Audio copy protection system
An apparatus for applying an anti-copy protection signal for use with switchable head recorders, such as Video Hi-Fi recorders, is described. A high frequency pulsed signal is added to an original signal. The pulses interfere with the compensator circuitry such that the switching of the record head cannot adequately be masked on recording of a signal. On playback, the switching of the recording heads can subsequently be heard. In Video Hi-Fi recorders, the effect of the high frequency signals also interferes with the compressor-expander circuits to worsen the sound of the head switching.
1. An apparatus to copy a received signal, comprising:
an input for receiving the signal, wherein the signal comprises an audio component;
a recording circuit configured to record the signal for later playback such that when the signal is recorded, at least the audio component of the signal is recorded with discontinuities; and
an output for outputting the protected recorded signal for playback, wherein a protection signal in the audio component of the received signal causes attempts to mask the discontinuities of the audio component to become audible on playback of the recorded signal.
2. An apparatus according to claim 1 , wherein the protection signal is a pulsed protection signal.
3. An apparatus according to claim 1 , wherein the protection signal comprises pulses that at least partially overlap with the discontinuities in the audio signal component.
4. An apparatus according to claim 3 , wherein the protection signal pulses at a rate that is identical or close to identical with that at which the discontinuities occur.
5. An apparatus according to claim 4 , wherein a rate at which the pulses occur is substantially the field synchronisation rate of a video picture.
6. An apparatus according to claim 4 , wherein a rate at which the pulses are arranged to occur is 50 Hz or 60 Hz±5 Hz.
7. An apparatus according to claim 1 , wherein a frequency of the protection signal is substantially at or beyond the upper end of the range of frequencies audible to the human ear.
8. An apparatus according to claim 7 , wherein the frequency of the protection signal is 19.5 kHz.
9. An apparatus according to claim 8 wherein the protection signal has a magnitude between 0.05 Vrms and 2 Vrms.
10. An anti-copy signal protection method for use by a recording device in which at least an audio component of the signal is recorded with discontinuities, and in which a compensating circuit is employed by the recording device to mask the discontinuities on recording and/or playback, the method comprising adding a protection signal to the audio component of the signal that impairs the operation of the compensating circuit of the recording device such that the attempts to mask the discontinuities in the recorded signal are audible on playback of a copy.
11. The method of claim 10 , wherein the protection signal is a pulsed signal.
12. The method of claim 11 , comprising adding the pulsed signal such that the pulses at least partially overlap with the discontinuities in the audio signal.
13. The method of claim 12 , wherein the frequency at which the pulses of the pulsed signal occur is arranged to be identical or close to identical with the frequency at which the discontinuities occur.
14. A method according to claim 12 , wherein the repetition rate of the pulses is substantially the field synchronisation rate of a video picture.
15. A method according to claim 10 , wherein a frequency of the protection signal is substantially at the upper end of the range of frequencies audible to the human ear.
16. The method of claim 10 wherein a signal is added to the audio component of the signal at or near an integer multiple of the line synchronisation rate of the video recorder to form protected signal, such that when the protected signal is recorded on a video recorder, one or more artifacts are produced which result in a reduced quality of the recorded signal on playback.
17. A detector for detecting an anti-copy protection signal in a received signal, the detector comprising:
a pulse detector arranged to identify pulses occurring in an audio portion of the received signal, wherein the audio portion of the received signal is recorded with discontinuities that are masked during playback and wherein the pulses are configured to interfere with the masking of the discontinuities;
an analyzer for determining values for one or more parameters of the pulses detected by the pulse detector;
control logic associating different values of each parameter with a control instruction for a recording device; and
an output for outputting a signal indicative of the control instruction associated with the values of the parameters of the analyzed pulse to the recording device.
18. The detector of claim 17 wherein the parameters include the location, the timing, the amplitude, the duration, the shape, or the frequency of the pulse.