JP2515809B2 - Digital transmission system - Google Patents

Description

The present invention relates to a digital transmission system, and more particularly to a transmission system suitable for transmitting digitally encoded voice with high quality.

[Conventional technology]

Currently, AM using the medium wave band as audio-only broadcasting
Broadcasting and FM broadcasting using the ultra-high frequency band are carried out.
On the other hand, with the spread of compact disc players,
Now that digital audio tape recorders are being put to practical use, there is an increasing demand for digitization in the field of audio-only broadcasting.

In such an era, the method of digitally encoding audio and broadcasting is reported in "Satellite Broadcast Receiver", Part 1 of the Satellite Broadcasting Receiving Technology Study Group Report published by the Institute of Radio Engineers of Japan, published in June 1983. However, satellite broadcasting requires a parabolic antenna with a diameter of about 1 m, so digital audio broadcasting that can be easily received like FM broadcasting using the ultra-high frequency band is desired.

In addition, as shown in the above "Satellite Broadcast Receiver", errors in transmission such as deterioration of the transmission signal CN ratio in digital audio are corrected using superposed error detection and correction codes. However, for those that cannot be completely corrected, the average value is interpolated from the preceding and following voice sample values, or the previous voice sample value is held as the previous value. Further, it is known that when the number of errors during transmission increases, the audio signal output is cut off.

[Problems to be solved by the invention]

In the above prior art, since no consideration is given to the distribution of the error rate between the high-order bit and the low-order bit after digitally encoding the transmission information, the CN of the transmission line becomes small and the error rate of the transmission digital code is high. In this case, an abnormal sound is generated or a reproduced sound is cut off, so that there is a problem that the content of the transmission information cannot be understood.

The present invention has been made to solve the above problems in a quadrature amplitude modulation digital transmission system in which two orthogonal carrier waves are amplitude-modulated by two sets of digital codes. That is, the present inventor has conducted research on this problem, and as a result, the occurrence of an error rate due to a decrease in the transmission CN ratio increases as the degree of multi-value quantization increases, and relatively significant bits, that is, higher order bits. Bits are intended to be solved by paying attention to the fact that it is necessary to reduce the occurrence of an error rate as compared with relatively unimportant bits, that is, lower bits.

Therefore, an object of the present invention is to reproduce the original information in a high quality state and reduce the transmission CN ratio in the quadrature amplitude modulation digital transmission system when the transmission CN ratio is large and the error rate of the transmission digital code is small. Therefore, even if the error rate of the transmitted digital code increases as a whole, it is possible to reproduce the transmitted information so that the content of the transmitted information can be understood by suppressing the occurrence of errors in the digital code part that have a significant effect on the reproduced information. To do.

[Means for solving problems]

In order to achieve the above object, in the quadrature amplitude modulation digital transmission system of the present invention, the multilevel signals for modulating orthogonal two-axis carriers are made different, and the multilevel signal of the axis having a low multilevel value is reduced. The upper bits of the analog-to-digital converted code are assigned to the required number of bits, and the remaining lower bits of the code are assigned as the multi-valued signal of the axis that is highly multivalued.

[Action]

When the transmission CN ratio of the transmission signal is small, the error rate of the lower bits transmitted by the multi-valued conversion is high, but the error rate of the high-order bits transmitted by the low multi-valued is small.

Since the error rate of the high-order bits is low, the amplitude is not greatly mistaken in the analog signal, and a terrible abnormal sound is rarely generated. can get.

〔Example〕

Hereinafter, as one embodiment of the present invention, quadrature amplitude modulation (hereinafter referred to as QA
(Abbreviated as M) The number of transmission bits is 4 bits, and the Q axis of 16QAM is 2
A description will be given by taking digitized 3-bit transmission as an example. FIG. 1 shows an embodiment of the receiving and reproducing apparatus of the present invention, in which 1 is an antenna, 2 is a tuning circuit, 3 is a first synchronous detection circuit, and 4 is a second.
Synchronous detection circuit, 5 is a carrier recovery circuit, 6 is a phase shifter, 7,
8 is an LPF (low pass filter), 9 is a first identification circuit, 10
Is a second identification circuit (four-value / two-value conversion circuit), 12 is a first reception side digital signal processing circuit, 13 is a second reception side digital signal processing circuit, and 14 is a digital / analog conversion circuit (hereinafter DAC). Abbreviated), 15 is a voice output. FIG. 2 shows an embodiment of the transmission signal generator on the transmitting side of the present invention,
21 is a voice input, 22 is an analog / digital conversion circuit (hereinafter abbreviated as ADC), 23 is a first transmission side digital signal processing circuit, 24 is a second transmission side digital signal processing circuit, and 25 is a binary-4 value. Conversion circuits, 26 and 27 LPFs, 28 carrier wave generation circuits, 29 phase shifters, 30 first modulation circuits, 31 second modulation circuits, 32 addition circuits, 33 amplifiers, 34 antennas is there. FIG. 3 shows an example of code arrangement of the transmission signal of the present invention, and FIG. 4 shows an example of bit allocation of the transmission signal of the present invention.

