JPH04270577A - Audio signal processing circuit and its method - Google Patents

Abstract

PURPOSE:To prevent frequent mute at the reception of a broadcast signal and to surely mute production of intermittent tone in a reproduced audio signal at the reception of a non-broadcast signal by switching a synchronization protection period of a frame synchronization protection circuit in response to the S/N of a MUSE video signal. CONSTITUTION:An S/N measurement circuit 15 measures the S/N from a MUSE signal converted into a digital signal by an A/D converter circuit 2 from 1st and 2nd lines. A frame synchronization protection circuit 14 decides the synchronization protection length from the S/N and generates a frame signal according to the frame frequency for a prescribed frame according to the synchronization protection length to protect the internal processing from mis-detection or undetection of a frame synchronizing signal and applies the signal to each circuit in the voice decoder. The frame synchronization protection circuit 14 selects the length of synchronization protection based on the S/N measured by the S/N measurement circuit 15.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to digital audio signal processing circuits, and more particularly to synchronization protection.

0002

[Prior Art] The audio signal transmission system of current satellite broadcasting is well known, as described in pages 30 to 37 of the Revised Edition Introduction to Satellite Broadcasting Reception, published by the Japan Broadcasting Publishing Association, November 30, 1986, for example. Are known.

By the way, as a method for band-compressing a high-quality video signal and transmitting it using a broadcasting satellite, a multiple sub-Nyquist sampling encoding method, MUSE (Multi
ple Sub-Nyquist Sampling
Encoding) method was proposed by NHK, and NHK
It is used in high-definition broadcasting by Satellite 2nd Television.

In high-definition broadcasting, audio signals are time-division multiplexed and transmitted during vertical marking periods of video signals.

[0005] As is well known, the audio transmission system for high-definition broadcasting uses differential PCM (DPCM), which transmits only the value of a change in an audio signal, as a band compression method. That is, the quasi-instantaneous companding method used in current satellite broadcasting is used, and the companding method is quasi-instantaneous companding DPCM using the above-mentioned DPCM method.

[0006] Regarding the demodulation of the MUSE audio signal,
An example is shown in JP-A-2-11076 (H04N7/00).

FIG. 3 shows a block diagram of a conventional MUSE audio decoder. (1) is the MUSE signal input terminal,
The analog format MUSE signal input to this terminal is A
After being converted into a digital signal by the /D conversion circuit (2), it is sent to the audio signal gate circuit (3).

By this audio signal gate circuit (3), A
The DPCM audio signal (hereinafter referred to as DPCM signal) in the MUSE signal after /D conversion is separated and extracted, and the extracted ternary format DPCM signal is converted from 16.2 MHz to 12.15 MHz by the frequency conversion circuit (4). After the sample rate is converted to , the ternary/binary conversion circuit (5)
is converted into a binary signal by

This binary DPCM signal is converted to 1.35 Mb/sec. by the time expansion circuit (6). The signals are converted into continuous signals, returned to the normal frame order by the interframe deinterleaving circuit (7), and sent to the bit deinterleaving circuit (8).

The bit deinterleaving circuit (8) performs bit deinterleaving on the interframe deinterleaved DPCM signal to perform bit deinterleaving in the row direction (
After returning the signals to aligned signals in the sample word direction), the signals are sent to the error correction circuit (9).

The error correction circuit (9) performs error correction on the input DPCM signal based on the BCH code, and then sends the signal to the word deinterleaving circuit (10). That is, since the error-corrected DPCM signal is still interleaved in the row direction and the sample word direction, the interleaving is canceled in this word deinterleaving circuit (10) and the signal is restored to a normal one frame signal. Thereafter, it is input to the DPCM decoding circuit (11).

[0012] Then, this DPCM decoding circuit (11
) decodes the input DPCM signal and reproduces a digital audio signal.

[0013] This digital signal is sent to a D/A converter (15).
After the audio is converted to analog audio, the mute circuit (16)
It is output after passing through.

