Coded data generation or conversion – Converter compensation
Patent
1998-01-06
1999-09-28
Hoff, Marc S.
Coded data generation or conversion
Converter compensation
341144, H03M 106
Patent
active
059595612
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a digital to analogue converter. More specifically the invention relates to a circuit for converting digital data representing audio as a multiplicity of sample values into an analogue audio signal.
Compact discs are now used extensively as a recording medium for audio signals. Compact discs provide a good, robust storage medium for and enable high quality playback of audio signals.
FIGS. 1 to 3 of the accompanying drawings illustrate generally the process by which an audio signal is converted into digital data. An original audio signal is represented by the plot in FIG. 1. The audio signal is continuous and has a zero value, i.e. it crosses the horizontal time axis 2 at an instant of time T.sub.0. As shown in FIG. 2, the analogue signal 1 is sampled at regularly spaced instances 3 to 6. Each sample has a level 7 to 10 which is converted into digital data having values corresponding to the levels. During playback the digital data is converted into a decoded analogue signal 11 such as shown in FIG. 3. The decoded analogue signal 11 is a stepwise approximation of the original analogue signal 1. It will, of course, be appreciated that the distance between adjacent levels represented by the digital data is in fact very small and that the steps shown in the decoded analogue signal of FIG. 3 have been exaggerated for the purpose of illustration only.
One of the limitations of representing an analogue signal in digital form is that some of the information in the original signal is lost. For example, frequency components greater than 22 kHz cannot be retrieved from the digital data. Also, the exact instances T.sub.0 at which zero crossings occur are not known. FIG. 4 shows one example of a signal 12 crossing zero at a point T.sub.0 and a decoded analogue signal 13 having a zero period P.sub.0 corresponding to the time interval between consecutive samples of the digital data. The zero crossing point T.sub.0 of the original analogue signal 12 happens to coincide with a sampling instant. In FIG. 5 an original analogue signal 14 has a zero crossing point which happens to fall between two sampling instances and has a slope which results in sample values of zero on each side of the zero crossing. Consequently, the decoded analogue signal 15 has a zero period P.sub.0 corresponding to two sampling periods. That is to say, in the situation shown in FIG. 5 zero period P.sub.0 corresponding to the zero crossing point T.sub.0 is exaggerated further as compared to the situation shown in FIG. 4.
Generally, it is believed that the loss of accurate zero crossings in the decoded analogue signal is unimportant. The human ear did not evolve to detect zero crossings and so a listener should not be able to sense them in the decoded audio during playback. However, research has shown that whilst zero crossings cannot be sensed, they are nevertheless perceived during playback on expensive, high quality audio equipment. The perception is that something is not quite right with the playback. Loss of a zero crossing results in a temporary loss of phase information and it may be that loss which is being perceived by the listener.
The present invention aims to overcome or at least reduce the above-discussed problems without the need to redefine the coding standard used in audio compact discs.
According to one aspect of the invention there is provided a circuit for converting digital data representing audio as a multiplicity of sample values into an analogue audio signal, the circuit comprising: a store for storing plural sample values; a signal generator for generating a correcting signal having a level depending on the sign of each of the stored sample values; a digital to analogue converter for creating an analogue signal from the stored sample values; and a combiner for combining the correcting signal with the analogue signal to produce a combined signal for output as the analogue audio signal.
According to another aspect of the invention there is provided a circuit in which digital data representing sample values o
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