A Sample Interframe Coder for Video Telephony

In a previous paper we described a way to halve the channel capacity required for the subjectively satisfactory transmission of an 8-bit PCM television signal by exchanging spatial and temporal resolution according to the amount of movement in the local part of the picture 1 Every second picture element ("pel") is sampled and in stationary areas of the picture the values of the unsampled pels are interpolated from adjacent temporal samples (reduced temporal resolution) ; in the moving areas the values of the unsampled elements are interpolated from neighboring sampled elements in the same line (reduced spatial resolution). We would like to apply this technique to a signal whose bit rate has already been reduced by an element-to-element differential quantizer (EDQ>, e.g., the Picturephone® codec. Unfortunately, halving the horizontal sampling rate, as in Ref. 1, increases the amplitude of the sample-to-sample difference signal which, in turn, requires a larger number of quantizing levels for adequate representation. There are two ways around this problem: 1877