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Pro Tools: Surround Sound Encoding Plug-ins

Which Software To Use?
By Mike Thornton

If you want other people to hear your surround mixes, you'll need to encode them into a consumer format. Pro Tools users have a choice of software for this purpose. Which should you choose?

Like many other DAWs, Pro Tools enables us to create multi-channel mixes, but unless we want to provide every consumer with a Pro Tools HD system so they can hear our work, we need a set of tools to put our surround mixes into a format that the end user can access and enjoy. In this article I'll be looking at some of the common formats available that allow the consumer to enjoy surround sound, and a selection of the software available for converting our Pro Tools surround mixes into one of those formats.

Pro Tools: Surround Sound Encoding Plug-ins

* DVD-Video (DVD-V)

This is by far the most common surround-capable format available to the consumer. The format can carry up to eight channels of audio, consisting of a six-channel surround mix and a separate stereo mix, as well as video content, menus and so on. However, for the DVD to be able to deliver all this data, everything must be data-compressed and encoded.

Dolby Digital, which is also referred to as AC3, is the mandatory audio format for DVD-V discs. Dolby Digital can be encoded at up to 448kbps at a maximum resolution of 24 bits and 48kHz, and can be used to encode both 2.0 (stereo) and 5.1 surround content. DTS Digital Surround is an alternative encoding format that can be included on a DVD-V along with the Dolby Digital track; some producers prefer the sonic qualities of DTS, as it can be encoded at up to 1536kbps at 24-bit, 96kHz.

* DVD-Audio (DVD-A)

This format offers much higher quality audio delivery at the expense of the video options. It can support up to six channels of 96kHz, 24-bit uncompressed PCM audio, as well as a separate high-resolution stereo mix. DVD-A discs can also include AC3 and DTS tracks for some DVD-V compatibility, but you need a DVD-A or 'universal' player and this makes the format much less popular than its DVD-V cousin.

* SACD (Super Audio Compact Disc)

An SACD is a dual-layer disc that offers a Red Book 16-bit stereo mix which conventional audio CD players can read, plus a second layer that carries a high-resolution six-channel mix, as well as a separate high resolution stereo mix. The benefits of this format over DVD-V are that it provides a disc that will play in a conventional CD player, and that SACD players don't need a video monitor attached to make it possible to navigate around the disc.

* Compact Disc

You can deliver multi-channel audio using any conventional two-channel digital delivery format, by encoding your surround content using one of the 'matrixing' systems. These are Dolby Surround and Pro Logic, both of which offer a four-channel surround system consisting of Left, Centre, Right and a mono Surround (rear) channel. Then there is Dolby Pro Logic II, which offers stereo surrounds as well as the three front channels. Finally, there is Circle Surround from SRS, which can provide a Dolby Pro Logic-compatible output or, with the matching SRS decoder in the consumer's system, offer an additional Centre Surround channel, so providing a 6.1 delivery format. The advantages of these formats are that they don't need any special delivery hardware, Pro Logic decoders can be found in most, if not all, domestic surround systems, and no data compression is used. The down side is that the matrixing system means the separation between the various channels will be compromised, and anyone without the correct decoder will hear the encoded audio, usually described as LtRt.

* Dolby E

Finally, Dolby E is a format developed by Dolby to enable broadcasters to distribute eight channels of audio through their existing two-channel AES3 digital infrastructure, and doesn't normally appear outside the broadcast production chain.

Alternatives

Neyrinck are now producing a DTS version of their Soundcode plug-in. Although Surcode For Pro Logic II is their first Pro Tools plug-in, Minnetonka Audio produce a range of stand-alone encoding products for the Pro Logic II, Dolby Digital and DTS formats, as well as the Disc Welder DVD-A disc-authoring packages.

Surround In The Pro Tools World

This brings us to the methods available in the Pro Tools environment to enable us to encode our surround mixes into one of the delivery formats described above. Other tools are available, but they are stand-alone packages that don't work as Pro Tools plug-ins (see the 'Alternatives' box for more details). There are three matrix encoding packages, and one Dolby Digital encoding package (see box). All require a Pro Tools TDM rig, since LE systems don't offer surround features.

Dolby Labs Dolby Surround Tools

Dolby Surround Tools is a TDM-only plug-in for Mac OS X (including Intel Mac support in the latest version) and Windows that enables you to matrix-encode your surround mixes into Pro Logic (LCRS) format, and supports sample rates up to 96kHz. Digidesign make it clear that you cannot use Dolby Surround Tools for any content destined for film release: film mixes must be handled in approved film dubbing theatres. The package is made up of three plug-ins: Dolby Surround Encoder, Decoder and Surround Panner. The Encoder and Decoder plug-ins are software models of Dolby's analogue Models SEU4 and SDU4, and DP563 and DP564 digital rackmount hardware processors.Pro Tools: Surround Sound Encoding Plug-insPro Tools: Surround Sound Encoding Plug-ins

If you want to use Dolby Surround Tools within a 5.1 Session, you'll need to create lots of extra tracks and Paths.

