Compression Study #5 – Urei 1176LN Ratio Curve

This post is a continuation of the previous blog post, exploring the shape of the gain reduction curve created by the 1176LN RevD at different frequencies and ratios.

Part 1 is available to view here

 The previous post covered how the 1176 gain reduction curves change over different frequencies (20Hz, 100Hz, 1kHz and 5kHz) and the program dependant nature of the GR curves. The following tests follow the same topic, but address how the gain reduction curves change depending on ratio. Each of the following graphs is a view of the transient 1kHz burst, as this gives us a clear image and allows graph to be much more focussed on a timeframe of roughly half a second. The only variable of these tests was the ratio. Attack and release remained set to 7 (fastest).

4:1 – I was fairly surprised with how quickly the 4:1 ratio scooped back on the release. Considering the knee of this ratio is the softest, as seen in the graph above (Urei Inc, 1989), I would have expected a much more gentle release. After compression of -10dB the release time was 140ms. 
Urei 1176 1kHz A7 Rl7 Rt4

8:1 – Compared to the 4:1 ratio, this is a much smoother curve, releasing over 260ms. The same scooping shape is present, but appears to have a short period of ‘hold’ after the level falls below the threshold, of around 30ms. 
Urei 1176 1kHz A7 Rl7 Rt8

12:1 – This ratio has an interesting shape that can be visibly seen change, in three phases. For the first 50ms of release the GR shows the ‘hold’ characteristic, similar to 8:1. From 50ms to 140ms the curve scoops upwards slightly, before finally arcing out of gain reduction until 330ms after the threshold had been passed.
Urei 1176 1kHz A7 Rl7 Rt12

20:1 – Considering the hard knee now applied at 20:1, the curve is much smoother than any of the other ratios thus far. After the brief ‘hold’ period, the release has a somewhat linear characteristic before arcing.
This shape release would sound the most transparent on percussive elements, such as overheads, by not throwing the ambient sound after the transient back at the listener, but smoothly reintroducing it.
Urei 1176 1kHz A7 Rl7 Rt20

All Buttons In – Different to all the other ratios shown so far, ‘All buttons in’ completely changes the patten we could see emerging. Whilst testing this mode, the GR needle could audibly be heard hitting the bottom of the meter, and the curve below illustrates why.
The exponential curve that ‘All buttons in’ mode creates is not subtle nor transparent. The whole release phase lasts 72ms. The last 20ms of release is an extremely steep slope, creating an explosive pumping and breathing. This mode is known as ‘British Mode’.
Urei 1176 1kHz A7 Rl7 RtAll

The following quite is from one of Universal Audio’s engineers, explaining some of the features of ‘All buttons in’:

“But in All-Button Mode, a few more things are happening; the ratio goes to somewhere between 12:1 and 20:1, and the bias points change all over the circuit. As a result, the attack and release times change. This change in attack and release times and the compression curve that results is the main contributor to the All-Button sound. This is what gives way to the trademark overdriven tone. The shape of the compression curve changes dramatically in All-Button.” (Shanks, 2003)


References:

Shanks, W. (2003). Compression Obsession: “All Buttons Mode”. Universal Audio. [Online]. Available at: http://www.uaudio.com/webzine/2003/april/index4.html [Accessed: 3 February 2016].

Urei Inc. (1989). Model 1176LN Peak Limiter. [Online]. Available at: http://www.jblproservice.com/pdf/vintage%20jbl-urei%20electronics/urei-1176lnmanual.pdf [Accessed: 2 February 2016].

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