On the other hand, the output of the interframe deinterleaving circuit (7) is also supplied to a frame synchronization detection circuit (13). This frame synchronization detection circuit (13) generates a frame detection signal every time it detects a frame synchronization signal, and this frame detection signal is supplied to a frame synchronization protection circuit (14). This frame synchronization protection is also disclosed in Japanese Patent Publication No. 2-10618 (H04L 7/00), Japanese Patent Application Laid-open No. 2-170632 (H04B 14/04), etc.

This frame synchronization protection circuit (14) prevents errors such as when the frame synchronization signal is temporarily lost due to deterioration of the C/S ratio of the broadcast MUSE signal due to radio wave conditions, or when it is detected in a relatively short cycle. At the time of detection, a frame synchronization signal according to the regular frame frequency is internally generated during a predetermined frame period to protect the frame synchronization signal, and to prevent errors even after the predetermined frame period (for example, 64 frame periods) has passed. If the detection state is not recovered, a mute signal is generated and the mute circuit (16) is controlled to mute the audio output. The timing signal generation circuit (17) creates timing signals for the circuits (8), (9), and (10) in the decoder based on the internal frame signal generated by the frame synchronization protection circuit (14).

Note that the muting is canceled by generating a regular frame detection signal. That is, in this circuit, when the frame detection signal is continuously generated over four frame periods, it is determined that the receiving state is normal, and muting is canceled. Note that this muting is canceled after a time corresponding to the processing delay in the audio signal circuits (8), (9), (10), (11), and (15). In particular, the DPCM decoder (11) usually has a built-in leakage integration circuit, and this leakage integration circuit alone needs to delay the output of the mute signal by about 200 msec. If no frame detection signal is input for 64 consecutive frames, a mute signal is output without delay.

[0017]

[Problem to be Solved by the Invention] The frame synchronization protection period described above is set on the assumption that broadcast signals are being received.
It is set to a relatively long period, such as a frame period, to prevent frequent muting. In other words, even if part of the audio data is lost due to deterioration of the C/N ratio when receiving a weak electric field, the noise component only increases, and audio that can be recognized as audio is output.

On the other hand, MUSE disc players and MUS
During normal playback of playback signals from a playback device such as an E-VTR, erroneous detection of simultaneous signals due to deterioration of the C/N ratio is less likely to occur, unlike during broadcasting.

However, when this MUSE disk player is used as a MUSE-VTR, several frames of audio data may be completely lost along with the synchronization signal due to dropouts or effects during search, and as described above, If the protection period was set to a long time, the mute would not be applied immediately and a loud noise would occur. However, if the protection period is shortened, muting will be applied immediately upon reception of a broadcast signal.

[0020]

[Means for solving the problem] In an audio signal processing circuit that decodes and reproduces an audio signal multiplexed with a video signal, frame synchronization protection is determined based on the S/N ratio of the input video signal. Changed the length.

[0021]

[Operation] When receiving strong electric field with good S/N ratio and VDP:VT
When reproducing from R, the period of synchronization protection is shortened so that muting is applied immediately upon dropout, and when receiving a weak electric field with a poor S/N ratio, the period of synchronization protection is lengthened.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a block diagram of the apparatus of this embodiment, and the same parts as in the conventional example are given the same reference numerals and will be explained.

FIG. 1 shows a block diagram of one embodiment of the invention. (15) is an S/N ratio measuring circuit. This S
/N ratio measuring circuit (15) is disclosed in, for example, Japanese Patent Application Laid-Open No. 62-2726.
As shown in No. 73 (H04N 5/21), the noise component of a predetermined signal portion is integrated and the S/N ratio is output. In addition, according to the current MUSE signal standard, clamp level 5
63, the 1125th line. Note that the noise detection lines include the first and second VI lines with the VIT signal.
The S/N ratio may be calculated from a source other than the T signal insertion section.

