The 8051 is an analogue compressor with six transformer balanced audio paths complemented by two side chains and a key input. Flexible side chain routing, trimming and filtering, make this a powerful and responsive tool for surround mixing.

The 8051 uses the same feedback topology as the 33609 compressor, the style of compression that has created the famous Neve Sound.

Each channel can be controlled from either Side Chain A or Side Chain B. The six signal paths can be split between the two side chains in any format required, for example with L, C, R, LS, RS on Side chain A and LFE on Side chain B, with L, C, R on Side chain A and LS, RS on Side chain B, as a pair of separate stereo compressors, etc.

Each audio channel can be routed to either or both side chains. When multiple channels are routed to the same side chain, the circuit responds to the highest signal present and resulting compression is applied equally to all channels avoiding any image shift across a surround mix. Side chains can be filtered, trimmed or processed externally.

A Key Input is provided to allow adjustable control from an external source. It can be connected in parallel with one of the inputs, for example to allow a Centre Channel to control separate compression of front and rear signals in a surround mix.

A 12dB per octave 80/100/120Hz low-pass filter can be inserted on any one channel to create a Sub-Bass or LFE channel.

The 8051 is available in Standard and Mastering stand alone versions, it can also be fitted to Neve 88R series consoles.

Is provided by gain reduction meters and switches which illuminate to indicate routing and change colour to indicate which signal is driving each side chain at any moment.

Most compressors apply gain reduction based entirely on the signal level at their inputs, this is known as a feed-forward system. The 8051 uses a feedback topology, monitoring the output of each gain cell and continually adjusting the control voltage returned to give the required gain reduction.

Each channel can be controlled from either Side Chain A or Side Chain B. The control Link switches determine which side chain and which gain makeup control affects each channel. The six signal paths can be split between the two side chains in any format required. For eample with L, C, R, LS, RS on Side Chain A and LFE on Side Chain B, with L, C, R on side Chain A and LS, RS on Side Chain B, as a pair of seperate stereo compressors, etc.

Each audio channel can be routed to Side Chain A, Side Chain B, or both side chains. When multiple channels are routed to the same side chain, the compression cicruit responds to the highest signal present. The resulting control voltage is then applied equally to all channels avoiding any image shift as compression is applied accross a surround mix.

Once above the side chain's threshold level, the link button for the largest signal on that side chain illuminates red. Any signal that is above threshold and within 2dB of the dominant signal will also illuminate.

Side chain levels can be trimmed on four controls grouping the six channels into two stereo pairs and two mono channels.

A high-pass filter can be inserted in the channel's side chain path, preventing unwanted compression due to LFE content without affecting the output sound. The frequency can be set to 80Hz, 100Hz, or 120Hz.

can be set between -20dBu and +16dBu and determines the signal level at which compression is applied.

controls the amount of gain reduction applied as the input level increases. Values available are characteristic, and its ratio gradually increases as the compression becomes heavier at all settings.

of 2ms to 165ms are available. With slower attack settings compression is more dependant on the average signal level, with faster settings it is more dependant on signal peaks.

between 100ms and 3s are available. Recovery compliments the attack control, longer recovery times prevent pumping effects where gain reduction follows the program material closely.

control provides a variable active pad across all six outputs, from 0dB to -5.5dB in 0.25 steps. The pad is outside of the relay bypass loopand so always controls all six channels together in order to maximize the units output level without clipping.

is used to restore the average level or loudness of a compressed signal. The gain is adjustable from -10 to +20dB.

The EXT buttons provide an insert point in the control voltage signal for each Side Chain. By using the send and return the signal controlling the compression may be externally processed. Alternatively the EXT mode can take a control voltage from the rear connector instead of the side chain outputs, allowing the unit to be slaved from a second 8051 or a 33609.

The bias control is used to balance the control voltages from the two side chains by applying up to 6dB of cut and boost, effectively modifying their ratios. If only a single side chain is in use, for example when used as a stereo compressor, the bias control allows ratio values between the standard settings to be selected with continuously variable control.

A Key Input is provided to allow adjustale control from an external source. Like the side chains the key input has a threshold level control ranging from -20dBu to +16dBu to set the intended level at which compression control is required.

It can be used several ways including ducking from a voice channel. It can also be connected in parallel with one of the inputs which allows that input to be routed onto both side chains with different compression thresholds, for example to allow a Centre Channel to control seperate compression of front and rear signals in a surround mix. The key input is provided with a selectable 12dB/octave high-pass filter to remove unwanted low frequencies from the input signal being used to control the compressor.

A 12dB per octave llow-pass filter can be assigned to any one channel. This filter is applied directly to the input before the relay-bypass point, and its -2dB frequency is selectable from 80, 100 or 120Hz. This can be used to create a Sub-Bass or LFE channel.

In addition to the standard model the 8051 can also be supplied as a specialized mastering version. This unit is slightly different to the standard 8051 in two ways. The Trim and Bias controls are indented to aid total mechanical recall of the unit, 1dB switches are fitted to the Threshold and Gain Make-Up controls to allow stepped switching.

If a channel routed to a side chain is bypassed, its connection to the side chain is maintained. This means that other channels controlled by that side chain are unaffected and allows a channel to control others without being compressed itself (as though it were another key input.) Two bypass modes are available, single or master. In single mode each switch bypasses its respective channel. In master mode all channels are bypassed together. Master mode is toggled by pressing and holding the channel 1 bypass switch and is indicated by the MST LED to the left of channel 1 being lit. If individual channels are placed in Bypass mode, the Master Bypass function then becomes a Group Bypass control for the remaining channels.