JPH05315984A - Am radio receiver - Google Patents

Abstract

PURPOSE:To receive AM radio waves with high sound quality free from interference noises. CONSTITUTION:When frequency interval between adjacent channels in AM broadcasting is DELTAF, an A/D converter 17 samples an output from an AM detector 16 at fs (>=DELTAF) and A/D converts the sampled data and a digital FIR filter 18 filters the data outputted from the A/D converter 17 to cut out a band component more than a prescribed frequency including noises between the adjacent channels. Although noises can be removed by the processing, a signal component in the band is also removed. Thereby the sampling rate of the output data from the filter 18 is reduced to 1/2, and while interpolating one digital data between digital data whose sampling rate is turned to 1/2, the digital data are converted into an analog signal by a D/A converter 20. Consequently the signal component cut in the band higher than the prescribed frequency can be apparently formed and added and AM radio waves free from interference noises and having high sound quality can be received.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AM radio receiver, and more particularly to an AM radio receiver which is free from adjacent interfering noise and can be received with good sound quality.

[0002]

2. Description of the Related Art FIG. 7 is a block diagram of a conventional AM radio receiver. 1 is an AM tuner, 2 is a local oscillator, 3 is an intermediate frequency filter (IF filter), 4 is an intermediate frequency amplifier, and 5 is an intermediate frequency filter. It is an AM detector. In such an AM radio receiver, the intermediate frequency filter 3 has a wide bandwidth (bandwidth
5KHz) or narrow band (bandwidth 2.0KHz to 2.5KHz).

[0003]

When the intermediate frequency filter 3 has a wide band (bandwidth 5 KHz), broadcasting can be received with good sound quality with little distortion. However, in AM broadcasting, broadcasting stations are placed every 9 KHz, so if there is a large station nearby, interference will occur (adjacent interference). FIG. 8 is an explanatory diagram of adjacent interference. When the intermediate frequency filter 3 has a wide band, interference occurs in the shaded area and adjacent station noise is generated. On the other hand, when the intermediate frequency filter 3 is set to a narrow band (bandwidth 2.0 KHz to 2.5 KHz), adjacent interference does not occur, but the band width becomes narrow, resulting in a muffled unclear sound. From the above, the object of the present invention is to
An object of the present invention is to provide an AM radio receiver having good sound quality without interference noise.

[0004]

According to the present invention, there is provided an AM tuner, a wide band intermediate frequency filter, and
When the frequency width between the M detector and the adjacent station is ΔF, A
An AD converter for sampling the output of the M detector at fs (≧ ΔF) and AD-converting it; and a digital filter means for filtering the output of the AD converter and cutting a band component of a predetermined frequency or higher that includes noise due to adjacent interference. , Between the means for halving the sampling rate of the data output from the digital filter means and the digital data output from the sampling rate ½ means
This is achieved by constructing an AM radio receiver with a DA converter that interpolates each piece of data and converts it into analog.

[0005]

When the frequency width between adjacent stations of AM broadcasting is ΔF, the output of the AM detector is fs (≧ Δ
In step F), sampling is performed and AD conversion is performed, and the AD converter output data is filtered by a digital filter to cut a band component of a predetermined frequency or higher that includes adjacent station noise. As a result, noise can be removed, but the signal component in the band is also removed. Therefore, the sampling rate of the data output from the digital filter is halved by the sampling rate 1/2 means, and D is added between the digital data whose sampling rate is halved.
The A converter converts one digital data into an analog signal while interpolating and outputs the analog signal. As a result, it is possible to apparently generate and add a signal component in the cut-off band above the predetermined frequency, and it is possible to perform AM radio reception with good sound quality without interference noise.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Overall Configuration FIG. 1 is a configuration diagram of a main part of an AM radio receiver according to an embodiment of the present invention, and FIG. 2 is a frequency characteristic diagram for explaining an AM receiving operation of the present invention. In FIG. 1, 11 is an antenna and 12 is A
M tuner including high frequency amplifier circuit, mixer, etc., 1
3 is a local oscillator, 14 is a wide band (bandwidth 5 KHz) intermediate frequency filter (IF filter), 15 is an intermediate frequency amplifier,
Reference numeral 16 is an AM detector. A wide band (bandwidth 5 KHz) filter is used as the IF filter 14. Therefore, the frequency characteristic is as shown in FIG. 2 (a), and the band of the signal output from the AM detector 16 is 2.5 KHz or more (hatched portion).
Includes noise due to adjacent interference. Reference numeral 17 denotes an AD converter that AD-converts the output of the AM detector. When the frequency width between adjacent stations is ΔF (= 9 KHz), the output of the AM detector is sampled at fs (= ΔF = 9 KHz) and AD-converted. .. A high frequency component higher than the voice signal appears repeatedly by sampling, and its frequency characteristic becomes as shown in FIG. 2 (b).

