio of pilot tone ger amplitude divided by stereo carrier amplitude or Two-tone carrier amplitude a predetermined threshold exceeds or falls below.

The predetermined threshold value E is also with a Hysteresis in the case of strongly fluctuating Pi Lotton-carrier amplitude values because of a strong dependency of video content.

Now falls below the ratio of pilot carrier amplitude a predetermined value for the stereo carrier amplitude value Threshold value E, it automatically switches to stereo playback connected.

Falls below the ratio of pilot carrier amplitude value to the two-tone carrier amplitude value the predetermined threshold value E, the system automatically switches to two-tone playback on.

However, the above conditions exceed the above determined threshold value E, the system automatically switches to mono Playback switched.

Under the most unfavorable reception conditions, a more secure one Reception achieved with analog multi-tone transmissions. This results from the independence of the absolute values of the parameters sounds from the transmission. Safe detection of stereo or two-tone transmission is within a relatively short over wearing times possible.

The invention is based on a table and Illustrated embodiment illustrated.

They represent:

Tab. 1 System parameters for the standard B / G television standard;

Fig. 1 amplitude / frequency diagram on the position of the sound carrier to the image carrier;

Fig. 2 amplitude / frequency diagram on the location of the two th sound carrier to the pilot carrier and the stereo / two sound carrier;

Fig. 3 flow chart.

Table 1 shows the known system parameters for the TV sound standard B / G. The first sound channel or sound carrier is 5.5 MHz and the second sound channel or sound carrier is 5.74 MHz below the image carrier, as is also shown in FIG. 1. The pilot carrier frequency is approximately ± 54.7 kHz from the second sound carrier. The stereo identification frequency is ± 117.5 Hz from the pilot carrier. If the sound is emitted as a dual or two-tone, the characteristic frequency for stereo is omitted and the characteristic frequency for dual or two-tone is 274.1 Hz from the pilot carrier.

In a television set with a detection circuit the two sound carriers as well as the pilot and the front existing characteristic frequency detected and their amplitude values A, B or C. determined. It becomes the ratio of the amplitude values of the pilot carrier to those of a characteristic frequency.

In Fig. 2, the ratio of pilot carrier amplitude A to the amplitude of the characteristic frequency B or C has z. B. the value '2'. In an evaluation circuit (not shown) in the television set, this ratio value is compared with a predetermined threshold value. The size of the threshold is variable.

In the present case, there is stereo or two-tone replay switched. If the ratio is exceeded or exceeded for a certain time, is switched to mono on. It also switches to mono when the Ampli the value of the pilot, the second sound or the Characteristic frequency a further predetermined threshold value F, G or fall below H.

Claims (5)

1. Method for mono, stereo or two-tone switching in a television set with a device for determining an amplitude value A of the pilot carrier and with a device for determining an amplitude value B of the stereo identification signal and / or for determining an amplitude value C of the two-tone identification signal , characterized in that it is switched to stereo or two-tone reproduction if the ratio A: B or the ratio A: C is less than or equal to a first predetermined threshold value E. 2. The method of claim 1, wherein the first predetermined Threshold value E is variable or for example in the case of strongly fluctuating pilot tone carrier amplitude values A has a hysteresis. 3. The method according to claim 1 or 2, in which on mono- Playback is switched when the ratio of A: B or A: C is greater than the first predetermined threshold value E is. 4. The method according to claim 1 to 3, in which on mono- Playback is switched when the amplitude value A of the pilot carrier a second predetermined threshold F falls below. 5. The method according to any one of claims 1 to 4, in which Mono playback is switched when the amplitudes value B of the stereo identification signal a third predetermined Threshold value G or the amplitude value C of the two-tone Identification signal a fourth predetermined threshold value H un terschreitet.