The Dolby Surround Tools plug-ins only work with the four-channel LCRS format, so if you are working in a 5.1 Session, you have to create sub-paths and use Aux tracks to convert your 5.1 mixes into LCRS. The user guide provides you with 'step by step' instructions for this process, which adds up to eight tracks to the Session, as well as loads of additional paths and sub-paths (see screens below).

The Encoder plug-in has very few controls, just like its hardware cousins. An input level trim control is used to set the optimum level into the Surround Encoder plug-in for the L, C, R and S input channels, and has a range of +6dB to ­12dB. Since it is a 'ganged' trim, all four input channels are boost or cut simultaneously. Most of the plug-in window is taken up by level meters, which indicate the input level to the Surround Encoder of the L, C, R and S signals. These meters feature peak-reading ballistics, with a peak-hold function to indicate momentary peak values, as well as clip lights which stay on until you clear them. Similar output level meters display the level of the Surround Encoder 's matrix-encoded Lt and Rt output signals.

There are also Dolby level indicators on both inputs and outputs, which look like little triangles, and display the optimum level, also known as the 'reference dialogue level', which corresponds to a sound pressure level (SPL) of 79dB for home video and other 'small room' releases. (These playback levels are 'C-weighted, slow' measurements made with an SPL meter from your mixing position.) This helps to make sure that everyone mixing in Dolby Surround is monitoring at similar levels; our hearing response changes with volume, which is why we should check a mix done at high volumes to see if it still 'works' at low playback volume too.

As the LCRS signals are combined into the LtRt output it is quite easy to overload the matrix, so as a matter of good working practice you should be aiming set the levels so that the Dolby level indicators are flashing regularly but you don't hit the peak level Indicators at all.

The Dolby Surround Decoder plug-in (above) decodes two-channel Dolby Surround-encoded audio (Lt/Rt) to recreate the four-channel Dolby Surround (LCRS) mix. You should use Surround Decoder to monitor the Dolby Surround mix, to make sure that the Surround-encoded two-channel mix will sound good in what ever format the listener chooses. Digidesign strongly advise that you do not monitor through a Dolby Surround Pro Logic amplifier or receiver to decode a Dolby Surround matrix-encoded signal. The reason is that Pro Logic consumer products are not built to the same professional specifications as the professional decoders and Pro Logic-equipped consumer products have auto-balancing features which, if used in a mix monitoring situation, may contribute to inappropriate mixing decisions.

Pro Tools: Surround Sound Encoding Plug-ins Surround Decoder lets you switch the monitor mode from Dolby Surround to stereo or just mono. Switching between these modes takes the Lt and Rt input signals and routes them to either one, two, or four playback channels to enable you to easily check for compatibility. In mono the LtRt signals are summed and routed to the Centre speaker, while in stereo the LtRt signals are routed direct to the left and right speakers without decoding. In Dolby Surround the LtRt signal is routed through the decoder matrix and the output is fed to the LCRS speakers.

The Decoder plug-in's pink-noise generator is used to generate and send band-limited pink noise to the LCRS output channels, which is useful for setting up your monitor system. The output level trims should be adjusted so that the pink-noise signal produces a sound pressure level of 79dB at the mix position for each speaker in turn.

The Surround Decoder plug-in also has a Surround Delay control, which delays the rear Surround channel in 10 millisecond increments, from 10ms to 100ms. This delay helps to ensure that any front-channel (L, C or R) information that has 'leaked' into the S channel is heard from the front speakers first. Delaying the Surround channel by just a small amount means that any common 'crosstalk' information will arrive at the listener after the main front signal, and as a result the information will be perceived to have emanated from the front speakers.

Pro Tools: Surround Sound Encoding Plug-insThe pre-trim meters indicate the output levels from Surround Decoder for each of the LCRS channels. The Decode Steering display has no user controls: it simply shows the direction in which the Pro Logic decoder output is currently being 'steered' by the mix. This perspective shows the position of the dominant channel, relative to your front (L, C and R) and rear (S) speakers. The Surround Decoder's adaptive matrix tracks the changes and makes the appropriate decoding adjustments as the steering direction of the mix shifts dynamically.