 For convenience, first, the operation will be described from the receiving side.

The transmitted radio waves are received by the antenna 1 shown in FIG. 1, and the channel selection circuit 2 selects a broadcasting station. The intermediate frequency signal after being tuned is output to the first by the output of the carrier recovery circuit 5 and the output of the phase shifter 6.
The synchronous detection circuit 3 and the second synchronous detection circuit 4 are synchronously detected in an orthogonal relationship, and LPFs 7 and 8 remove unnecessary signals. As its output, an eye pattern having a binary value on the Q axis and a quaternary value on the I axis is obtained. A binary digital code is obtained from the output of the clock recovery circuit 11 and the first discrimination circuit 9 and the second discrimination circuit 10 from the eye pattern. Then the first
The digital signal processing circuit 12 and the second digital signal processing circuit 13 perform digital signal processing for demodulating digital transmission such as detection and correction of errors occurring during transmission, deinterleaving, etc. obtain.

Next, the operation of the transmitting side will be described with reference to FIG. Second
The figure is a block diagram of an apparatus for generating a transmission signal to be reproduced by the above receiving and reproducing apparatus. The analog signal from the voice input 21 is binary-coded by the ADC 22 and the first
By the digital signal processing circuit 23 and the second digital signal processing circuit 24, a code for detecting and correcting an error occurring during transmission is added, and interleaving is applied to avoid a burst error. After that, on the I-axis, the binary output of the second digital processing circuit 24 is converted into four values, so that it is applied to the binary-to-four-value conversion circuit 25, the unnecessary band is removed through the LPF 27, and the carrier wave generation circuit 28 outputs. Output phase shifter
The second modulation circuit 31 using the signal phase-shifted by 90 via 29
Is modulated by On the other hand, on the Q axis, the binary output of the first digital processing circuit 23 is directly applied to the LPF 26 to remove the unnecessary band, and is modulated by the first modulation circuit 30 using the output of the carrier generation circuit 28. In this embodiment, since the degree of multi-valued Q-axis is binary, the binary-multivalued conversion circuit like the I-axis is omitted. Those modulation circuits 3
The outputs of 0 and 31 are added by an adder 32, amplified by an amplifier circuit 33, and transmitted as an electric wave from an antenna 34.

FIG. 3 shows the code arrangement of the QAM signal in which the I-axis is 4-valued and the Q-axis is 2-valued. The horizontal axis of FIG. 3 is the Q axis, which is a binary value of 0 and 1, and the I axis is a quaternary value of 00, 01, 10, and 11, and 3-bit data can be transmitted simultaneously in the same time slot. This is shown in FIG. 3 in the order of (Q, I ₁ , I ₂ ). Where Q
There is a three-fold difference between the inter-symbol distance on the axis and the inter-symbol distance on the I-axis, and the transmission signal CN ratio at which the bit error rate is the same may be 10 dB less on the Q axis. Conversely, in the case of a signal transmitted at a certain CN ratio, the Q axis has a smaller error rate.

Now, as shown in FIG. 4, it is assumed that the audio signal to be transmitted is quantized with N bits, for example, 12 bits per sample, and the data is sequentially D ₁ , D ₂ ,
D _3, ... and D _12, and error detection correction code E ₃ for E _2, D ₇ ~D ₉ for E _1, D ₄ ~D ₆ for the top M bits, for example 3 bits D ₁ to D _3. At this time, in the time of time slots T _{1 to} T ₅ , Q is D _{1 to} D ₄ and E ₁ , and I ₁ and I ₂ are D _{5 to} D ₁₂ and E ₂ .
When the CN ratio of the transmission signal deteriorates by allocating E ₃ , the error rate is low because the upper bit side is assigned to the Q axis, and the error rate is high because the lower bit side is assigned to the I axis. Become. As a result, even if the CN ratio of the transmission signal is extremely deteriorated and most of the lower bits are erroneous, the error rate of the upper bits is small and a voice signal can be reproduced to some extent.

Although the error detection / correction codes of E _{1 to} E ₃ are shown as the parity for one sample, the upper 3 bits of several samples may be put together and the error detection / correction code of several bits may be added.

