JPS61180902A - FM audio playback circuit - Google Patents

Abstract

PURPOSE:To improve the sound quality by performing the preholding, band limitation and muting for the demodulation signal in response to the drop-out period of the FM signal. CONSTITUTION:The drop-out of the FM signal is detected by a drop-out detect ing circuit DOPET9. Then a subsequent preholding circuit 6 of a DEM 5 is controlled and the demodulated audio signal is held at the value obtained imme diately before the drop-out of the FM signal. For the drop-out of a comparative ly long period, a band limit detecting circuit 10 for a comparatively long-period drop-out is used for actuation of a band limiting circuit 7. The circuit 7 drops the high band component for smoothening and deletes the harmful pulsative tones. Furthermore an MUTE circuit 8 following the circuit 7 is controlled by an MUTE detecting circuit 11 for discontinuation of output against a longer a drop-out period.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a playback circuit for bx FM audio;
The present invention relates to a circuit that obtains high-quality reproduced audio signals even when signal dropouts occur.

[Background of the invention]

When reproducing and demodulating an audio signal modulated by magnetic tape KFM, the F
If there is a missing part of the M signal, the demodulated signal becomes noise as shown in b), leading to a deterioration in the audible sound quality.

In addition, for those related to this type of device, for example,
No. 5-72220 is mentioned.

[Purpose of the invention]

An object of the present invention is to provide a high-quality FM audio signal demodulation circuit that removes the noise.

[Summary of the invention]

In order to achieve the above object, the present invention performs pre-holding, band limiting, and meeting operations on the demodulated signal according to the dropout period of the FM signal, thereby improving the perceptual sound quality.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

A magnetic tape 1 in FIG. 1 records a Hi-Fi audio signal of about 20 Hz to 20 KHz that is FM-modulated with a carrier around 1.5 MHz.
The FM signal reproduced by is amplified by an FM reproduction preamplifier 3, passes through a BrF 3, which passes the FM signal in a band, and is demodulated into an audio signal by an FM demodulator (DgM) 5.

'82 Figure 17) a) is the demodulated signal of the FM signal b), c) o and d) c. If the FM signal is unevenly coated with magnetic powder on the tape, the head touches the tape, there is instant dust, etc., an FM signal dropout will occur as in the '1m't+'3 section.

This FM signal dropout is detected by the dropout detection circuit DODET connected to BPF4 in FIG.
EM5's subsequent pre-hold circuit is controlled, and "1" in Figure 2 C) is
．． The demodulated audio signal is held at the value immediately before dropout as in the tt interval. At this time, if the dropout is short compared to the period of the demodulated signal, as in the t1 interval, there is no audible problem with this pre-holding operation, but for a slightly longer period of dropout tz, the t1 interval waveform in C) Since the discontinuous sections are audibly noticeable, the band limit detection circuit 7 is operated in the band limit detection circuit 10 for detecting dropouts in a relatively long period shown in FIG. This reduces high-frequency components, smooths them out, and removes pulsative sounds that cause problems with hearing.

For a longer dropout section t3 caused by bending of the tape, etc., the MU'I shown in FIG.
”Fli detection circuit 11 connects the band-limiting circuit to the subsequent MUT.
Control the E circuit 8 and stop the output.

To determine the dropout period, a CR time constant circuit is connected to the transistor base, and the length of the input dropout detection signal is determined.

The audio signal subjected to the signal processing at the time of dropout is sent to a noise reduction circuit for 8/N improvement, an output amplifier 15, and an output terminal 14.

When there is no dropout at all or when there is a short dropout, the FM signal in a) in Figure 2 is reproduced by a wide band 20H1 to 20KHz amplifier, as shown in d), and when a relatively long dropout occurs, After pre-maintaining, the band is limited, for example, by playing with a frequency characteristic of 2oHz to 5KHz, and when there is a long dropout, no sound is emitted, and Hi-Fi replay is performed without causing any unnatural hearing. It is.

〔Effect of the invention〕

According to the present invention, as described above, deterioration in sound quality due to dropouts peculiar to magnetic tapes can be prevented, and it is effective in converting to k''MM audio SPT.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a signal waveform diagram. 5...FM demodulator, 6...Pre-hold circuit, 7...Band limit circuit, 8...MUTE circuit, 9...・Dropout detection circuit.

Claims (1)

[Claims] 1. In a magnetic recording and reproducing circuit that records and reproduces FM audio signals, the head, FM reproduction amplifier, FM signal bandpass filter, FM demodulator, prehold circuit, band limit circuit, MUTE circuit, noise reduction circuit, and output amplifier are A DODET that detects short-term dropouts of the FM signal is connected to the BPF output, and a band limit detection circuit that detects relatively long-term dropouts is connected to the DODET output, and an MUT that detects even longer-term dropouts.
E detection circuit is connected, DODET is connected to the pre-holding circuit of the demodulated signal, the band limit detection circuit is connected to the band limit circuit of the demodulated signal, and the MUTE detection circuit is connected to the MUTE circuit of the demodulated signal, and the drop of the FM signal is connected. An FM audio playback circuit characterized in that it performs pre-hold, band limit, and MUTE operations depending on the out period to obtain a playback signal of good audible quality.