That is, this S/N ratio measurement circuit (15) measures the S/N ratio from the first and second lines of the MUSE signal converted into a digital signal by the A/D conversion circuit (2).

[0025] The frame synchronization protection circuit (14)
The synchronization protection length is determined from the /N ratio, and a frame signal according to the frame frequency is internally generated between fixed frames according to the synchronization protection length in order to protect internal processing from detection errors or non-detection of the frame synchronization signal. and supplies it to each circuit in the audio decoder.

[0026] The frame synchronization protection circuit (14)
The synchronization protection length is switched based on the S/N ratio measured by the S/N ratio measurement circuit (15).

FIG. 2 shows an example of switching the synchronization protection length. In the example shown in FIG. 2A, the synchronization protection length is determined in stages based on the S/N ratio measured by the S/N ratio measuring circuit (15). The length is determined to be long when the S/N ratio is low, and short when the S/N ratio is high.

The example shown in FIG. 2(b) shows the S/N ratio measured by the S/N ratio measuring circuit (15) with a predetermined threshold as the boundary.
Two types of synchronization protection lengths are selected based on the N ratio. In other words, when the S/N ratio is low, a long frame synchronization protection length is used.
When the N ratio is high, select a short synchronization protection length.

[0029]

Effects of the Invention According to the present invention, when receiving broadcasting with a good C/N ratio, and when using a MUSE-VTR or MUSE-VTR with a good S/N ratio,
When reproducing from a VDP, the period of frame protection is shortened, so that it is possible to suppress the occurrence of noise in the reproduced audio signal due to effects such as special reproduction, rewinding of the VTR, and movement of the VDP head. When receiving a weak electric field with a poor S/N ratio, the frame protection period is lengthened, so that the broadcast audio is less likely to be muted and interrupted midway.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a MUSE audio decoder in one embodiment of the present invention.

FIG. 2 is a diagram for explaining switching of synchronization protection length.

FIG. 3 is a block diagram of a conventional MUSE audio decoder.

[Explanation of symbols]

16 Mute circuit 13 Frame synchronization detection circuit 14 Frame synchronization protection circuit 17 Timing signal generation circuit 15 S/N ratio measurement circuit

Claims (4)

[Claims] 1. An audio signal processing circuit having a frame synchronization protection circuit that decodes a MUSE audio signal based on a frame synchronization signal in the MUSE audio signal and protects the erroneous detection of the frame synchronization signal for a predetermined frame period, An audio signal processing circuit that switches a synchronization protection period of the frame synchronization protection circuit based on a value of an S/N ratio of a MUSE signal. [Claim 2] A MUSE audio signal is input, and the MUSE
The audio signal processing circuit that decodes and reproduces the DPCM audio signal in the USE signal includes an S/N ratio measurement circuit (15) that measures the S/N ratio of the MUSE signal, and multiple frame synchronization protection lengths that can be selected. A frame synchronization protection circuit (14) having a synchronization protection length determining means, and switching the synchronization protection length based on the S/N ratio measured by the S/N ratio measurement circuit (15). signal processing circuit. 3. In an audio signal processing circuit to which a video signal is input and which decodes and reproduces an audio signal multiplexed with the video signal, the audio signal is adjusted according to the S/N ratio of the video signal. Audio signal processing circuit that switches the frame synchronization protection period. 4. After converting the analog audio signal into a digital audio signal on the transmitting side, a frame synchronization signal is added and interleaved, and the video signal is frequency- or time-division multiplexed and transmitted, and the receiving side converts the frame synchronization signal into a digital audio signal. In an audio signal processing method for reproducing the digital audio signal by deinterleaving in synchronization with the signal and further reproducing the analog audio signal, means for determining the S/N ratio of the video signal received at the receiving side. (15): An audio signal processing method characterized in that the protection period for the frame synchronization signal is changed according to the determination result.