Method and apparatus for auto-tuning of a radio fm-receiver

IS 2 300 580 T3

DESCRIPTION

Method and apparatus for auto-tuning an FM radio receiver.

The invention relates to a method of automatically tuning or auto-tuning an FM radio receiver [frequency modulated] by scanning the frequency band of the receiver until an FM signal is received that satisfies the criteria for identifying the frequency. signal as having a predetermined quality, particularly as originating from a valid FM station or station.

EP-A-0 430 469 describes an example of such a method.

One of the essential features of a broadcast receiver is the automatic tuning function, or auto-tuning. This allows a user to scan the receiver's frequency band for available signals, tune them, and store them in presets, all at the push of a button. Particularly in combined FM television-radio receivers, the television frequency and FM radio signals are converted into baseband video and audio signals by means such as a self-tuning function. This auto-tuning function is important, for example, for manufacturers of television cards for PCs (“personal computer”) who wish to provide computer users with the possibility of enjoying television and radio broadcasting while they work on their PC's.

The concept of self-tuning for television is well established and relatively error-free in that a television signal constitutes a unique type of modulated signal that, among other characteristics, has a periodic sync pulse. Thanks to it, it is always possible to discriminate between a television signal and a non-television signal, including short-term non-periodic noise.

However, in the case of FM radio, relatively error-free auto-tuning is not entirely possible. An FM discriminating device is not inherently capable of differentiating between a true FM radio signal and an interfering non-FM signal, which may be frequency stable or random. This results in a case where well known auto-tuning systems can pick up numerous signals and store them, although perhaps only a small part of them are signals that are desired. Tuning in the rest of the presets will only provide irritating noise.

The first problem lies in the sensitivity of the receiving system, which is capable of detecting signals that are close to the noise floor. This means that the probability of mistakenly storing a random noise signal that exists at the time of auto-tuning is relatively high. Furthermore, this also means that a true FM signal that is very weak could also be picked up. In reality, once an FM radio signal falls below a certain signal strength, commonly known as the FM threshold, there is little point in storing a channel, as the sound quality is simply too poor to handle. be intelligible.

The practice of applying two conditions to be satisfied when deciding whether a signal is a valid FM station or station is known. The FM-IF signal level must be high enough. This can be done, for example, by setting or setting a bit in a status register that is available to the user. This criterion is indicated in the following as "intensity". When this condition is met, the AFC (automatic frequency control) status should be checked; if the received signal is within the IF bandwidth, the second condition is met and the signal is then stored as a valid station. For the 10.7 MHz IF band, the received signals must be within ± 75 kHz of the 10.7 MHz IF. This second criterion is referred to below as the "AFC window". It is already known, for example, from the European Patent Application EP-A-0 430 469 the concept of auto-tuning an FM radio receiver by scanning the frequency band of the receiver until an FM signal is received. that meets the above criteria to identify the signal as having a predetermined quality. When a received FM signal meets both criteria, this is indicated below as "a positive test result."

However, in the event that sufficiently strong random interference reaches the demodulator of the FM radio receiver, this may also be stored by mistake, since it may appear momentarily within the AFC window.

The purpose of the invention is to avoid picking up such random interference in frequency.

Consequently, according to the invention, the method as described in the opening paragraph is characterized in that, at least during tuning, it is permanently established whether the FM signal meets the criteria or not, after which the Results of this are read a predetermined number of times, and the FM signal is only stored if the criteria are met at least most of the time. It has been found that good results are obtained when these results are read 10 times and the FM signal is only stored if at least 8 times the criteria are met.

The invention further relates to an automatic tuning or self-tuning device having means for registering whether or not an FM signal, received at an FM radio receiver, meets criteria for identifying

ES 2 300 580 T3 the signal as having a predetermined quality, particularly as originating from the valid FM station, and computational means intended to record, in an interval of a predetermined number of times in which it is recorded whether the FM signal meets the criteria or not, the number of times the FM signal meets the criteria. The invention also relates to an FM radio receiver for carrying out the above method, comprising the above self-tuning device.

The invention further relates to an algorithm for processing or treating FM signals in a self-tuning device according to the foregoing, to a computer program capable of being executed on signal processing means of an FM radio receiver or cooperating with an FM radio receiver comprising the above auto-tuning device, and with an information-carrying device, that carries instructions to be executed by signal processing means, so that the instructions are such that they allow said processing means to carry out the above method.

The invention will be further clarified by means of a specific embodiment and with reference to the drawing, which shows a flow chart of the method according to the invention.

The sweep of the FM band, which in most of the world is the frequency range from 87.5 MHz to 108.5 MHz, is carried out by means of a local oscillator (LO) signal that is traversed a range from 98.2 MHz to 119.2 MHz in steps of 50 kHz. In the flow chart, this is indicated by block 1. At the beginning of each LO run, a counter is reset to count positive test results. As soon as an FM signal is received, it is checked whether or not this signal meets the signal quality criteria "intensity" and "AFC window [automatic frequency control -" automatic frequency control "]". This is carried out in 10 stages of iteration. When the counter is reset, the number of iterations is also reset. The iteration procedure starts at 1 (block 3), whereas each time the received FM signal is checked, the number of iterations is increased by 1. The number of iterations in this example is limited to 10. Always and when the number of iterations has not reached the value of 10, indicated in block 4, it is checked whether or not the FM signal meets the criteria "intensity" (block 5) and "AFC window" (block 6). When the FM signal meets both criteria, the positive check result counter is incremented by 1 (block 7) and, after a short delay, a subsequent check of the FM signal is performed (block 4). When, after 10 iteration stages (block 4), a positive check has been recorded 8 times (block 8) in this example, the FM signal found comes from a real station or station and the corresponding radio frequency of the station. itself is stored (block 9). Once 10 iteration stages have been carried out and there are no 8 positive results (blocks 4 and 8), the LO frequency value is increased in two stages, that is, by 100 kHz (block 10). If the end of the LO frequency run has not yet been reached (block 11), the counter is reset again and a search for a valid FM station on an offset frequency is initiated. Once the end of the LO frequency run has been reached, the procedure ends and can be started again. When the first criterion, "intensity" (block 5), is not satisfied, the local oscillator signal is increased, after which the procedure is repeated or terminated as described. When the first criterion, “intensity” is met, but not the second criterion, “AFC window”, the number of iteration stages is increased, but, of course, not the number of positive results. However, after that, a positive check result can be obtained, as it is still possible that 8 positive check results are obtained in 10 iteration steps.

This procedure is carried out on an auto-tuning device that is part of an FM radio receiver that has means to record whether an FM signal received on an FM radio receiver meets the "intensity" criteria or not. and "AFC window". Whether or not the FM signal meets the criteria can be recorded permanently, or at least during tuning, in the auto-tuning device, for example in a status register contained therein. This status register will be read 10 times during each check procedure. Consequently, the tuning device will comprise computing means intended to record in an interval of a predetermined number of times (10 times) in which it is recorded whether the FM signal meets the criteria or not, the number of times that the signal FM meets the criteria.

The above verification procedure will be carried out by means of an algorithm in the form of a computer program capable of being executed in signal processing means existing in the FM radio receiver or that cooperate with the FM radio receiver comprising the auto-tuning device.

The invention is not limited to the embodiment that has been described. Modifications are possible. In particular, the number of iteration stages and the number of positive test results within the number of iteration stages can be chosen to be different from 10 and 8 respectively. Furthermore, it can be appreciated that the scanning procedure as described above can be applied for an FM band in the frequency range from about 75 MHz to about 91 MHz, as is the case in Japan.