Reference numeral 18 denotes a digital FIR filter that filters the output data of the AD converter 17 to cut off components in the noise band of 2.5 KHz or more. The frequency characteristic of the data output from the digital FIR filter is shown in FIG. As shown in c). By cutting the components above 2.5 KHz, noise due to adjacent interference is removed, but the signal components above 2.5 KHz are also removed. Reference numeral 19 denotes a thinning processing circuit that reduces the sampling rate of digital data output from the digital FIR filter to 1/2.
Sampling rate is set to 1 by thinning out every 1 data
It can be / 2. 1/2 sampling rate
As a result, aliasing around fs' (= fs / 2) occurs, and the frequency characteristic of the output data is shown in FIG.
It becomes as shown in (d). The digital FIR filter 1
8 and the thinning-out processing circuit 19 can be configured by a DSP (digital signal processor). Reference numeral 20 is a DA converter that interpolates one data between the data whose sampling rate is halved and converts the data into the data of the frequency fs to convert it to analog. This DA converter 20
According to the above, the digital FIR filter 18 cut 2.
It is not possible to generate exactly the same signal component of 5 KHz or more, but it is possible to generate and add a signal equivalent to the signal component in appearance. This makes it possible to output an audio signal that is equivalent to the original audio signal in terms of envelope and has no noise. FIG. 2E shows the frequency characteristic of the analog audio signal output from the DA converter 20, and the characteristic can be adjusted as shown by the arrow by changing the interpolation function waveform described later.

DA Converter FIG. 3 is a block diagram of the DA converter 20. 21 is the latest m + 1 digital data X generated at every predetermined time T
A digital data storage unit 22 stores j (j = 0 to m) while shifting, and 22 divides the unit pulse response signal (interpolation function signal) SP shown in FIG. 4 at a predetermined time interval T (m +
1) When (m is 8) partial signals Sj (j = 0 to 8), each partial signal Sj (j = 0 to m) has a period T / n (n
Is stored in ROM22-0 to 22-m for n time-series numerical data (see the table below) obtained by sampling in 8), and is sequentially stored in ROM22-0 to 22-m for each T / n. A partial signal time-series data generator that reads time-series numerical data from the digital signal and digitally repeatedly generates each partial signal Sj (j = 0 to m),
Reference numeral 23 denotes a digital operation unit that multiplies the time-series numerical data of each partial signal output from the partial signal time-series data generation unit 22 and digital data corresponding to the partial signal, and adds the multiplication result, and 24 indicates the addition result. It is a digital-to-analog converter that converts to analog.

[0009]

[Table 1]

In the digital data storage unit 21, M ₀
˜Mm is a shift register that stores m + 1 latest digital data Xj generated at each time interval T while sequentially shifting, and in the digital operation unit 23, K0 to Km are predetermined times of each partial signal S _{0 to} S _m ( i-1). Time-series numerical data c _0i , c _1i , c _2i , c _3i , ..., C _mi at T / n and digital data X 0 to stored in the shift register corresponding to each partial signal. Xm is a multiplier that multiplies each time T / n, SUM is an adder that adds the multiplication results,
From the adder, digital data Y _{i represented} by the following formula Yi = C _0i · X ₀ + C _1i · X ₁ + C _2i · X ₂ + C _3i · X ₃ + ... + C _mi · X _m (i = 1 to n) Is output every T / n.

The interpolation function signal SP has the waveform shown by the dotted line in FIG. 5 (a), and n = 4 and m = 2, and the three partial signals S
The four time-series numerical data of _{0 to} S ₂ are S ₀ : c ₀₁ , c ₀₂ , c ₀₃ , c ₀₄ S ₁ : c ₁₁ , c ₁₂ , c ₁₃ , S ₁₄ S ₂ : c ₂₁ , c ₂₂ , If c ₂₃ and c ₂₄ , one unit digital data input, then sequentially from the adder SUM c ₀₁ → c ₀₂ → c ₀₃ → c ₀₄ → c ₁₁ → c ₁₂ → c ₁₃ → c ₁₄ → c
₂₁ → c ₂₂ → c ₂₃ → c ₂₄ is output. That is, the interpolation function signal SP of FIG.
The time series numerical data of is sequentially output from the adder SUM. Therefore, when the digital data string Xj having the period T is input as shown in FIG. 5B, the interpolation function signal SP is multiplied by Xj to produce the signal SPj for each T as shown in FIG. 5C. , By adding these at every time interval T / 4, the data strings Y _{1 to} Y _{4 in} which three pieces of data are interpolated between the input data are sequentially output from the adder SUM of the digital operation unit 23, and digital-analog conversion is performed. The analog signal AS is output from the unit 24.

As an embodiment for applying this DA converter to the DA converter 20 of FIG. 1, the interpolation function signal SP has a waveform shown by a dotted line in FIG. 5 (a), and n = 2, m.
= 2, the two time-series numerical data of the three partial signals S _{0 to} S ₂ may be S ₀ : c ₀₁ , c ₀₃ S ₁ : c ₁₁ , c ₁₃ S ₂ : c ₂₁ , and c _23. .. By doing so, it is possible to output an analog audio signal while interpolating one piece of digital data between digital data having a sampling rate of fs / 2 and returning the sampling rate to fs. The interpolation function waveform, in other words, the time series numerical data c ₀₁ , c ₀₃ , c ₁₁ , c ₁₃ ,
By changing the values of c ₂₁ and c ₂₃ , the frequency characteristic of the output audio signal can be adjusted as shown by the arrow in FIG. 2 (e).

