This invention relates to the quantizing of a real value to an approximate binary representation. More particularly, it is concerned with apparatus for obtaining binary quantized values of increasingly closer approximation to the real value being quantized.
Quantization is the process of representing a sample, possibly an infinitely valued analog sample, by an approximate value from a finite set of allowed values. The sample is compared with a set of quantization threshold levels corresponding to a set of allowed reconstruction levels, and an output symbol, usually in the binary format, is produced to indicate which pair of threshold levels the sample is located between. To reconstruct the sample, or dequantize, the output symbol in binary format is employed to retrieve the closest approximate reconstructed value from a set of allowed values.
In some applications, such as variable-rate transmission and progressive transmission for video and speech signals, embedded quantization is desired. The embedded quantizer allows a coarse approximation of the sample to be transmitted initially and the approximation to be improved successively as additional information bits become available. With embedded quantization the previous output is not altered as it is changed from coarse resolution to finer resolution. For example, if (b.sub.1, b.sub.2, b.sub.3) is the binary output from a 3-bit quantizer, the output symbol for the 4-bit quantizer should be (b.sub.1, b.sub.2, b.sub.3, x), where "x" is a new binary digit appended to the previous output. In other words, the output symbol of the 3-bit quantizer is embedded in that of the 4-bit quantizer. Therefore, a partial output from a quantizer can be used to make a coarse approximation of the input sample and, also, additional information bits can be appended to an existing output to achieve finer approximation.