Pro Tools mixer

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The Pro Tools mixer is a component of the Audio Editor - Software Pro Tools . It is used to summarize, output and technically and tonally process the audio and MIDI signals. The structure can be individually adapted to current needs. Five different mixer tracks are available for the different signals: sound track, aux input, instrument track, MIDI track and master fader. The mixer tracks, with the exception of the MIDI track, can be single-channel ( mono ) or multi-channel ( stereo , surround sound ). In Pro Tools LE systems, the calculation is carried out natively and in Pro Tools TDM systems on the audio cards by the digital signal processors (DSP). The structure of the Pro Tools mixer is very similar to the structure of an analog mixer . The maximum size of the mixer is limited; the maximum number of sound tracks is determined by the available Voices of the system. Voices are required to record or play back audio signals using the soundtrack. In TDM systems, large mixers are distributed across several DSP processors.

Mixed environment in Pro Tools TDM systems

Using the mixing environment, the number of summing levels (mixer tracks) is variable. Pro Tools creates a TDM mixer when you open or create a new session (project). The term mixer channel refers to all track types, with the exception of the master faders, which do not require any additional DSP power. If a certain number of “mixer channels” is exceeded, an additional DSP is integrated to provide additional capacity for the mixing process. Within the Pro Tools environment it is possible to put together your own mixer structure with regard to the number of summing levels. In this way, the required DSP power and the number of time slots used are adapted to the actual needs of the mixer.

Mixer add-on module

The summing levels are loaded as required by the DAE (Digidesign Audio Engine) in the form of mixer modules . There are two different types of mixer modules. On the one hand the stereo and on the other hand the additional surround sound module. A total of three different mixer modules are available for TDM-I technology : the surround sound mixer module, the 24-bit optimized mixer and the 16-bit optimized mixer. Four different mixer modules are available for TDM-II technology : Stereo, Surround, Stereo Dithered and Surround Dithered. By dithering the maximum number is reduced slightly at entrances.

Stereo mixer

The stereo version of the mixer can mix a variable number of inputs to a maximum of two outputs. According to this definition, the mixer is in the form of Nx2, where N stands for the variable number of inputs and the number 2 defines the maximum number of outputs. For example, if eight mono audio tracks are assigned to a stereo output (two channels), a simple 8 (N) x2 mixer is required.

Surround mixer

The surround mixer can have a variable number of inputs and a variable number of outputs. For a 7.1 surround sound mix consisting of 10 mono sound tracks and two stereo sound tracks, a surround sound mixer module with the format 14 (N) x8 is required.

Main mixer and submixer

Each mixer has a total capacity (size) that depends on the selected sampling rate . Each main mixer needs its own DSP processor assignment. If the number of channels to be summed exceeds the total capacity of a main mixer, additional main mixers and so-called submixers are automatically created. A submixer adds up the outputs of several main mixers to the number of output channels defined with the main mixers. This means that if a maximum of 68 mixer inputs and 2 mixer outputs are possible (68 (N) x2) and the number of inputs is exceeded, a new main mixer (68 (N) x2) and a submixer (4 (N) x2), according to the Outputs of the main mixer. The additional main mixer now enables the use of more than 68 inputs. The submixer sums the two stereo outputs of the main mixer to the stereo output of the submixer. The maximum positive range of the channel faders can be set by the user. There are two ways to do this (+6 dB and +12 dB), which you can choose yourself when creating a Pro Tools session. The maximum control range of the channel faders has a direct influence on the headroom of the system.

24-bit mixer structure with TDM I.

The mixer modules in the Pro Tools MIX systems offer a headroom of 30 dB. With this mixer it is possible to override both the inputs and the outputs of the summing stages. By using a master fader, it is possible to reduce the signal level at the outputs and thereby prevent overmodulation. However, reducing the output level has no effect on the inputs of the summing stage. As a result, overloads that occur at the inputs of the summing stage can only be prevented by reducing the input level.

48-bit mixer structure with TDM II

The mixer modules are calculated on a reserved area of ​​the DSPs. This makes it possible to sum the signals with a constant 48-bit resolution. Even if the signal level is reduced by the fader, the full resolution is retained. The 48-bit resolution enables a headroom of 48 dB. Even with a fader position of +6 dB, no clipping can occur on the input side of the summing bus. In contrast to this, the output side of the summing stage, which outputs the signals to a physical output of the hardware or the TDM bus, can nevertheless override. In order to prevent this overload at the mixer outputs, a master fader can be used, which does not result in any loss of quality.

Dithered mixer at TDM II

With the dithered mixer, an uncorrelated dither is used on each summed output, both on the physical outputs and on the internal buses. The dither prevents audible artifacts from occurring at the outputs to the 24-bit TDM bus or 24-bit physical output when the signal leaves the 48-bit mixer level. The possible artifacts can arise if, for example, low signal levels are cut off during the conversion from the 48-bit to the 24-bit level. The artifacts are now covered by the dithering with uniform white noise .

The addition of the uncorrelated dither requires more DSP power. The maximum number of mixer channels is reduced by about 15%. The Pro Tools software allows you to choose which mixer module to use.

The dithering of the mixer plug-in is designed for 24 bits. In the event that a further bit depth conversion is indicated, it can be useful to use an additional dither module for the finished mix,

How the 48-bit TDM mixer works

The function of the mixer can be explained in more detail based on a signal that has been digitized by an analog-to-digital converter at an interface with 24 bits and therefore has a theoretical dynamic range of around 144 dB (1 bit enables 6.02 dB to be displayed) become.