Reception operation The spectrum of the audio signal output from the AM detector 16 is as shown in FIG. 6 (a). It should be noted that noise is included above 2.5 KHz, but it is omitted. In order to remove the noise component included in the output signal of the AM detector 16 by the digital FIR filter 18, the output of the AM detector 16 is sampled at the sampling frequency fs (=
It is sampled at the frequency width between adjacent stations (9 KHz) and converted to digital data. Thereafter, the digital FIR filter 18 cuts off frequency components of 2.5 KHz or higher. This eliminates noise due to adjacent interference.

However, the signal component of 2.5 KHz or more is also eliminated, and the hatched portion in FIG. 6B is cut off from the audio signal spectrum. When there is no music signal component in the shaded area, noise is not included but the voice becomes unclear. Therefore, it suffices to produce this cut audio signal portion, but it is obvious that the exact same thing cannot be produced. Therefore, in the present invention, the thinning processing circuit 19 and the DA converter 20 are used to generate and add the audio signal portion of the apparently cut band. That is, the thinning-out processing circuit 19 thins out the FIR filter output data one by one to reduce the sampling rate to 1/2 (= 4.5 KHz).
By this thinning-out processing, the spectrum of the digital data becomes as shown in FIG. 6 (c). Then, DA converter 2
At 0, while interpolating one data between digital data (the sampling speed of digital data after interpolation is f
s = 9KHz), convert to analog. With this, 2.5KHz
A component that has been called aliasing noise up to 5 KHz is added as an apparent component. In this case, the interpolation function waveform of the DA converter 20 is adjusted so that the level of the apparent component to be added becomes similar to the spectrum when there is no noise. As a result, the spectrum as shown in FIG. 6 (d) can be realized, and the voice can be clearly heard without noise.

As a derivation of the above idea, conversely, it is also possible to construct so that a higher quality voice is output when the reception state is good, even if there is no problem even through a wide band IF filter. In this case, the sampling rate of the AD converter 17 may remain 10 KHz, and down sampling by the intermediate thinning processing circuit 19 is not necessary. Finally, the DA converter 20 interpolates one data between the data of 10KHz to make the data of 20KHz, and then performs the DA conversion to add the spectrum of 5KHz or more, and output the high-quality voice. it can. In the above, the sampling rate fs in the AD converter was made equal to the frequency width ΔF (= 10 KHz) between adjacent stations, but fs ≧ Δ
It may be F. Although the present invention has been described above with reference to the embodiments, the present invention can be variously modified according to the gist of the present invention described in the claims, and the present invention does not exclude these.

[0016]

As described above, according to the present invention, when the frequency width between adjacent stations is ΔF, the AM detector output is sampled at fs (≧ ΔF) and AD-converted, and the AD-converted output is filtered. , A band component having a frequency equal to or higher than a predetermined frequency including noise due to adjacent interference is cut, and thereafter, the sampling rate of the filtered data is halved, and the sampling rate is halved.
Since it is configured to convert each piece of data to analog while interpolating, noise due to adjacent interference can be removed, and the signal component of the removed band can be apparently generated and added, and there is no interference noise and sound quality. Can receive good AM radio.

[Brief description of drawings]

FIG. 1 is a configuration diagram of essential parts of an AM radio receiver that is an embodiment of the present invention.

FIG. 2 is a frequency characteristic diagram illustrating an AM radio receiving operation of the present invention.

FIG. 3 is a configuration diagram of the DA converter in FIG.

FIG. 4 is an explanatory diagram of time series numerical data in a unit pulse response signal.

FIG. 5 is an explanatory diagram of digital-analog conversion of a DA converter.

FIG. 6 is a spectrum characteristic diagram illustrating an AM radio receiving operation of the present invention.

FIG. 7 is a block diagram of a conventional AM radio receiver.

FIG. 8 is an explanatory diagram of adjacent interference.

[Explanation of symbols]

 12 ・ ・ AM tuner 14 ・ ・ Wide band (bandwidth 5KHz) intermediate frequency filter 16 ・ ・ AM detector 17 ・ ・ AD converter 18 ・ ・ Digital FIR filter 19 ・ ・ Decimation processing circuit 20 ・ ・ DA converter

Claims (1)

[Claims] 1. An AM radio receiver equipped with an AM tuner, a wide band intermediate frequency filter and an AM detector, wherein when the frequency width between adjacent stations is ΔF, the AM detector output is fs (≧ ΔF). An AD converter that performs sampling and AD conversion, digital filter means that filters the AD converter output and cuts band components of a predetermined frequency or higher that include noise due to adjacent interference, and sampling rate of data output from the digital filter And a DA converter for interpolating one data between the digital data output from the sampling rate 1/2 means and converting it to analog.
M radio receiver.