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Deconstructing the home theater pre-processor

by Nyal Mellor February 11, 2013

Ahhh....the home theater pre-pro....my thoughts on them are basically summarized by the phase "can't live with them, can't live without them"! Modern pre-pros are the brain of all modern home theaters. An AVR (audio video receiver) is essentially a pre-pro and multi-channel power amplifier in one box. Pre-pro's do a number of essential things without which it would be virtually impossible to watch movies in surround sound. They are not without their limitations though. The main thing that seems to suffer in lower priced units from Marantz and Integra is sound quality. What do you really expect if the the pre-pro manufacturers at this end of the market are basically caught up in a features war that each year adds more functionality to them that most people probably don't want or need. Audio and video quality is pretty much secondary to discussions of Audyssey XT vs XT32 and 7.1 vs 9.1. The other main problem with pre-pros is that the pace of change, particularly in video, media streaming and room correction is rapid, and makes new pre-pros into obsolete and expensive boat anchors very quickly. Top notch pre-pros are expensive - for example the DataSat RS20i probably classifies as state-of-the-art: 16 channels of output capability, Dirac Live room correction, exceptional DACs...but the retail is >$15k!  width= A state-of-the-art pre-pro with a $$$ price tag: the DataSat RS20i[/caption]   What if there was a better way to go about including the functionality of a pre-pro in your system? Could it be done in such as way that would make it modular and upgradeable when new improvements in video processing and room correction make their way to market? Many people say a pre-pro is like a mini audio and video computer. If so could we use a home theater PC as a pre-pro? Before we delve into this subject let's step back and take a look at what the core functions of a pre-pro are.

The functions of a pre-pro

Core. These are the pieces of functionality that any pre-pro must have in order to do it's job. We're assuming that the source signal is digital from a DVD or Blu-Ray.
  • Source selection
  • Volume control
  • Multi-channel digital to analog conversion
  • Delays and volumes for individual speakers
  • Upmixing, from 5.1 to 7.1 for example
  • Bass management
Secondary. There are a couple of items here which are 'nice to have' but are not necessary for a pre-pro to function.
  • High resolution audio format decoding (all blu ray players can decode these formats)
  • Room correction
  • Video scaling
Tertiary. Many Japanese pre-pros have a host of other features which are, in our opinion, not really necessary and many people may not even use.
  • 2nd and 3rd zones
  • Media streaming
  • Internet radio

The problem with pre-pros

With all of this functionality packed into one box typically something has to give. It seems like it is typically sound quality. It's interesting to review the pre-pro market. At one end there are the units from the Japanese manufacturers like Marantz and Integra that include everything but the kitchen sink. It seems features are added every year and many of them are not core to audio and video replay. In fact they most likely hinder reproduction quality as every additional piece of functionality added must take budget away from that dedicated to the audio and video circuitry.  width= The archetypal Japanese 'kitchen sink' pre-pro[/caption]   At the other end there are the units from Classe, Bryston and Anthem that cost ~9k or so. These units are typically streamlined and focus on audio and video playback quality. Many people, however, are wary of purchasing a high end pre-pro as they have a demonstrable history of becoming obsolete very rapidly. Units purchased before the advent of Blu-Ray and HDMI sell for <20% of their retail price. Most of these changes are driven by improvements in video technology, which moves a lot quicker than audio and where changes in HDMI specifications, for example, happen every few years it seems. The core issue I see day to day is that customers make do with a lower performance unit because they do not want to repeat the mistake of having an expensive boat anchor!  width= Bryston SP3: audio quality first![/caption]  

Deconstructing the pre-pro part 1: using an outboard video processor

Top manufacturers recognize these fundamental barriers to purchase with higher end pre-pros and have converged on a solution which involves removing video processing. The pre-pro keeps HDMI source switching but does no processing on it. The Bryston SP3, Cary Audio Cinema 12 and Krell Foundation all embody this thinking. Home theaters taking this approach may not include any video processing functionality beyond that included in the display. Those needing processing to optimize their display or upscale / deinterlace HDTV would likely use a Lumagen processor. Their higher end units incorporate multiple legacy and HDMI inputs, likely enough to mean that most customers do not need their pre-pro to include ANY video source switching. People using a dedicated video processor have a relatively low cost upgrade path when any new video display technology is released - for example Lumagen's 3D upgrade cost Lumagen XS3D - a state of the art video processor![/caption]  

