I had an enquiry recently about how to setup a 361 to perform “The Dolby Trick”. This is the basic setup:
Audio routing
- DAW (effects send, bus, etc) —> audio interface —> Dolby “line in”
- Dolby “to rec” output —> audio interface —> DAW channel line in
Send the signal to the Dolby & record the output. You’ll need to compensate for the audio interface latency
Dolby 361 back panel
Hardware level calibration
- send tone to Dolby in Rec mode, NR out
- set output level for unity gain using multiturn output level pot behind front panel
Hardware setup for Dolby Trick
- Set NR in and Rec on (white & red buttons pushed in)
Dolby 361 front panel
Notes
If you’re using a standard Cat.22 Dolby A card the output of the 361 will be the encoded signal – ie the original signal plus the processing. This means balancing the effect will take a bit of juggling with levels to get the desired effect
For a deeper effect & easier setup a modified Cat.22 is a useful tool
How it works
As you reduce the level going into the Dolby the effect will increase. Dolby A encoding uses a multiband expander so more level = less effect, the inverse of a compressor, and the inverse of the decode process. For the Dolby Trick there is no decode process, just encoding
The NR in / out switch should give an obvious effect if you monitor the return signal. If you hear no effect, reduce the level into the Dolby
Theory
Dolby A passes the audio signal through a four-band compander. A sidechain determines the level of encoding for each band. In the encode process the sidechain-generated signal is summed with the audio, to create the encoded signal, and recorded to tape. On playback the sidechain recreates the encoded signal and subtracts it, decoding the signal
As a compander, Dolby A raises low-level signals during recording and lowers them again on playback. This has the effect of reducing tape noise by the same amount. There’s a lot of subtlety in the details – in the way the bands are chosen, the amount of expansion & compression and how these are optimised for maximum perceived effect and minimum artefacts
Hardware
The 361 (and earlier 360) are hardware interfaces with no audio electronics but with input & output transformers. They have the power supply and switching to route audio to and from the Cat.22
The Cat.22 has audio electronics, input & output stages, sidechain and four bands of processing on one card. It’s all Class A, transistor-based circuitry
The separation of audio path and support hardware was a highly significant move. It meant Cat.22 noise reduction cards could be used in other hardware interfaces such as the M16 & M8 multitrack units and film playback systems. It also allowed the next generations of noise reduction, Dolby SR, to use existing interfaces
Ray Dolby was a clever guy, a true audio pioneer
