Briefly looking at the Dolby A CAT 22 card, I'll assign numbers to the 
blocks. It might  be useful to draw out blocks in this pattern and 
connect them

1       2        3        7        11       12
                    4        8
                    5        9
                    6      10

The signal comes in via

(1) an input amplifier and low-pass filter (4 transistors).

(2) Then there is a filter driver and output 1 driver section (3 

(3), (4), (5), (6) There are four filters, each fed off (2) (with 2, 2, 
4, and 3 transistors each -- 11 transistors)

Each filter feeds  (i.e. (3)-(7), (4)-(8), etc)

(7), (8) (9), (10) a dedicated compressor for each band (10 transistors 
each -- 40 transistors)

These four feeds are summed into

(11) Noise reduction Signals Addition Stage (2 transistors)

There is an

(12) Output 2 amplifier (3 transistors).

which is fed from (2) in parallel with (3), (4), (5), and (6).

This uses 63 transistors (per channel) to this point!

The Record/Play switch takes a feed from (11) and switches it either to 
(2) or (12).

The output for recording is taken off (2) and the output for playback is 
taken off (12)

It is in phase (I believe) when switched to (2) and out of phase when 
switched to (12), thereby adding during the record cycle and subtracting 
during the play cycle.

What is interesting is that during record, the compressed signal sum 
(from (11)) goes back through the filters and compressors (via (2)), but 
is apparently subtracted in (12).

There are several other modules that are not part of the basic signal flow
(13) Line Output Amplifier (9 transistors)
(14) Dolby Tone Oscillator and Logic (8 transistors)
(15) Meter amplifier (2 transistors)
(16) Power stabilizer and midpoint reference (4 transistors)
for another 23 transistors--a total of 86 transistors per CAT22 card.
Couple that with several "Adjust on test" resistors and we're in a fine 

As I understand it, the side chains only start to become active at about 
-20 dBu.

I've never thought it a fun project to duplicate one of these <smile>.



On 2012-01-11 1:24 PM, Roderic G Stephens wrote:
> I thought that Dolby decoding was a process done in real time, so a DAW version would be too static.  In other words, doesn't a Dolby decoder adjust to the incoming data and change its compression and EQ accordingly?

Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada           (905) 713 6733     1-877-TAPE-FIX
Quality tape transfers -- even from hard-to-play tapes.