[Discuss-gnuradio] Cheap ADC at 35MHz

[Adrian Godwin]

Hi all, new reader here ..


I've looked through the archives of this list and elsewhere,
and can't find any references to the Bt848; so if this is
an FAQ I apologise.

The 848 (and similar 878 etc) are cheap, popular framegrabbers.
In order to do this they capture composite video at 8x the
colourburst frequency and decode it digitally. It's also 
possible to capture chroma (from s-video) in a second ADC.

In addition to the TV-specific signal path, they also have 
a dumb capture mode for use in single-frame grabbing and
video blanking interval data capture. This is therefore an
8-bit 35Mhz PCI capture card for about 30 quid (or a lot
less, at boot sale prices : this is mass-market stuff).  
It's easy to change the crystal if you want 20Mhz (and
I suspect 40Mhz would work, too).

There are disadvantages : 8 bit is woefully low resolution,
and they have dc-restoration circuits that really demand 
a line-synchronised dc clamp (injecting a 15kHz spike into
the signal), but the price is hard to beat. They also have
I2C interfaces and often tuners. The later Bt878 has audio
digitising, too. Is this worth pursuing as a low-end
input card ?

Just to show that a signal need not be video-like to 
operate with these boards, here's a hex dump of /dev/vbi. 
The phono input is fed from a sig gen, and it's in PAL mode 
so the period of the signal you can see is about 7 samples 
at 35.46Ms/s or 5MHz. Actually, it was 5.5MHz but 7 samples 
is rather roughly counted !

0000A300 : 6B 00 00 00 00 64 88 2C 00 00 00 2D 88 62 00 00 :
k....d.,...-.b..
0000A310 : 00 00 6C 86 21 00 00 00 37 8A 5A 00 00 00 00 71 :
..l.!...7.Z....q
0000A320 : 80 16 00 00 00 42 8C 53 00 00 00 01 78 7C 0A 00 :
.....B.S....x|..
0000A330 : 00 00 48 8B 4A 00 00 00 0B 7E 78 00 00 00 00 53 :
..H.J....~x....S
0000A340 : 8B 41 00 00 00 15 81 72 00 00 00 00 5C 8A 36 00 :
.A.....r....\.6.
0000A350 : 00 00 21 86 6C 00 00 00 00 63 88 2D 00 00 00 2C :
..!.l....c.-...,
0000A360 : 88 63 00 00 00 00 6A 86 22 00 00 00 35 8A 5E 00 :
.c....j."...5.^.
0000A370 : 00 00 00 71 7F 16 00 00 00 3F 8B 55 00 00 00 00 :
...q.....?.U....
0000A380 : 78 7D 0A 00 00 00 48 8A 49 00 00 00 0A 7D 78 01 :
x}....H.I....}x.
0000A390 : 00 00 00 53 8C 3F 00 00 00 11 7F 72 00 00 00 00 :
...S.?.....r....
0000A3A0 : 5A 8B 38 00 00 00 1F 85 6C 00 00 00 00 62 88 2D :
Z.8.....l....b.-
0000A3B0 : 00 00 00 2C 88 63 00 00 00 00 6A 86 22 00 00 00 :
...,.c....j."...
0000A3C0 : 35 8A 5C 00 00 00 00 70 81 16 00 00 00 41 8B 54 :
5.\....p.....A.T
0000A3D0 : 00 00 00 00 77 7D 0C 00 00 00 49 8B 4A 00 00 00 :
....w}....I.J...
0000A3E0 : 0A 7D 77 00 00 00 00 53 8B 41 00 00 00 16 81 72 :
.}w....S.A.....r
0000A3F0 : 00 00 00 00 5A 8A 37 00 00 00 21 85 69 00 00 00 :
....Z.7...!.i...

The need to fix the DC restoration circuit is fairly obvious from
this data (loss of lower part of signal) and since the vbi device
only captures data during the vertical interval it's necessary to
change the bttv driver to get full capture mode. It does give an
indication that it could work, though.


Here's a capture of a 9MHz input :


0000A300 : 67 00 00 3E 5F 00 00 47 5A 00 00 50 52 00 00 56 :
g..>_..GZ..PR..V
0000A310 : 4B 00 00 5E 3F 00 00 64 39 00 00 6A 2E 00 00 6D :
K..^?..d9..j...m
0000A320 : 26 00 00 72 1A 00 00 75 10 00 00 79 03 00 00 79 :
&..r...u...y...y
0000A330 : 00 00 00 7A 00 00 00 79 00 00 07 78 00 00 10 75 :
...z...y...x...u
0000A340 : 00 00 1B 72 00 00 26 6E 00 00 2F 6A 00 00 38 63 :
...r..&n../j..8c
0000A350 : 00 00 42 5E 00 00 4A 56 00 00 52 4F 00 00 59 45 :
..B^..JV..RO..YE
0000A360 : 00 00 61 3F 00 00 66 34 00 00 6C 2B 00 00 70 20 :
..a?..f4..l+..p 
0000A370 : 00 00 74 17 00 00 77 0B 00 00 79 01 00 00 79 00 :
..t...w...y...y.
0000A380 : 00 00 79 00 00 00 78 00 00 0B 75 00 00 14 73 00 :
..y...x...u...s.
0000A390 : 00 21 6E 00 00 2A 6C 00 00 35 65 00 00 3C 61 00 :
.!n..*l..5e..<a.
0000A3A0 : 00 48 59 00 00 4E 53 00 00 57 4C 00 00 5C 42 00 :
.HY..NS..WL..\B.
0000A3B0 : 00 64 3A 00 00 67 31 00 00 6E 26 00 00 70 1B 00 :
.d:..g1..n&..p..
0000A3C0 : 00 74 12 00 00 77 04 00 00 78 00 00 00 79 00 00 :
.t...w...x...y..
0000A3D0 : 00 79 00 00 06 78 00 00 10 74 00 00 1B 73 00 00 :
.y...x...t...s..
0000A3E0 : 23 6D 00 00 2F 6A 00 00 39 65 00 00 42 5E 00 00 :
#m../j..9e..B^..
0000A3F0 : 4A 57 00 00 52 4F 00 00 59 48 00 00 5F 3E 00 00 :
JW..RO..YH.._>..


-adrian