Deconstructing the pre-pro part 2: using an Oppo with HDMI inputs

Just what are Oppo doing with their new blu-ray players, the BDP-103 and BDP-105? Their thinking is very solid I think: most people only have one or two sources beyond blu-ray (a game console and a cable box for example)...why don't we add a couple of HDMI inputs and start to make the blu-ray player a pre-pro. Oppo's year-on-year inclusion of functionality traditionally reserved for pre-pros (volume control, bass management, media streaming) make this a trend that seems likely to continue. Next year I forsee them introducing 5.1 to 7.1 upmixing. Maybe then we'll see room correction. It's interesting that they are already working with the Dirac Live algorithm in their new phone. width= Oppo's BDP103 - the first Blu-Ray player with HDMI inputs[/caption]  

Deconstructing the pre-pro part 3: a HTPC redux

HTPCs have to date generally been used to allow storage and rapid access to content in a way which is easier to use than a disc player and a physical DVD or Blu-Ray. You can rip all of your movies and TV shows to the HTPC and also use it to record cable TV, replacing a DVR. Nearly everyone, maybe 99.9% of users, are using their HTPCs with a pre-pro. They hook up their pre-pro or AVR to the PC via HDMI. The HTPC is basically a clever media storage device. But what if it could do more? What if it could replace a pre-pro entirely? This is the question that we at Acoustic Frontiers are going to investigate. Until recently there was no software available that could replicate the core functions of a pre-pro. This has changed with the availability of JRiver's recent releases. JRiver now has bass management, upmixing and 64 bit digital volume control capabilities. It has inbuilt parametric EQ and can support room correction / crossover convolution files from third parties such as AudioLense and Acourate. Although Dirac Live Room Correction Suite does not come in a plug in version you can run that on your PC too. There is even a plugin - MadVR - that seek to replicate what an outboard video processor like the Lumagen does. The stage is now set for a HTPC to replace the pre-pro in your system!  width= JRiver's Theater View[/caption]   There is of course one major component missing that your hardware must provide - a multi-channel digital-to-analog converter. There are a couple of ways to go about this - an internal soundcard within your computer or an external multi-channel DAC. There are many units available from the pro side of the business that could work including the Metric Halo LIO8, Lynx Aurora as well as the ExaSound e28. Or you could use a digital interface and hook up a number of 2 channel DACs, perhaps using different quality units for your left / right and remaining speakers. With all this power and flexibility the core questions are:
  • Can a HTPC replace a pre-pro?
  • Can a HTPC work reliably in this capacity or will it crash, freeze during playback, etc?
  • Can it sound better than a high end pre-pro when using an external high quality DAC?
  • Can it replace an outboard video processor?

The experiment begins...an Acoustic Frontiers HTPC proof of concept!

In our demo room we have a 7 channel surround system using Procella Audio speakers. The screen speakers (left, center and right) are the P610s which need an external crossover. There are four subs in a front and rear array, each of which requires a separate signal. The front is comprised of two Procella P10s and the rear of two JL F112s. We therefore need 12 output channels! Our first attempt at getting the demo room up and running was to use a Marantz AV7701 and a Xilica XP-4080. Whilst this resulted in good sound quality it was far short of the DEQX HDP-4 when it was used to drive the left and right speakers in a stereo setup. It was clear the audio quality of the Marantz and Xilica combo wasn't up to par with the DEQX, and nor should you expect it to be since the HDP-4 is almost double the retail price of the Marantz / Xilica combination. We therefore wondered what other solutions there might be. A candidate was DataSat, with their RS20i. However that retails for >$15K! We wondered if we could do the same for less, and with greater flexibility. Once we started looking around we came across the HTPC as a potential solution. It would even be possible to try out the same room correction algorithm as DataSat uses - Dirac Live, since they have just released a room correction package that runs on the PC. So begins the HTPC experiment. Our PC build is based on the CAPS Zuma but uses a larger chassis with an optical drive to allow DVD and Blu-Ray playback from optical media. We are using a Lynx AES16e card and to start with our external DACs will be a DEQX HDP-4 and a Metric Halo LIO8. Can the Acoustic Frontiers HTPC do 12 channels with room correction at 24/96 without crashing? Will it sound better than a high end pre-pro? All these questions and more will be answered in future blog articles.

Nyal Mellor
Nyal Mellor


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Nyal Mellor, Founder, Acoustic Frontiers

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