The Surround Decoder plug-in includes options for different centre speaker configurations: Large, Small and None. You should choose Large if your Centre speaker is matched or closely matched, in terms of low-end frequency response, with your Left and Right speakers. This is the optimum setup, but not everyone's system is like this, hence the two other options. The Small option redirects low-frequency content from the Centre channel below 100Hz to the left and right speakers, while with None selected the Centre channel output signal is attenuated by 3dB and added equally into the Left and Right output channels, to deliver a correct 'phantom centre' image of your Centre channel information.

The Dolby Surround Panner plug-in is now redundant, in favour of the more flexible surround panning available in Pro Tools 5.1 or later, and is included for backwards compatibility with Pro Tools Sessions created with older versions of Dolby Surround Tools.

Soundcode For Dolby Digital

The plug-ins described in the main text are all matrix-based encoders that use variants of the Dolby Surround or Dolby Pro Logic II systems. Neyrinck's Mac OS X (including Intel Mac) and Windows-compatible Soundcode plug-in, by contrast, uses the non-matrix-based Dolby Digital or AC3 process. This does away with the narrowing of the soundfield that is typical of matrixing, but it uses data compression, so there are different side-effects to take into consideration.

Pro Tools: Surround Sound Encoding Plug-insThe integrated help features are a nice touch.The integrated help features are a nice touch.Neyrinck's system consists of an Audiosuite-based encoder and decoder, plus an RTAS version of the decoder for easier checking of playback material. To use the encoder, you first need to produce a bounced file of your surround mix. You then highlight the bounced file and select Soundcode Encoder from the Soundfield sub-menu of the Audiosuite menu. Next, you set the Dolby Digital encoding parameters, the file name and file format, and the encoding happens off-line. You can then decode the resulting file and import the results back into Pro Tools for quality checking, by using the Encode and Import button at the bottom. Dolby Digital encoding has many features and options, and you should read the documentation that comes with the software package before embarking on any serious encoding. Fortunately, the plug-in has integrated help (right), which can be accessed by clicking on a text description.

An inspired feature of this plug-in is the ability to drop in and patch a short section of the encoded file. This will save you loads of time if, for example, you have already encoded a two-hour film soundtrack for a DVD, but a last-minute change is made to replace one line of dialogue. Obviously, you have to match the encoder settings when dropping in, and drop-ins can only be done at 48kHz sample rate, not 44.1kHz.

The decoder plug-in operates as a real-time RTAS plug-in (in stereo-to-mono, stereo-to-stereo or stereo-to-surround versions) or as a non-real-time Audiosuite plug-in. There are also HTDM versions for Pro Tools 6.9 users, though Neyrinck recommend using Pro Tools 7. The decoder has integrated help, just like the encoder, and again, you would be very wise to read the documentation before using it in anger, in order to understand the implications of all the different settings.

It is possible to use the decoder to decode a digital stream from a DVD player straight into the Pro Tools environment, which is useful when you want to compare your mix against a commercial DVD without leaving Pro Tools. You simply need to connect the DVD's digital output to the S/PDIF input on your Pro Tools interface, and pick up that digital input from within the plug-in. Conversely, the decoder can also be used to output a Dolby Digital AC3 file as a bitstream from a Pro Tools interface digital output, so you can make a reference CD or DVD disc to check your mix and the encoding on various consumer playback systems, assuming you have a suitable authoring package.

This is very definitely a professional's tool and not something the beginner should tackle. You need to read up and understand what all the different metadata settings are for. That said, the ability to create reference DVDs and CDs and then play them on various consumer systems to check out how your mixes hold out through the AC3 encoding and encoding processes, before commiting your project to manufacture, is great. Neyrinck include a comprehensive set of documentation to help you to get your head round the AC3 process and its settings.

SRS Circle Surround

Circle Surround TDM Pro v2.0 (above) comes from the creators of 'Circle Surround', SRS Labs. It requires a TDM rig running Pro Tools 7, and there is currently no Windows or Intel Mac version, though version 1 of Circle Surround is still available for VST hosts on Windows and Mac. The format is similar to Dolby Pro Logic II in that it is another matrix encode/decode process and offers backward compatibility with mono, stereo and any matrix surround decoder, but is different in that it offers up to 6.1 channels, whereas Pro Logic II is 5.0. SRS also claim that Circle Surround-encoded content can be delivered over any two-channel carrier, such as stereo television, HDTV, HD and FM radio, standard Red Book CDs and CD/DVD Dual Discs, as well as AAC and MP3 files for download or streaming.

Pro Tools: Surround Sound Encoding Plug-ins

The Circle Surround encoder takes up to a maximum of six discrete inputs and a subwoofer channel, and produces a two-channel LtRt mix. It is not necessary to use all inputs, though: you can use the encoder to prepare LCR, LCRS, and 5.1-encoded mixes as well.

