Re: PAL Video

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Fri, 8 Nov 1996 09:15:39 -0600
>From: "P3D  Joerg Meyer Inf.]"  <jmeyer@xxxxxxxxxxxxxx>
>Subject: Re: PAL Video

>>I believe someone once told me that PAL requires the storage of a line of
>>video information for the error correction to work. If this is correct,
>>do you recall how this was stored in the days before digital memories?

>A PAL circuit contains a delay line, which delays the signal
>of a line until the next line is transmitted. The two signals
>are superimposed and thus compensate for errors due to
>transmission interference. This procedure is serial, so no
>storage device is required.

Thanks for the information.

A delay line *is* a storage device - whatever part of the signal is currently
in transit through the delay line is in storage. Very simple delay lines
store very little data - more sophisticated ones can store large amounts.
If a PAL system compares the video signal at each pixel, that would involve
storing a whole line of video data, which would be a challenging task in the
days before digital memories.

NIST had one of the very early computers - SEAC. That machine used
recirculating acoustic mercury delay lines as its main working memory
(where we would use DRAM in modern designs).

Advanced video data storage may be beneficial to stereo image transmission
in the near future.

John R


------------------------------