On instantaneously adaptive delta modulation and encoding of video signals
2013-06-04T12:33:27Z (GMT) by
Conventional pulse-code and differential pulse-code modulators for encoding video signals are difficult to realise economically. To alleviate this problem, a technique which divides the modulators into two stages is proposed. The first stage is a two-bit instantaneously adaptive delta modulator operating at a high clock rate and using low-precision components. Two-bit signals conveying polarity and magnitude information are produced by this delta modulator and used as the input to the second stage, a code converter. The code converter transforms, digitally, delta modulated signals into Pulse Code Modulation (PCM) or Differential Pulse Code Modulation (DPCM) signals. The resolution of the final PCM or DPCM encoder depends on the performance of the delta modulator used as the input stage. For that reason, the performance of the two-bit Instantaneously Adaptive Delta Modulation (2BIADM) encoder is evaluated. This evaluation is made in two steps. First, a semi-empirical anaysis of the High Information Delta Modulation (HIDM) is made, because the 2BIADM system is derived from the HIDM. Then the performance of the 2BIADM is derived considering the HIDM as a reference. For the HIDM and 2BIADM modulators operating at the same sampling frequency, the 2BIADM presents an improvement in peak signal-to-noise ratio (SNR) Of 6 to 8 dB. Expressions are established to enable SNR to be calculated for the HIDM as a function of the encoding parameters. The expressions also apply to Constant Factor Delta Modulation, and represent the only known method of estimating numerically the SNR for instantaneously adaptive delta modulators. The 2BIADM was tested, built and operated at a low sampling rate. This gave an insight into the operation of the proposed system, and complemented the computer simuLation analyses. The principles for the code conversion from the 2BIADM to PCM or DPCM are fully discussed. The 2BIADM does not impose restrictions on the values that the coefficients of the digital low-pass filter required in the code converter can assume. For low bandwidth expansion rates, it was verified that a 2BIADM-to-PCM conversion filter with 5 stages performs better than a HIDM-to-PCM conversion with a filter having 256 stages (both encoders operating at the same word-rate). A generalization of the 2-bit encoder to a N-bit adaptive DPCM system is outlined.