The input-level indicators are much more basic than those of the Dolby Surround Tools encoder. They light green when an input signal reaches ­60dBFS and go red at or above 0dBFS. This makes them OK for checking that the encoder is receiving signal, but that is all. The encoder output, on the other hand, does have a more conventional bar-graph level meter.

SRS have also included optional bass management in the encoder process. When Bass Management is on, any information that enters the satellite channels (L, R, C, Ls, Rs, Cs) will go through an 80Hz high-pass filter, and any information that goes through the LFE will go through a corresponding low-pass filter. Using Bass Management allows you to roll off frequencies below 80Hz on all non-sub channels, thereby reducing the amount of low-end build up, but this feature flies in the face of conventional practice. Best practice is to leave bass management to be handled by the playback system, where it can be correctly designed for that particular system.

SRS include a matching decoder in their package, so you can check for compatibility through the encoding and decoding process, with input-level and output-level indicators that work in the same way as those on the encoder. The decoder operates in two modes: Monitor mode is for monitoring a Circle Surround-encoded two-channel mix and Xtract mode is for extracting dual-mono and stereo signals into surround sound. We will take a closer look at how well this works in a future survey of 'unwrapping' or 'upmixing' plug-ins.

Within Monitor mode, there are five different options available for monitoring a Circle Surround-encoded two-channel mix. SRS Circle Surround Cinema and SRS Circle Surround Music are separate modes that are sometimes found in consumer receivers, while in LCRS mode the rear channels are folded down to mono and rolled off above 7kHz. This is useful for monitoring in a way that emulates older decoders such as those found in early AV receivers. You can also monitor in stereo and mono, and there are options for monitoring with or without front and rear centre speakers.

Two post-processing options emulate features on SRS consumer products. SRS TruBass is a bass-enhancement algorithm using proprietary psychoacoustic techniques, which can be directed either to the LFE or the front L and R channels, while SRS Dialog Clarity is designed to improve dialogue intelligibility from all source material. These options are only available in SRS Circle Surround decode mode.

As in the Dolby Surround Tools decoder plug-in, a pink-noise test signal is available to identify channels and enable you to balance up your monitoring system using the Output Trim controls.

Minnetonka Surcode For Dolby Pro Logic II

The most recent addition to the field is Surcode For Pro Logic II from Minnetonka Audio (right), an RTAS plug-in that enables you to matrix-encode your surround mixes direct from 5.1 into Pro Logic II. It is cross-platform, but again there's no Intel Mac version yet. Unlike the others, Surcode is a single plug-in rather than a suite of tools. This makes it the most compact of the three 'matrix type' plug-ins; the encoding, decoding and monitoring are all handled in one plug-in, making comparison and compatibility checks as easy as one mouse click. The graphical interface won't win any awards for prettiness, but it is simple and clean, and one very useful feature of the display is the help that pops up in the bottom left-hand corner of the plug-in window as you roll over each control.

Pro Tools: Surround Sound Encoding Plug-ins

The instructions are basic, and I suspect they have been written for the experienced professional rather than being aimed at the beginner, as is the case with the other two plug-in manufacturers. This has its advantages, as there is less info to wade through to get to the important bits, but it does sometimes mean that useful stuff is missing, such as the best place to insert the plug-in in the signal chain.

Three buttons in the upper-left corner set the modes for encoding and for monitoring. When the Source button is selected, the Pro Logic II encoder is turned off and the inputs are connected directly to the outputs. When the Encode button is selected, the Pro Logic II encoder is activated, the encoded stereo signal is sent to the first two channels of your Pro Tools output bus, and the other four outputs are muted. This enables you to monitor the LtRt signal to check compatibility for listeners who don't have the appropriate decoders.

When the Encode and Decode buttons are both selected, the Pro Logic II encoder and decoder are both activated, enabling you to monitor through the encode and decode process and check the six channels of decoded audio that the end listener will hear in their home through their decoder. When you are mixing for delivery in Pro Logic, you should always monitor through the encode and decode loop rather than listening to the 'raw' 5.1 mix, as you will find signal imaging will change — for example, the front image will tend to be narrower than in the original, discrete 5.1 mix.

There is an overload indicator to show when the matrix encoder is overloaded. There is also a Limiter option to take care of the occasional overload, but it only has a range of 4dB, and Minnetonka recommend that if the overload shows more than three or four times on a track you should reduce input levels accordingly. This is done using the Input Trim controls, which are sensibly arranged so that the 0dB point is halfway up, with most of the travel covering +6dB to -12dB. They can all be ganged together or reset, but when I Alt-clicked the faders (the normal default keyboard shortcut to reset a control to its default setting) the input trims went to -0.02dB rather than zero.

