JP2568562B2 - Illegal copy prevention system - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system for preventing illegal copying of audio signals.

2. Description of the Related Art In order to prevent illegal recording from commercially available compact discs (CDs) and digital audio tapes (DATs), illegal copy prevention systems are being studied (for example, JAS Journal '87 May May issue). 5-14
page). As a method of encoding a copy code into an audio signal, a method of removing a frequency component of 3840 Hz ± 125 Hz has been proposed. As a method of detecting a copy code, a consumer's recording device examines a frequency component near and around 3840 Hz, and determines that there is a copy code when there is no such component alone.

Problems to be Solved by the Invention However, in the illegal copy prevention system as described above,
For music where the frequency component around 3840 Hz is important, there is a problem that the sound quality is seriously degraded.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides an illegal copy prevention system using a copy code with little deterioration in sound quality.

Means for Solving the Problems In order to solve the above problems, an illegal copy prevention system according to the present invention comprises a copy code encoder including a delay unit, an amplifier, and an adder, a low-frequency component removal filter, a normalized autocorrelation. It has a configuration called a copy code detector composed of a function calculator, a maximum function value calculator, an average function value calculator, and a level difference determiner.

Operation The present invention has the advantage that, with the above-described configuration, a slight echo is added to an audio signal with a copy code, and deterioration in sound quality is small.

Embodiment An illegal copy prevention system according to one embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 (a) shows an embodiment of a copy code encoder of the illegal copy protection system of the present invention. 1 is a delay unit, 2 is an amplifier, and 3 is an adder. The original audio signal is delayed by the delay unit 1 for T seconds. This output is amplified K times by the amplifier 2. However, K is a positive value smaller than 1. This output is added to the original audio signal in the adder 3. This output is an audio signal with a copy code.

FIG. 1B shows an embodiment of a copy code detector of the illegal copy protection system. Reference numeral 4 denotes a low-frequency component removal filter, 5 denotes a normalized autocorrelation function calculator, 6 denotes a maximum function value calculator, 7 denotes an average function value calculator, and 8 denotes a level difference determiner. A general audio signal including an audio signal with a copy code passes through a low-frequency component removing filter 4. This is the normalized autocorrelation function R (τ)
In order to reduce the portion other than τ = 0. Next, a normalized autocorrelation function calculator 5 calculates a normalized autocorrelation function R (τ). This calculation method will be described below. The output of the filter 4 is defined as A (t). The case of an audio signal without a copy code is defined as a (t). Let the autocorrelation function of a (t) be r (τ). Also, t takes a discrete value.

Here, N is the length of the section for taking the autocorrelation.

Here, it is assumed that τ is sufficiently smaller than N.

When A (t) = a (t) Becomes This is shown in FIG. Where the vertical axis is R
(Τ), the horizontal axis is τ. The graph shown by the solid line is R (τ) in this case.

Next, a case where a copy code is attached will be described.

At this time, A (t) = a (t) + k · a (t−T) (4) If a (t) has ergodicity, From k≪1, | r _N (τ) | ≦ 1, R (τ) r _N (τ) + k ｛r _N (τ−T) + r _N (τ + T)｝ (7) The second is the effect of the copy code.

When τ = T or τ = −T, R (τ) r _N (T) + k (8) The effect of the copy code is shown by the dotted line in FIG. Let ΔT ₁ <ΔT ₃ and ΔT ₂ <ΔT ₄ . These values are smaller than T.

The maximum function value calculator 6 calculates the maximum value of R (t) in T−ΔT ₁ ≦ τ ≦ T + ΔT ₂ .

The average function value calculator 7 calculates an average value of R (τ) in T−ΔT ₃ ≦ τ ≦ T + ΔT ₁ and T + ΔT ₂ ≦ τ ≦ T + ΔT ₄ .

Subtracting the output of the calculator 7 from the output of the level difference determiner 8, calculator 6 to see if the result is in the between k-.DELTA.k ₁ of k + Δk _2. However, Δk ₁ and Δk ₂ are smaller values than k. If it is during this time, it is determined that there is a copy code. Otherwise, it is determined that there is no copy code. This result is output.

As described above, according to the present embodiment, significant deterioration in sound quality can be prevented by encoding a small echo component as a copy code.

FIG. 1A illustrates an example in which one delay device 1 and one amplifier 2 are used. These may be plural sets in parallel. However, in this case, a plurality of sets of the calculator 6, the calculator 7, and the determiner 8 are used on the detector side. However, in principle, it is the same as this embodiment.

Further, a filter equivalent to the filter 4 may be inserted in series with the delay unit 1 and the amplifier 2. The sound quality will change slightly, but will not change as the detector output. The filter 4 may be a band pass filter or a high pass filter.

Effect of the Invention As described above, the present invention provides a delay unit, an amplifier, and an adder as a copy code encoder and a normalized autocorrelation function calculator as a copy code detector, thereby preventing illegal copy code with little deterioration in sound quality. The system can also be configured.

[Brief description of the drawings]

FIG. 1 (a) is a configuration diagram of a copy code encoder in an embodiment of the present invention, FIG. 1 (b) is a configuration diagram of a copy code detector, and FIG. 2 is a graph showing a normalized autocorrelation function. It is. DESCRIPTION OF SYMBOLS 1 ... Delay device, 2 ... Amplifier, 3 ... Adder, 4 ... Low frequency component removal filter, 5 ... Normalized autocorrelation function calculator, 6 ... Maximum function value calculator, 7 ... Average function value calculator 8, level difference determiner.

Claims (1)

(57) [Claims] 1. An echo signal generating means comprising: a delay unit having an input of an audio original signal and having a delay time T; and an amplifier having an amplification factor K times that of the input signal; A copy code encoder having an adder for adding the signal and the output of the adder as an audio signal with a copy code; and a low-frequency signal inputting an audio signal including the audio signal with the copy code and removing a low frequency component. A frequency component elimination filter, a normalized autocorrelation function calculator that receives the output of the low frequency component elimination filter as input, and calculates a normalized autocorrelation function R (τ); and a delay that receives the output of the normalized autocorrelation function calculator as input. A maximum function value calculator for calculating a maximum value of a normalized autocorrelation function R (τ) in a region including T as time; An average function value calculator that receives an output of the autocorrelation function calculator as an input and calculates an average value of a normalized autocorrelation function R (τ) in a region that does not include T as a delay time; When a value obtained by subtracting the average function value calculator output from the maximum function value calculator output with the function value calculator output as an input is within an area including K, it is determined that a copy code exists, and the copy code is not included. An illegal copy prevention system comprising: a level difference determiner that determines that there is no copy code in the case; and a copy code detector that uses the output of the level difference determiner as a copy code presence / absence determination result.