#151 Drift in decoding raw VBI data

[nigel o\'brian]

I've been using the zvbi library (via a modified version of the
'capture' utility in the test directory) to access channel 31
(datacasting) teletext data, using an old Pinnacle bt848 (PAL) tv
card.

The data packets have a payload of 36 bytes including a 2 byte
CRC checksum. Using the CRC I can detect and correct any single
1 bit error per packet.

I am seeing quite a lot of errors, but they are not uniformly
distributed throughout the packet. Here is a table showing
number of single bit errors in each of the 36 byte positions:

00: 8 12: 138 24: 729
01: 28 13: 65 25: 899
02: 15 14: 157 26: 1564
03: 23 15: 133 27: 870
04: 21 16: 154 28: 1938
05: 50 17: 224 29: 1362
06: 46 18: 194 30: 2268
07: 79 19: 409 31: 1801
08: 43 20: 304 32: 1922
09: 61 21: 636 33: 2855
10: 110 22: 435 34: 3438
11: 77 23: 737 35: 10810

This is out of around 360,000 packets. Any idea what is causing
this, and if there might be a simple fix?

Picture reception seems quite good, and ordinary teletext using
alevt seems to work OK. The VBI waveform looks reasonably
clean, with fairly distinct rising and falling edges.

[nigel o\'brian]

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Added later:

I tried switching to a newer saa7134 based card, and this problem
mostly disappeared. The VBI waveform does not look that different,
except the sampling rate is lower - about 4 bits per data bit instead of
5. I'm now seeing very few errors - only a couple every 10,000 lines.