Pro Tools: Surround Sound Encoding Plug-ins

Surcode For Pro Logic II provides four different decoder modes. Pro Logic is the original Pro Logic decoding system, which is helpful to make sure your mixes will work when decoded through the original Pro Logic decoders. Movie represents the standard setting for content with video, and is based on the original Pro Logic decoding scheme, but with the single surround channel separated into left and right surround channels. Matrix is basically the same as the original passive Dolby Surround decoder, without the directional steering provided by Pro Logic. Finally, the Music mode is the most versatile, and unlike the other modes, offers control over Panorama, Dimension and Centre Width options (though, like all decoder settings, these affect only the decoder part of the plug-in path and so will have no affect on the encoded signal).

Further options set the output Channel Configuration to match your monitor speaker configuration (see screen above), and allow you to attenuate the output of the Decoder to match the signal level without affecting the encoder path, making possible easier comparisions between the original and encoded/decoded mixes.

The owner's manual gives no guidance on how to use this plug-in, other than explaining that, when in Encode mode, it routes the encoded output to the L&R outputs of the plug-in. This means that to create a stereo encoded file you need to do a bounce with the Bounce Source set to the L&R outputs, having created a stereo front sub-path from the main Surround path. It would be helpful to have more guidance for less experienced users.

On-line Resources

Dolby Digital Professional Encoding Guidelines

www.dolby.com/assets/pdf/tech_library/46_DDEncodingGuidelines.pdf

Dolby Technical Library

www.dolby.com/resources/tech_library/index.cfm

Dolby Surround Mixing Manual

www.dolby.com/assets/pdf/tech_library/44_SuroundMixing.pdf

Pro Tools Surround Sound Mixing by Rich Tozzoli

www.backbeatbooks.com

Comparing Matrix-based Systems

All three of the matrix systems I tried work well, and they seem very compatible with each other: material encoded with each decoded well with all the others. They all suffered from a narrowing of the surround sound field, which is to be expected of any matrix-based encode/decode process, but I only really noticed this when I switched back to the original 5.1 mix. Then I got the sense of extra space, both in terms of image width and 'air' — almost as though the walls of the room expanded.

All three of the plug-ins have their own virtues and limitations. Dolby Surround Tools has an excellent manual, with comprehensive guidance on mixing in surround and working with the restrictions of a matrix-based system. The plug-in has comprehensive metering and a full professional decoder section. It is easy to use if you're mixing in LCRS but all the additional routing is a pain if you are mixing in 5.1 and need to provide a Pro Logic II mix as well. This plug-in really needs updating to add Pro Logic II and/or support for 5.1 tracks.

The SRS Circle Surround package includes a comprehensive range of plug-ins to suit the different input and output options, from LCRS to 6.1, consequently saving you from having to create aux inputs and paths to get the routing to work. It is easy to use, although I preferred the more detailed level-metering in Dolby Surround Tools. The claims of backward compatibility with other matrix-based systems seem to hold up well.

Minnetonka's Surcode For Pro Logic II is a compact plug-in that only needs one instance and makes it easy to compare source, encoded output and encoded/decoded signals. At first I thought it would be a problem that there is no way to monitor in surround when bouncing the LtRt signal, but on reflection it's useful to hear the complete track in LtRt and know there no problems for those listening without a decoder. The manual could be more comprehensive, but if I had to choose one of the three packages, it would be Surcode For Pro Logic II, for both simplicity and sonic quality. 

Prices & Contact Details

Dolby Surround Tools £558.13.

Digidesign UK +44 (0)1753 655999.

+44 (0)1753 658501.

infouk@digidesign.com

www.digidesign.com

www.dolby.com

SRS Circle Surround TDM Pro 2 £528.75.

Unity Audio +44 (0)1440 785843.

+44 (0)1440 785845.

sales@unityaudio.co.uk

www.unityaudio.co.uk

www.srslabs.com

Minnetonka Surcode For Dolby Pro Logic II £349 (VST version £385).

Et Cetera +44 (0)1706 285650.

+44 (0)1706 829457.

info@etcetera.co.uk

www.etcetera.co.uk

www.minnetonkaaudio.com

Neyrinck Soundcode For Dolby Digital £699.13.

Digidesign UK +44 (0)1753 655999.

+44 (0)1753 658501.

infouk@digidesign.com

www.digidesign.com

www.neyrinck.com

All prices include VAT.

Published June 2007