Signal flow to the mixer

Bit resolution in the signal curve in a TDM-II 48-bit mixer

The now digitized signal is placed on the 24-bit TDM-II bus and transported to the DSP processors on the audio cards. The number of signals that can be transported at the same time is defined by the number of time slots. The signal can now be processed on the DSP processors using additional modules (EQ, compressor, etc.). The modules work according to the so-called “double precision” principle. A 24-bit signal (144 dB) is processed with 48 bits (288 dB). The expanded dynamics provide more options for displaying the signal processing. This effect is particularly noticeable through the recalculated decimal places of the signal. With every signal change, for example a tone control, compressor, change in signal level, etc., the original signal is multiplied by the changes. The more display options there are, the higher the sound quality can be. After the module, the signal is dithered and then sent back to the 24-bit TDM bus.

Signal curve in the mixer

Level and dynamic range of the 48-bit mixer

The mixer is an additional module that is loaded by the DAE (Digidesign Audio Engine). The mixer processes the signal with 48 bits and therefore offers processing dynamics of 288 dB. In the event that the mixer has to be distributed over several DSP processors, this dynamic is constantly maintained. For the connection of several mixer modules, two time slots on the 24-bit TDM bus are used for each connection.

The 24 additional bits allow additional dynamic range for the channel faders above and below the original 24-bit resolution. This allows 128 correlated tracks to be summed with full modulation (24 bit) and maximum channel fader position (+ 12 dB) without overdriving the input of the summing stage. Conversely, it is possible to adjust the channel faders to close to −90 dB before the signal can no longer be displayed with full 24 bits.

At the input of the mixer, the 24-bit original signals are multiplied by 24-bit levels and panorama coefficients. This produces a 48-bit result. The original 24-bit information is contained in the 48-bit word that has now been created. With the mixer register of 56 bits it is now possible to increase or decrease the signal level to a limited extent without losing quality. The errors only arise for signals that have a signal level of around −240 dB. The resulting quantization error with the “double presicion” technique is around one millionth of a dB. The 56-bit register not only provides 24 additional bits to represent low-level signals, but 8 bits are provided as headroom for the summation.

With the Pro Tools mixer, it does not matter whether the levels at the input (channel fader) or output (master fader) are changed, as this change has no effect on the signal display and transmission.

Comparison with a 24-bit mixer

Level and dynamic range of a 24-bit mixer

A mixer with a maximum word length of 24 bits has no headroom and no additional area for displaying negative level changes. This further reduces the area of ​​the display for small signal levels when adding up several signals. If the number of signals is doubled, the dynamic range is reduced by 1 bit. The reason is that when the number of correlated signals is doubled, the level is raised by 6.02 dB.

With the 48-bit mixer structure, it was possible to sum 128 correlated signals without any loss of quality. The reason lies in the dynamic range in which the summed signal can be represented with the original 24-bit word length. This range extends from + 48 dB to 0 dB (headroom) and from 0 dB to −90 dB.

Compared to the previous example (48-bit mixer structure), a 24-bit mixer architecture with the same basic requirements looks a little different. With 128 correlated signals, the summed signal level will increase by 42.17 dB. Since no headroom is available, must be the entire signal level reduced by 42.17 dB. There are no additional bits available for the correct representation of the level-reduced signal, so the last eight bits are cut off and the original 24-bit signal becomes a 16-bit signal.

Signal curve from the mixer

At the mixer output, the signal from the 48-bit mixer level is converted back to the 24-bit level. The original 24-bit signal component, which is between a maximum of +48 dB and a minimum of −90 dB, is output in full quality on the TDM-II bus. Via the TDM bus, the signal is sent either directly to additional DSP processors or to a physical output via a D / A converter .

See also

literature

  • Markus Ammer, Christian W. Huber: Music production with Pro Tools 7. Wizoo, Bremen 2006, ISBN 3-934903-55-X .
  • Mike E. Collins: Pro Tools LE and M-Powered. The Complete Guide. Focal, Amsterdam et al. 2006, ISBN 0-24-051999-X .
  • Digidesign (Ed.): Pro Tools 201: Pro Tools Production Essentials . 2nd edition. Digidesign Training & Education Program, 2004, PN: 910216758-00 REV. B.
  • Digidesign (Ed.): Pro Tools 210M: Music Production Techniques . 2nd edition. Digidesign Training & Education Program, 2003, PN: 910612760-00 REV. A.
  • Digidesign (Ed.): Pro Tools 210P: Post Production Techniques . 2nd edition. Digidesign Training & Education Program, 2002, PT 210P REV B 020627.
  • David Leathers: Pro Tools Bible. (Covers up to Pro Tools 6.1 and Pro Tools HD. Software. Hardware. Plug-ins. Midi. Pro Tools 6.1 and beyond). McGraw-Hill Publishing Co., New York NY et al. 2004, ISBN 0-07-141234-4 .
  • Ken C. Pohlmann: Principles of Digital Sound. 4th edition. McGraw-Hill Professional, New York NY et al. 2000, ISBN 0-07-134819-0 .
  • José Valenzuela: The Complete Pro Tools Handbook. Pro Tools / HD, Pro Tools / 24 mix, Pro Tools LE for home, project, and professional studios. Backbeat Books, San Francisco CA 2003, ISBN 0-87930-733-1 .

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