Conventional data processing systems may provide digital signal processing capabilities for converting an analog signal into digital form. When sampling an analog signal, such as audio, to convert it into digital form for storage, the number of samples taken per second (sampling rate) determines the quality of the signal that is generated when the digital signal is converted back to an analog form. According to the Nyquist Theorem, a band-limited analog signal, x(t), can be reconstructed from its sample values, x(nT), if the sampling rate, 1/T, is greater than twice the highest frequency, f, present in x(t). The sampling rate, 2*f, is referred to as the Nyquist rate.
The sampling rate in a conventional data processing system is either determined by the designer or is left to the user to decide. If a sampling rate smaller than the Nyquist rate is chosen (under sampling), information will be lost from the original analog signal, decreasing the fidelity of the digital signal when it is played back. Over sampling (selecting a sampling rate much higher than the Nyquist rate), on the other hand, will cause the sampling rate of the sampled signal to be larger than what is needed to reconstruct the signal from its samples, thus wasting valuable storage space. Conventional systems do not determine the Nyquist rate of the analog signal, and thus cannot prevent the information loss caused by under sampling or the storage waste caused by over sampling. This problem is compounded in a multimedia data processing system which may be converting multiple analog signals, all with differing Nyquist rates.
Prior to U.S. Pat. No. 5,302,950 entitled Method of and Apparatus For Providing Automatic Determination of Information Sampling Rate, invented by the same inventors and assigned to the same assignee as the present invention and incorporated herein by reference, the prior art approaches presented the user with seven difficulties. The first difficulty was that the prior art approaches did not automatically determine the Nyquist rate of an analog signal. The second difficulty was that the prior art approaches did not automatically determine a sampling rate for an analog signal based on the analog signal's Nyquist rate. The third difficulty was that the prior art approaches did not inform a user that a loss of information due to under sampling may occur. The fourth difficulty was that the prior art approaches did not inform a user that a waste of storage due to over sampling may occur. The fifth difficulty was that the prior art approaches did not provide a user a choice of alternative actions when such a loss of information or a waste of storage may occur. The sixth difficulty was that the prior art approaches did not adjust a sampling rate to prevent either a loss of information or a waste of storage. The seventh difficulty was that the prior art approaches did not adjust a sampling rate to available storage.
The previously mentioned U.S. Pat. No. 5,302,950 solved these difficulties by (1) determining a highest frequency component of an analog signal; (2) calculating a Nyquist rate responsive to the highest frequency component; (3) altering a sampling rate responsive to the Nyquist rate by informing a user that the sampling rate and the Nyquist rate were not equal and allowing the user to select an appropriate action wherein (4) the sampling rate is determined automatically by a size of a user specified storage repository. Thus, the invention of the '950 patent solved the seven and other problems of the prior art by attacking the first part of the process of sampling and storing an analog signal. The sampling rate was modified to accommodate the highest determined frequency so that no information was lost and to accommodate available memory space.
Often it is difficult to determine how best to sample an audio signal, particularly when storage resources are limited and frequency range of the audio information is not known. An efficient method for sampling audio information is therefore needed when variables governing sample size are not known.
Another problem in storage efficiency arises when sound to be digitally recorded varies from very low frequencies to very high frequencies, particularly out of the range of human hearing. Sampling the audio signal must be performed according to the highest frequency in an entire recording session to ensure all sound is captured. A method which allows dynamically changing the sampling rate during a session for the purpose of not over-sampling is needed. Excessive sampling wastes storage by having duplicated or redundant information within a time interval.