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Currently Being Developed:

8-Channel Fully Digital Parametric Equalizer

( Deutsche Version: 8-kanaliger, voll-digitaler parametrischer Equalizer)

Option: 8-Channel ADCs and DACs

Preface

The development of this equalizer is very far advanced. Now it's about finding out the interest in such a device. Whether it is worth to risk the costs for further prototypes and a small series. My fear is that the need for such a device is too low, because DAWs or digital mixing consoles are sufficiently equipped with equalizers and other applications rarely exist. That is why I want to present this development here and perhaps also demonstrate the laboratory sample on one or the other occasion. Maybe I can even arouse enthusiasm with the features of the equalizer?

Features

Concerning its settings this equalizer is similar to the 2-channel analog equalizer TwinEq-1 MK2 of the Estec company. Great emphasis has been placed on the "analog feeling". Most settings are made with analog rotary encoders which have such a high resolution that in spite of the very fine level settings a quick and precise adjustment in real time is possible as with potentiometers in analog devices.

The selected frequency responses are displayed in real time during the setting. On the one hand this display serves as visualisation of the settings currently being made. On the other hand it also serves for quick orientation, e.g. after retrieving preset values or after undo and redo actions, to assess where and to which extent the equalizer affects the signals now. The graphics from frequency responses act like symbols providing a high recognition value for the associated application. This is of course considerably more informative than, for example, the tick marks on a multitude of potentiometers.

Basically, digital circuits and software solutions, if they are done correctly (which is not as self-evident as one should think), have the properties to behave very, very widely ideal, for example:

• There is no danger of internal overflow. In practice it is not possible to set the equalizer in a way that internal clipping occurs.
• There is no danger of internal underflow. In practice it is not possible to set the equalizer in a way that internal noise develops.
• The settings are precise and not just so similar as the positions of the potentiometers of an analog equalizer can tell.
• Linear frequency responses are really strictly linear.

Additionally the digital technology offers further possibilities, e.g. the storage and retrieval of preset values or the synchronous setting of several channels simultaneously, which are not impossible in the analog world, but can only be realized with great effort.

Front View

Preliminary design of the front panel (Click to enlarge)

The front panel, from left to right:

• 8 illuminated keys for preselection of channels
• Graphic display
• Key panel and rotary encoder: Configuration, storage and retrieval of preset values, undo and redo function
• Low- and high-cut filter
• 4 parametric filters, two of which are usable as shelf filters
• Master gain, key to switch on the limiter above
• Headphone section

Inputs and Outputs

• 8 digital input channels, 4 x AES3 (AES-EBU), transformer-coupled, switchable from XLR inputs or a 25 pin D-Sub connector
• 8 digital output channels, 4 x AES3 (AES-EBU), transformer-coupled, to XLR outputs and a 25 pin D-Sub connector
• Headphone output, 6.3 mm TRS jack on the front panel
• LAN (RJ-45), RS-232 (D-Sub 9) and USB on the rear

Channel Preselection

Channels can be set individually or in groups. Within groups all channels receive identical settings. A typical group consists of e.g. two channels being joined to one stereo pair.

Building groups: Groups are built by preselecting and holding the key of the channel whose settings shall be copied. Then each further desired channel key is pressed. The corresponding keys are flashing. The copying of the settings and the building of the group takes place in the moment when any setting is changed in this state. Then all keys of the group are illuminated.

Preselecting groups: When a channel assigned to group is preselected, the whole group is preselected. All keys of the group are illuminated.

Disbanding groups: By pressing a key of an already preselected group only this channel is preselected. All other keys of the group go out. The channel is disbanded from the group in the moment, when any setting is changed in this state. It is also possible to disband several channels from a group simultaneously. In doing so a new group is created.

Equalizer Channel

• 8 identical channels
• Filter update frequency (number of changes of the frequency response, when settings are changed continuously): Approx. 800 Hz
• All frequency settings are performed from approx. 10 Hz to approx. 20 kHz in 1580 steps of 1/144 octave or 1/12 semitone per step resp.
• All gain settings are performed from -20 dB to +20 dB in 400 steps of 0.1 dB each (Master Gain: -40 dB to +20 dB in 600 steps of 0.1 dB each)
• All Q-settings (filter width or quality) are done from 0.1 to 10 in 800 logarithmically divided steps

Settings for each equalizer channel

• 1 low-cut, frequency adjustable from approx. 10 Hz, steepness 6, 12, 18, 24, 30 and 36 dB/octave or linear (off-state)
• 1 high-cut, frequency adjustable to approx. 20 kHz, steepness 6, 12, 18, 24, 30 and 36 dB/octave or linear (off-state)
• 1 bell/low-shelf filter, adjustable: Frequency = 10 Hz - 20 kHz, Q = 0.1 to 10, gain -20 dB to +20 dB or linear (off-state)
• 2 bell filters, adjustable: Frequency = 10 Hz - 20 kHz, Q = 0.1 to 10, gain -20 dB to +20 dB or linear (off-state)
• 1 bell/high-shelf filter, adjustable: Frequency = 10 Hz - 20 kHz, Q = 0.1 bis 10, gain -20 dB to +20 dB or linear (off-state)
• 1 master gain, adjustable from -40 dB to +20 dB or 0 dB (off-state)
• 1 limiter, switchable on/off, attack time 0 (1 sample), release time selectable by configuration

Equalizer Operation

• 14 high-resolution analog rotary encoders with approx. 1000 steps/turn and approx. 800 updates/second "feeling" like analog potentiometers in practice
• 2 digital rotary encoders
• 18 illuminated keys (another 8 illuminated keys for channel preselection)

Low- and high-cut:

• 1 high-resolution analog rotary encoder each for frequency
• 1 digital rotary encoder each for steepness
• 1 green illuminated key each for switching on/off

