1. Technical Field
The present invention relates to an echo attaching circuit and an audio device using the same and, particularly, to an improvement of a circuit for producing an echo signal from an audio signal and attaching the echo signal to the audio signal, that is, an echo attaching circuit to be used in an audio device such as Karaoke device, component stereo device, radio cassette device or VTR, etc.
2. Background Art
Almost all of the recent audio devices are equipped with the so-called Karaoke function with which an audio input from a microphone can be mixed or synthesized with another reproduced signal, etc., and output externally. For such function, an echo attaching function for attaching an echo effect is indispensable for which the echo signal must be internally produced. Echo signal is usually produced by delaying an audio signal, which is obtained by converting an audio information such as voice and/or music into an electric signal, by a certain constant time. In order to enhance the echo effect, the echo attaching circuit operates to attenuate the audio signal and the echo signal and feeds back them to the side of the audio signal to produce a signal which is a mixture of the audio signal and the fed back signal, and the delaying and mixing operations are repeated for thus the produced signal.
FIG. 5 shows an example of a construction of a conventional echo attaching circuit. In FIG. 5, an echo attaching circuit 8 includes a waveform synthesizing circuit 1, a low-pass filter (LPF) 2, a delay circuit 3 using a BBD (Bucket Brigade Device), a low-pass filter 4, attenuators (ATTs) 5 and 6 and a waveform synthesizer 7.
An audio signal A input to the echo attaching circuit 8 is synthesized or combined in the waveform synthesizer 7 with a signal which is obtained by attenuating an echo signal B down to a suitable level by the attenuator 6, resulting in an audio signal C to which the echo is attached. The audio signal C is amplified and output from a loudspeaker as sound or recorded on such as tape by means of a recording circuit.
The echo signal B is produced by a feedback loop including the attenuator. That is, the echo signal B output is attenuated by the attenuator 5 down to a suitable level and fed back to an input side in which it is mixed with the audio signal A by the waveform synthesizer 1. Then, the synthesized signal is passed through the low-pass filter 2, resulting in a signal D. A delay signal E is produced by delaying the signal D by a constant time by the delay circuit (BBD) 3 and the echo signal B is produced by passing the signal E through the low-pass filter 4. The low-pass filters 2 and 4 are provided in an upstream and a down stream of the BBD 3 to prevent an aliasing distortion by allowing components whose frequencies are lower than a half of a sampling frequency of the BBD 3 to pass through. By removing a noise component of the delay signal E by means of the low-pass filter 4, the delay signal E becomes the echo signal B. The audio signal becomes the echo signal by further attenuating the echo signal E thus produced by delay and this is circulated while being attenuated and the echo effect is obtained by repeating this procedure.
The BBD 3 having construction for delaying an analog signal itself is noise-sensitive and easily degraded with time. Therefore, its accuracy and reliability are insufficient. On the other hand, demand of improvement of function of the audio device by users or customers is becoming severe more and more, and, for example, a dynamic range of about 90 dB is required for an audio signal with echo signal in Karaoke function.
In order to satisfy such demand, it is a recent tendency that the portion of the delay circuit 3 is constituted with, instead of BBD, a digital circuit capable of improving the function.
FIG. 6 shows a block circuit of a delay portion of the delay circuit 3 when it is replaced by a digital circuit. The delay portion includes an A/D converter 31, a memory control circuit (CTRL) 32, a memory 33 and a D/A converter 34. In this delay circuit, a signal D from the low-pass filter 2 is converted by the A/D converter 31 into a digital value. The memory control circuit 32 stores the digital value in the memory 33 in sequence. The memory control circuit 32 reads out from the memory 33 sequentially digital values stored at a time prior to a current time by a constant time and sends them to the D/A converter 34. The D/A converter 34 converts them into analog values and outputs them to the low-pass filter 4 as the delay signal E.
By employing such digital circuit, influence of the sensitivity to noise and the degradation with time can be prevented.