As described above, according to this embodiment, when the transmission CN is large and the error rate of the transmission digital code is large, 12
Bit reproduction sound is obtained, and even if the CN ratio becomes smaller and worse, the upper 3 to 4 bits can be obtained with few errors,
There is an effect that a reproduced sound that can be understood as voice is obtained.

Here, the required transmission bandwidth when transmitting with 3 bits is calculated. 12 bits × 32K / S × 2ch × 1.3 = 998.4Kbps 998.4Kbps (Kbits / sec), assuming that the number of quantization bits is 12 bits, the sampling frequency is 32KHz, the audio is 2 channels (stereo), and the error correction code superimposition is 30%. Since 3 bits are transmitted at the same time, 332.8 Kbps is achieved, and it is possible to transmit with a bandwidth of 332.8 KHz. This bandwidth is about the same as current FM broadcasting, and it can be transmitted in the ultra-high frequency band.

On the other hand, the carrier recovery circuit 5 is important for obtaining a reproduction orthogonal axis, and includes data (0,0,0), (0,1,1), (1,0,0) and (1,1,1). A method of performing negative feedback so that the amplitudes on the I-axis and the Q-axis are the same can be considered based on only the case of four values. In the case of 16QAM, this circuit is explained in the reference carrier recovery circuit shown in pp134 to 135 of "Digital Microwave Communication" published by Planning Center Co., Ltd. in May, 1984.

Although the audio signal has been described above, the same effect can be obtained even when the upper bit has important information such as an image signal.

In the above explanation, 4 bits of 16QAM are transmitted as 3 bits of 8 states, but transmission of 6 bits of 64QAM is changed to 5 bits of 32 states with an I axis of 8 values and a Q axis of 4 values. Similar effects can be obtained with other QAMs. In this case, the transmitting side needs a binary-to-eight-value conversion circuit for the I-axis and a binary-to-four-value conversion circuit for the Q axis. A similar inverse conversion circuit is required on the receiving side.

〔The invention's effect〕

As described above in detail with reference to the embodiments, according to the quadrature amplitude modulation digital transmission system of the present invention, the degree of multi-value conversion of the multi-valued signals respectively modulating the carrier waves of the two orthogonal axes is made different, and the degree of multi-value conversion. The high-order bits of the A / D-converted code are allocated as the multi-valued signal of the axis with few values, and the remaining low-order bits of the A / D-converted code are allocated as the multi-valued signal of the axis with the high degree of multi-value conversion. Therefore, high quality reproduction is possible when the transmission CN ratio is large and the transmission conditions are good, and even under bad conditions where the transmission CN ratio is low, the increase in the error rate of the upper bits compared to the lower bits can be suppressed as much as possible. As a result, an excellent effect is obtained such that reproduction can be performed to such an extent that the transmission information content can be understood.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of a receiving and reproducing apparatus used in the present invention, FIG. 2 is a block diagram of an embodiment of a transmission signal generating apparatus on the transmitting side of the present invention, and FIG. 3 is a transmission used in the present invention. FIG. 4 is a diagram showing an example of code arrangement of signals, and FIG. 4 is a diagram showing an example of bit allocation of transmission signals used in the present invention. 3,4 …… Synchronous detector 9,10 …… Multi-level code identification circuit (4 level-binary conversion circuit) 11 …… Clock recovery circuit 12,13,23,24 …… Digital signal processing circuit 14 …… Digital・ Analog conversion circuit 21 …… Voice input terminal 22 …… Analog / digital conversion circuit 25 …… Binary-four-value conversion circuit 30,31 …… Quadrature modulation circuit 32 …… Adding circuit.

Claims (3)

(57) [Claims] 1. A high-order predetermined number of bits of a digital code is taken out as a multilevel signal having a relatively low degree of multilevel conversion, and the remaining number of bits of the digital code is a multilevel signal having a relatively high level of multilevel conversion. Code conversion means for extracting as a signal,
Quadrature amplitude modulation means for amplitude-modulating and synthesizing two orthogonal carrier waves orthogonal to each other by the multilevel signal having a relatively low level and the multilevel signal having a high level. And a means for sending the output of the means to a transmission line. 2. A means for receiving a signal from the transmission line, a means for demodulating a received quadrature amplitude modulation digital signal about two axes, a demodulated multi-valued signal having a relatively low degree of multi-valued and comparison. A code reverse conversion means for extracting a multi-valued signal having a large degree of dynamic multi-valued conversion as a binary digital code, and a digital-analog conversion circuit for converting the binary digital code into an analog signal. The digital transmission method according to item 1. 3. The digital transmission according to claim 3, wherein a digital signal processing circuit for detecting and correcting an error occurring during transmission in the binary digital code is provided before the digital-analog conversion circuit. method.