DETAILED DESCRIPTION Briefly, this and other objects are accomplished by a spectral error correcting system the operation of which is based on the fact that an isolated bit error in a pulse code modulated digital television signal produces a spike in the video having spectral energy well beyond the frequency band of a composite color television signal.
The principle applies to television signals of any standard, including NTSC, PAL and SECAM, but will be described herein as applied to the NTSC standard.
It is assumed that the sampling frequency, F.
sub.
s, is higher than twice the highest video signal F.
sub.
v.
Digital composite NTSC signals are typically sampled at frequencies equal to or greater than 2.
5 F.
sub.
v.
In theory, an error-free digital NTSC television signal lacks energy in the neighborhood of F.
sub.
s /2 and odd multiples thereof.
Isolated bit errors, however, generate significant levels of energy in the region of F.
sub.
s /2; thus, by measuring the energy appearing at F.
sub.
s /2 the presence of bit errors can be established.
Also, errors in the more significant bits of a sample generate higher energy than do errors in less significant bits; therefore, the amplitude of the signal measured in the region of F.
sub.
s /2 is proportional to the significance of the bit in error, which identifies the bit and makes it correctable.
In accordance with the invention, the digital television signal is converted to analog form, and such energy as is present in the region of F.
sub.
s /2 due to isolated bit errors passes through a linear-phase, sharp bandpass filter having a center frequency of F.
sub.
s /2 and produces frequency bursts.
The output from the filter is synchronously demodulated to produce pulses corresponding to the envelope of the frequency burst, the peak of which is representative of the location of the sample in error and the amplitude of which is indicative of the significance of the bit in error.
The envelope pulses are processed to determine the location of the errors in the digital television signal, and the significance of the bits in error, and correction is applied to the digital signal by simply inverting the appropriate bit in the sample that was found to be in error
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Image Analysis 