Bell and bell/shelf filter:

• 3 high-resolution analog rotary encoders each for frequency, Q and gain
• 1 green illuminated key each for switching on/off
• 1 orange illuminated key each for the two bell/shelf filters to switch between bell and shelf

Master gain:

• 1 high-resolution analog rotary encoder
• 1 green illuminated key for switching on/off

Limiter:

• 1 blue illuminated key for switching on/off

Headphones

• Gain digitally adjustable up to +20 dB with a conventional potentiometer
• Max. output voltage approx. 7 V_eff
• Channel selection for headphone "downmix" manifoldly adjustable with a digital rotary encoder:
  - Each channel separately in mono
  - In stereo the channel pairs 1/2, 3/4, 5/6 or 7/8
  Several stereo channels combined to one stereo signal:
  - Channels 1+3/2+4 or 5+7/6+8
  - Channels 1+3+5/2+4+6 or 3+5+7/4+6+8
  - Channels 1+3+5+7/2+4+6+8
  Stereo downmix of surround signals (center and LFE in the middle):
  - Channels 1+3+4+5/2+3+4+6 and 1+3+5+6/2+4+5+6 (channels 3 and 4 or 5 and 6 resp. are center and LFE)
  - Channels 3+5+6+7/4+5+6+8 and 3+5+7+8/4+6+7+8 (channels 4 and 5 or 7 and 8 resp. are center and LFE)

Graphic Display

Top left:

• Peak-meter, shows output levels for 8 channels
• Red triangle: Clipping, the minimum time duration of the clipping display is selectable by configuration
• Blue triangle: The limiter is limiting, the minimum time duration of the limiter display is selectable by configuration
• Channel number highlighted in blue: The limiter is on

Top center:

• Display of the numerical setting of the 6 equalizer filters and of the master gain of the currently selected channels
• Highlighted in white: The filter stage is switched off, gain is 1 (0 dB)
• Highlighted in green: The filter stage is working with characteristic bell
• Highlighted in orange: The filter stage is working with characteristic shelf

Top right:

• Headphone amplification, in the example set to -0.97 dB (probably very loud, therefore red)
• Selection of the channels for headphone-downmix, in the example channel 1 on the left and channel 2 on the right

Middle Area:

• The frequency responses of the 8 channels, for demonstration purposes the example shows 8 settings being rather nonsensical in practice
• The currently set frequency responses are displayed in white, all others are displayed in colours
• To the right in white highlighting the channels currently selected for set-up, in the example channel 1 only

Bottom left:

• Input D-1 is connected, synchronized (green) and it is the master input (frame)
• Input D-4 is connected, but asynchronous (red)
• Inputs D-2, D-3 and WC (word clock) are grey, i.e., these signals are not connected
• The sample rate is 192.0 kSa/s in the example and can be resynchronized by the VCXOs (green)

Bottom center:

• Scheduled for storage and retrieval of preset values

Bottom right:

• Undo and redo state (example):
• Blue: There are 5 previously saved undo states
• Green: The last saved undo state
• Yellow: The current setting is not saved in the undo memory

The Synchronous System

The equalizer works with sample rates of 16, 22.05, 24, 32, 44.1, 48, 64, 88.2, 96, 128, 176.4 and 196 kSa/s. The equalizer can generate these sample rates in master mode or resynchronize resp. de-jitter them as slave by means of VCXOs (Voltage Controlled Crystal Oscillators) within +/-100 ppm. It can also work with all sample rates in between, but then there is no de-jitter and the frequency responses are shifted as well. For sample rates which cannot be resynchronized the sample rate display is highlighted in yellow.

As a slave the equalizer can be synchronized via word clock or one of its 4 AES inputs. The selection is done automatically: The word clock input has priority, followed by the AES inputs, adjustable in the order of 1 to 4 or 4 to 1, so that e.g. input 4 can be used for DARS synchronization. Without a digital input signal the equalizer works with the preset default sample rate, which in practice is relevant only with existing ADCs.

Option 8-Channel ADCs and DACs

• 8 electronically balanced inputs or outputs to D-Sub 25
• ADC and DAC reference level (i.e., the input sensitivity resp. the output voltage for 100% FS) from +12 dBu to +24 dBu in 1 dB steps, with 4 levels of 3 dB each being switched electronically distortion-free and the intermediate levels being digitally calculated
• Fully balanced analog signal path
• Strictly linear frequency response
• ADC: Typically 114 or 120 dB dynamic range A-weighted at +15, +18, +21 und +24 dBu input sensitivity
• DAC: Typically 120 dB dynamic range A-weighted at +15, +18, +21 and +24 dBu output voltage
• Routing can be configured, i.e., the sources of the equalizer channels are either the AES inputs or the ADC outputs, the DACs' sources are either the outputs of the equalizer channels or the AES inputs

Miscellaneous

• Latency 10 samples (e.g. 227 µs at 44.1 kSa/s and 52 µs at 192 kSa/s)
• Design 19", 2 HU, approx. 160 mm deep
• Power supply 100 - 240 V AC, approx. 20 W

In Planning

• Remote control for retrieval of preset values via USB and LAN
• Loading and retrieving set-up data via USB and LAN

State of the Development

The development of this equalizer is very advanced. All settings concerning the acoustic properties are fully available in the laboratory sample and can be demonstrated. Essentially the basic configuration settings are still missing, like e.g. default sample rate, the input and output routing and ADC and DAC reference level. Most of these configuration settings, however, only concern the ADC/DAC option. The operation with ADCs, DACs and word clock synchronization is prepared, but cannot be demonstrated in the laboratory sample at the moment.

Photo of the equalizer's laboratory sample (Click to enlarge)