The present invention concerns a system for resampling, and in particular, to a digital resampling system which resamples signals using a quantized phase.
Sample re-conversion systems are well known for converting information bearing signals from one format to another format. Such systems are commonly used for data telemetry, audio signal processing, video signal processing, and video signal standards conversion.
An exemplary video signal processing system is described in U.S. Pat. No. 4,774,581 entitled TELEVISION PICTURE ZOOM SYSTEM, issued to Shiratsuchi, which is hereby incorporated by reference for its teachings on digital resampling techniques.
Another exemplary video system standards conversions system is described in U.S. Pat. No. 5,057,911 entitled, SYSTEM AND METHOD FOR CONVERSION OF DIGITAL VIDEO SIGNALS, which is hereby incorporated by reference for its teachings on digital resampling techniques.
The system described in the SYSTEM AND METHOD FOR CONVERSION OF DIGITAL VIDEO SIGNALS patent converts video signals having a normal sample rate of 13.5 MHz into a digital signal having a sample rate of 14.31818 MHz, (hereinafter 14.3 MHz) which is compatible with the National Television Standards Committee (NTSC) standard. The ratio of these two sample frequencies is exactly equal to the ratio of 33 to 35.
Accordingly, the standards conversions system described in the referenced patent generates 35 output samples for every 33 input samples it receives. This is accomplished using 35 interpolation filters which generate interpolated samples at 35 respective positions between any two of the input samples. The filter, X(i+1), which is selected to generate the i+1.sup.th output sample is defined by equation (1). EQU X(i+1)=(X(i)+33) MOD 35 (1)
In equation (1) 33 is the number of input samples, 35 is the number of output samples, MOD is an abbreviation of a modulo operation, X is the interpolation phase location for the current sample in the sequence, and i is the time index. The interpolation phase location defines a particular frequency response characteristic to be used to filter the input samples in order to generate the particular resampled output sample.
This technique of sample rate conversion works well when the input and output sample rates can be represented as a ratio of two relatively small numbers. When, however, the ratio of these rates cannot be represented to a high degree of precision by a manageable ratio, this conversion process may be inaccurate or unduly expensive to implement.
For example, consider a resampling circuit which converts from the CCIR (601) standard having a nominal sample rate of 13.5 MHz to the Phase Alternate Line (PAL) standard which has a sample rate of 17.734475 (hereinafter 17.7) MHz. The smallest exact ratio between these two sample frequencies is 540,000 over 709,379. Thus, if the scheme described above were used to resample the CCIR (601) signal in a PAL digital signal, 709,379 digital filters would be required. At the current state of the art, it is not practical to implement a video standards conversion system having 709,730 filters.
An alternative sampling scheme may be used where the resampling function is approximated by limiting the number of possible interpolation points. As a result, the actual interpolated point is estimated by choosing the filter that is closest to the desired interpolated point. This is known as sample phase quantization. This technique introduces a phase shift error in the signal. However, with proper quantization, this error may be acceptable.
A system employing sample phase quantization would include a fixed number of filters or a filter with a fixed number of frequency characteristics which number is designated as NPHASES. Next, the number of filters in the ideal system is quantized into the number of filters, NPHASES, for the system. This is accomplished using equations (2) and (3) below. EQU X.sub.i+1 =(X.sub.i +A) MOD B (2) EQU Q=X.sub.i+1 * NPHASES/B (3)
In equations (2) and (3), B is the number of samples in the output line of the filter, A is the number of samples in the input line of the filter, and Q is the quantized phase. The quantized phase is used to determine which filter in the system is to be selected to produce the new sample.
Although equations (2) and (3) may be used to quantize the filters and phases, the equations require division and multiplication operations which are computationally expensive.